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Sacramento Model — (Hydrology) A dynamic River Flow Model or Water Budget Model, run on a computer at various intervals, which accounts for all water entering, stored in, and leaving a Drainage Basin. Though many parameters are used in this water balance accounting process, precipitation has the main impact on runoff.

Sacrificial Anode — An easily corroded material deliberately installed in a pipe or intake to give it up to corrosion while the rest of the water supply facility remains relatively corrosion-free.

Safe Drinking Water Act [SDWA] (Public Law 93–523) — An amendment to the Public Health Service Act which established primary and secondary quality standards for drinking water. The SDWA was passed in 1976 to protect public health by establishing uniform drinking water standards for the nation. In 1986 SDWA Amendments were passed that mandated the U.S. Environmental Protection Agency (EPA) to establish standards for 83 drinking water contaminants by 1992 and identify an additional 25 contaminants for regulation every 3 years thereafter. See Drinking Water Standards, Drinking Water Standards [Nevada], Primary Drinking Water Standards, and Secondary Drinking Water Standards. [Also see Appendix B–3, Nevada Drinking Water Standards.]

Safe Water — Water that does not contain harmful bacteria, toxic materials, or chemicals, and is considered safe for drinking even if it may have taste, odor, color, and certain mineral problems.

Safe Yield —
(1) The rate at which water can be withdrawn from supply, source, or an aquifer over a period of years without causing eventual depletion or contamination of the supply.
(2) A rate of extraction that does not deplete the basin over time.
(3) (Groundwater) The amount of water that can be withdrawn from an aquifer without producing an undesired effect. (4) (Surface Water) The amount of water than can be withdrawn or released from a reservoir on an ongoing basis with an acceptably small risk of supply interruption (i.e., reducing the reservoir storage to zero.) More commonly referred to a Perennial Yield and Sustained Yield. Generally consists of the rate of Natural Recharge, Artificial (or Induced) Recharge, and Incidental Recharge.

Sag Pipe — A section of a sewer line that is placed deeper in the ground than normal in order to pass under utility piping, waterways, rail lines, highways, or other obstacles. The sewer line is raised again after passing under the obstacle. Also referred to as Inverted Siphon.

Sag Pond —
(1) A small body of water occupying an enclosed depression or sag formed where active or recent fault movement has impounded drainage.
(2) (California) One of many ponds and small lakes along the San Andreas Fault.

Salina —
(1) A salt marsh, spring, pond, or lake.
(2) An area of land encrusted with salt.

Salination — The process whereby soluble salts accumulate in the soil.

Saline —
(1) Term used to describe waters containing common salt, or sodium chloride.
(2) Of or containing any of the salts of the alkali metals or magnesium.

Saline/Poor Quality Aquifer — An aquifer containing water that is high in total dissolved solids, and is unacceptable for use as drinking water.

Saline Marsh — A saturated, poorly drained area, intermittently or permanently water covered, having aquatic and grasslike vegetation whose water chemistry contains various dissolved salts.

Saline Seeps — Wet areas in non-irrigated soils where soluble salts accumulate from the evaporation of the seeping water and crop or grass production is reduced or eliminated.

Saline Sodic Land — Soil that contains soluble salts in amounts that impair plant growth but not an excess of exchangeable sodium.

Saline Soil — A nonalkali soil containing soluble salts in such quantities that they interfere with the growth of most plants.

Saline Water — Water containing dissolved solids; generally referring to solid contents in excess of 1,000 parts per million (ppm) Total Dissolved Solids (TDS). The U.S. Geological Survey (USGS) classifies the degree of salinity of these more mineralized bodies of water as follows:
[1] Slightly Saline — 1,000–3,000 ppm;
[2] Moderately Saline — 3,000–10,000 ppm;
[3] Very Saline — 10,000–35,000 ppm; and
[4] Brine — More than 35,000 ppm.

Salinity —
(1) The concentration of dissolved salts in water or soil water. Salinity may be expressed in terms of a concentration or as an electrical conductivity. When describing salinity influenced by seawater, salinity often refers to the concentration of chlorides in the water.
(2) The relative concentration of salts, usually sodium chloride, in a given water sample. It is usually expressed in terms of the number of parts per thousand (‰) or parts per million (ppm) of chloride (Cl). Although the measurement takes into account all of the dissolved salts, sodium chloride (NaCl) normally constitutes the primary salt being measured. Salinity can harm many plants, causing leaves to scorch and turn yellow and stunting plant growth. As a reference, the salinity of seawater is approximately 35‰ or 35,000 ppm. See Salts for comparative salt concentrations in water.

Salinity Control — The physical control, management, and use of water and related land resources in such a way as to maintain or reduce salt loading and concentrations of salt in water supplies.

Salinity Intrusion — The movement of salt water into a body of fresh water. It can occur in either surface water or ground water bodies.

Salinization — The accumulation of salts in soil to the extent that plant growth is inhibited. This is a common problem when crops are irrigated in arid regions; much of the water evaporates and salts accumulated in the soil. Also see Leaching Requirement.

Salmonella Typhosa — A waterborne microorganism which is the causative agent of typhoid fever.

Salmonellosis — The bacterial disease caused by the presence of bacteria of the genus Salmonella. The disease is a type of food poisoning characterized by a sudden onset of gastroenteritis involving abdominal pain, diarrhea, fever, nausea, and vomiting. A variety of foods, such as sweets, meats, sausages, and eggs, can be the mode of infection. Pet turtles and birds can also transmit the bacteria.

Salmonid — Fish family that includes salmon, trout, whitefish and grayling.

Salts — Salts are the minerals that water picks up as it passes through the air, over and under the ground, and through household and industrial uses. A compound composed of the positive ion from a base and the negative ion from an acid; i.e., a metal ion and a nonmetal ion, such as KBr (Potassium Bromide). The proportions or concentrations of salts in water is a determining factor in evaluating its quality. The concentration of salts in fresh water is, on the average, so small that it is expressed in parts per million (ppm). The table below presents examples of average concentrations of dissolved salts in different types of water:
Sources of Water Salt Concentrations (ppm)
Distilled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . .. . . 0
Rain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Lake Tahoe (California/Nevada) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Truckee River (California) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Suwannee River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 150
Lake Michigan . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 170
Missouri River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 360
Pecos River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,600
Pyramid Lake (Nevada – 1997) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5,000
Walker Lake (Nevada – 1997) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12,500
Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35,000
Brine Well . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 125,000
Dead Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250,000

Salvage Harvest — Removal of dead and dying trees resulting from insect and disease epidemics or wildfire.

Salvaged Water — The part of a particular stream or other water supply that is saved from loss, in respect to quantity or quality, and is retained and made available for use.

Sample — (Statistics) The sample consists of a number of randomly-selected, representative observations about an economic phenomenon; a part of a population taken to estimate a parameter of the whole population. The underlying assumptions are that we do not have the means to measure the entire population of events and that the sample, if properly selected, will accurately represent the behavior and characteristics of the entire population (of events) within specified limits of probability. Types of samples include:
[1] Random — A sample drawn without bias from a population in which every item has an equal chance of being drawn;
[2] Representative — A sample drawn in such a way that it gives a true value for the population from
which it was drawn.

Sand Filter — (Water Quality) A device used to remove particles from drinking water prior to distribution to customers. The water is allowed to Percolate through a chamber containing sand of various grain sizes, with the finest grain size located on the top. The particles in the water are removed at the surface of the sand and later discarded by reverse flushing.

Sandstone Aquifer — The type of aquifer supplying groundwater to large parts of the United States upper Middle West, Appalachia, and Texas. The water-bearing formation is often contained by shale strata, and the water has high levels of iron and magnesium.

Sand Trap — (Irrigation) A device, often a simple enlargement in a ditch or conduit, for arresting the heavier particles of sand and silt carried by the water.

Sanitary Water — Water discharged from sinks, showers, kitchens, or other nonindustrial operations, but not from commodes. Also referred to as Gray Water.

Saprobien System — (Water Quality) A European system of classifying organisms according to their response to organic pollution in slow-moving streams. Classifications include:
[1] Alpha–Mesaprobic Zone — Area of active decomposition, partly Aerobic, partly Anaerobic, in a stream heavily polluted with organic wastes;
[2] Beta–Mesaprobic Zone — That reach of a stream that is moderately polluted with organic wastes;
[3] Oligosaprobic Zone — That reach of a stream that is slightly polluted with organic wastes and contains the mineralized products of self-purification from organic pollution, but with none of the organic pollution remaining;
[4] Polysaprobic Zone — That area of a grossly polluted stream that contains the complex organic wastes that are decomposing primarily by anaerobic processes.

Sapropel —
(1) A mud rich in organic matter formed at the bottom of a body of water.
(2) A fluid slime found in swamps and bogs as a product of Putrefaction.

Saprophyte — An organism, especially a fungus or bacterium, that grows on and derives its nourishment from dead or decaying organic matter that help natural decomposition of organic matter in water.

Saturated Flow — The liquid flow of water in soils that occurs when the soil pores in the wettest part of the soil are completely filled with water and the direction of flow is from the wettest zone of higher potential to one of lower potential.

Saturated Thickness (Aquifer) — The thickness of the portion of the aquifer in which all pores, or voids, are filled with water. In a Confined Aquifer, this is generally the aquifer thickness. In an Unconfined Aquifer, this is the distance between the water table and the base of the aquifer.

Saturated Zone —
(1) The part of a water bearing layer of rock or soil in which all spaces, large or small, are filled with water.
(2) The zone in the earth’s crust, extending from the water table downward, in which all open pore spaces in the soil or rock are filled with water at greater than atmospheric pressure. A termed used synonymously with the Zone of Saturation.

Sausage Dam — A dam composed of loose rock that has been wrapped with wire into cylindrical bundles and laid in a horizontal or vertical position.

Saxitoxin — The primary toxin produced by dinoflagellate protozoans during blooms known as Red Tides in marine waters. The genus or protozoan involved in the generation of the red color in the water is Ganyaulax.

Scabland — (Geography) An elevated area of barren, rocky land with little or no soil cover, often crossed by dry stream channels.

Scarp —
(1) A line of cliffs produced by faulting or by erosion. The term is an abbreviated form of Escarpment, and the two terms commonly have the same meaning, although “scarp” is more often applied to cliffs formed by faulting.
(2) A relatively steep and straight, cliff-like face or slope of considerable linear extent, breaking the
general continuity of the land by separating level or gently sloping surfaces lying at different levels, as along the margin of a plateau, mesa, terrace, or bench.

Scenic Rivers — A classification under the national Wild and Scenic Rivers Act to include those rivers or sections of rivers that are free of impoundments, with shorelines or watersheds still largely primitive and shorelines largely undeveloped, but accessible in places by roads. The following represents restrictions applying to such designated rivers:
(1) Timber Production – A wide range of silvicultural practices could be allowed if such practices are carried on so that there is no substantial adverse effect on the river and its immediate environment. The river area should be maintained in its near natural environment. Timber outside the boundary but within the visual scene area should be managed and harvested in a way that provides special emphasis on visual quality.
(2) Water Supply – All water supply dams and major diversions are prohibited.
(3) Hydroelectric Power – No development of hydroelectric power facilities would be allowed.
(4) Flood Control – Flood control dams and levees would be prohibited.
(5) Mining – Subject to regulations (i.e., 36 CFR 228) that the Secretaries of Agricultural and Interior may prescribe to protect the values of rivers included in the national system. New mining claims and mineral leases could be allowed and existing operations allowed to continue. However, mineral activity must be conducted in a way that minimizes surface disturbances, sedimentation and pollution, and visual impairment.
(6) Road Construction – Roads may occasionally bridge the river area and short stretches of conspicuous or longer stretches on inconspicuous and well screened roads or screened railroads could be allowed. Consideration will be given to the type of use for which roads are constructed and the type of use that will occur in the river area.
(7) Agriculture – A wider range of agricultural uses is permitted to the extent currently practices. Row crops are not considered as an intrusion of the ‘largely primitive” nature of scenic corridors if there is not a substantial adverse effect on the natural-like appearance of the river area.
(8) Recreational Development – Larger scale public use facilities, such as moderate size campgrounds, public information centers, and administrative headquarters are allowed if such structures are screened from the river. Modest and unobtrusive marinas also can be allowed.
(9) Structures – Any concentrations of habitations are limited to relatively short reaches of the river corridor. New structures that would have a direct and adverse effect on river values would not be allowed.
(10) Utilities – New transmission lines, gas lines, water lines, etc., are discouraged. Where no reasonable alternative exists, additional or new facilities should be restricted to existing right-of-way. Where new rights-of-ways are indicated, the scenic, recreation, and fish and wildlife values must be evaluated in the selection of the site.
(11) Motorized Travel – Motorized travel on land or water may be permitted, prohibited or restricted to protect the river values.

Science Advisory Board (SAB) — An independent body established by the Administrator of the U.S. Environmental Protection Agency (EPA) in 1974 and by Congress in 1977. Its purpose is to review the scientific merits of EPA research and the scientific basis for the agency’s proposed regulations and standards.

Scour —
(1) To clear, dig, or remove by or as if by a powerful current of water.
(2) The erosive action of running water in streams, which excavates and carries away material from the bed and banks. Scour may occur in both earth and solid rock material.
(3) The powerful and concentrating clearing and digging action of flowing air or water, especially the downward erosion by stream water in sweeping away mud and silt on the outside curve of a bend, or during time of flood.
(4) A place in a stream bed swept (scoured) by running water, generally leaving a gravel bottom.
(5) The process by which flood waters remove soil around objects that obstruct flow, such as the foundation walls of a house.

Scouring Sluice — An opening in a dam controlled by a gate through which the accumulated silt, sand, and gravel may be ejected.

Seasonal Adjustment Factors — (Data Analysis) Mathematical indexes used to adjust for the intra-year normal seasonal fluctuations in time-series data. In its simplest form, such indexes may be derived from dividing the value of each observation by a simple moving average centered over each data observation with such an average encompassing the six-month period before and after the observation. More sophisticated seasonal adjustment processes may also account for the placement of weekends and holidays and other factors affecting data behavior.
Also see Seasonal Factors.

Seasonal Application Efficiency (SAE) — The sum of the Evapotranspiration of Applied Water (ETAW), the Leaching Requirement (LR), and cultural practices (CP) (e.g., frost protection, heat protection, weed control), divided by the total applied water (AW), expressed as a percentage, or,
SAE = (ETAW + LR + CP)/AW.

Seasonal Factors — (Data Analysis) Factors such as weather conditions, agricultural production and irrigation requirements, business and tourism patterns, school schedules and holidays, etc., which cause normal annual fluctuations in business activity within individual and/or combinations of industries. These factors affect the demand for commercial, residential, and irrigation water use. Also see Seasonal Adjustment and Seasonal Adjustment Factors.

Seasonal or Intermittent Streams — Streams which flow only at certain times of the year when it receives water from springs, rainfall, or from surface sources such as melting snow. Also see Stream.

Seasonality — (Statistics) Periodic, repetitive, and generally predictable patterns in time series data. Typically, forecast models must explicitly incorporate seasonality in the estimation process either through Differencing techniques (transformations), through the introduction of seasonal Dummy Variables to explicitly account for these patterns, or by seasonal smoothing (Winters Model).

Secchi Depth — A relatively crude measurement of the turbidity (cloudiness) of surface water. The depth at which a Secchi Disc (Disk), which is about 10–12 inches in diameter and on which is a black and white pattern, can no longer be seen.

Secchi Disc (Disk) — A circular plate, generally about 10–12 inches (25.4–30.5 cm) in diameter, used to measure the transparency or clarity of water by noting the greatest depth at which it can be visually detected. Its primary use is in the study of lakes.

Secondary Data — (Data Analysis) Typically, data acquired from published sources as opposed to data acquired from direct observation or measurement such as a survey. Contrast with Primary Data.

Secondary Drinking Water Regulations — Non-enforceable regulations applying to public water systems and specifying the maximum contamination levels that, in the judgement of the U.S. Environmental Protection Agency (EPA), are required to protect the public welfare. These regulations apply to any contaminants that may adversely affect the odor or appearance of such water and consequently may cause people served by the system to discontinue its use. Term may be used synonymously with Secondary Drinking Water Standards.

Secondary Drinking Water Standards — Non-enforceable standards related to the aesthetic quality of drinking water such as those relating to taste and odor; generally set by the U.S. Environmental Protection Agency (EPA) or state water-quality enforcement agencies based on EPA guidance. Term may be used synonymously with Secondary Drinking Water Regulations. Also see Drinking Water Standards and Drinking Water Standards [Nevada]. [See Appendix B–3, Nevada Drinking Water Standards for a listing of Nevada’s current drinking water primary and secondary quality standards.]

Secondary Maximum Contaminant Level (SMCL) — The maximum concentration or level of certain water contaminants in public water supplies set by the U.S. Environmental Protection Agency (EPA) to protect the public welfare. The secondary levels are written to address aesthetic considerations such as taste, odor, and color or water, rather than health standards. Also see Primary Drinking Water Standards, Maximum Contaminant Level (MCL), and Maximum Contaminant Level Goal (MCLG).

Secondary Porosity — The porosity that results from fractures and solution channels.

Secondary Standards — Allowable amounts of materials in air or water that are set to retain environmental qualities not related to the protection of human health. Secondary water standards are set for, among other things, taste, odor, and color, and some secondary air standards define concentrations that will not be harmful to plant life. Compare to Primary Standards.

Secondary Succession — The orderly and predictable changes that occur over time in the plant and animal communities of an area that has been subjected to the removal of naturally occurring plant cover. This type of succession occurs when agricultural fields are taken out of use or when forested areas are subjected to severe fires that destroy all vegetation. In both cases the top soil remains for the regrowth of natural plant communities. Compare to Primary Succession.

Secondary Wastewater Treatment — Treatment (following Primary Wastewater Treatment) involving the biological process of reducing suspended, colloidal, and dissolved organic matter in effluent from primary treatment systems and which generally removes 80 to 95 percent of the Biochemical Oxygen Demand (BOD) and suspended matter. Secondary wastewater treatment may be accomplished by biological or chemical-physical methods. Activated sludge and trickling filters are two of the most common means of secondary treatment. It is accomplished by bringing together waste, bacteria, and oxygen in trickling filters or in the activated sludge process. This treatment
removes floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection is the final stage of secondary treatment.

Section — A unit of land area, generally equal to one square mile or 640 acres (259 hectares). Thirty-six sections also comprise a Township (6 miles long by 6 miles wide). The section is part of a description of the location of land using the survey system (Public Land Survey System — PLSS) of the United States Government and includes the 40–acre subdivision within a quarter, section, township and range. The public land survey system is based on the concept of a township as a square parcel of land six miles on each side. Its location is established as being so many six-mile units east of a north-south line (called the meridian) and so many six-mile units north or south of an eastwest line (called the baseline). The township is described by township and range, e.g., T.4N, R.23E. Each township is further divided into 36 parts called sections one mile square (each section measuring 5,280 feet on each side). A typical section containing 640 acres may be further subdivided into quarters (measuring 2,640 feet on each side). The quarter of a section is equal to 160 acres and described as the NW1/4 or the NE1/4 or the SW1/4 or the SE1/4 of that particular section. The quarter section may be divided into four quarters, each being 1/16 of a section, or 40 acres (measuring 1,320 feet on each side). Each sixteenth is described as the NW1/4 or the NE1/4
or the SW1/4 or the SE1/4 of that particular quarter section. A person must determine which is north on a map (usually at the top) in this case the easterly direction would be to the right, south to the bottom, and west to the left. The written location of a specific 40–acre parcel of land would be NW1/4 of the NW1/4 of Section 13, T.4N., R.23E. Some sections may contain more or less acreage than 640.

Secure Maximum Contaminant Level — The maximum permissible level of a contaminant in water delivered to the free flowing outlet of the ultimate user, or of contamination resulting from corrosion of piping and plumbing caused by water quality.

Sediment —
(1) Soil particles that have been transported from their natural location by wind or water action; particles of sand, soil, and minerals that are washed from the land and settle on the bottoms of wetlands and other aquatic habitats.
(2) The soil material, both mineral and organic, that is in suspension, is being transported, or has been
moved from its site of origin by erosion (by air, water, gravity, or ice) and has come to rest on the earth’s surface.
(3) Solid material that is transported by, suspended in, or deposited from water. It originates mostly from disintegrated rocks; it also includes chemical and biochemical precipitates and decomposed organic material, such as humus. The quantity, characteristics, and cause of the occurrence of sediment in streams are influenced by environmental factors. Some major factors are degree of slope, length of slope, soil characteristics, land usage, and quantity and intensity of precipitation.
(4) In the singular, the word is usually applied to material in suspension in water or recently deposited from suspension. In the plural the word is applied to all kinds of deposits from the waters of streams, lakes, or seas, and in a more general sense to deposits of wind and ice. Such deposits that have been consolidated are generally called sedimentary rocks.
(5) Fragmental or clastic mineral particles derived from soil, alluvial, and rock materials by processes of erosion, and transported by water, wind, ice, and gravity. A special kind of sediment is generated by precipitation of solids from solution (i.e., calcium carbonate, iron oxides). Excluded from the definition are vegetation, wood, bacterial and algal slimes, extraneous lightweight artificially made substances such as trash, plastics, flue ash, dyes, and semisolids.

Sediment Control — The control of movement of sediment on the land, in a stream or into a reservoir by means of manmade structures; such as debris dams, wing dams, or channelization; land management techniques, or natural processes.

Sediment Load —
(1) The soil particles transported through a channel by stream flow.
(2) The total sediment, including bedload plus suspended sediment load, is the sediment being moved by flowing water in a stream at a specified cross section.

Sediment Oxygen Demand (SOD) — The amount of dissolved oxygen removed from the water covering the sediment in a lake or stream because of microbial activity.

Sediment Pool — The reservoir space allotted to the accumulation of submerged sediment during the life of the structure.

Sediment-Transport Curve — Usually the relation between water discharge and Suspended-Sediment Discharge, but it can be between water discharge and Bed-Load Discharge, Unmeasured Sediment Discharge, or Total Sediment Discharge.

Sediment Yield — That amount of sediment transported by a stream system that may be measurable at a particular location. Usually expressed in volume or weight per unit of time.

Sedimentary Cycle — The biogeochemical cycle in which materials primarily are moved from land to sea and back again.

Sedimentary Rock — (Geology) Rock formed of sediment, especially from mechanical, chemical, or organic processes, and specifically:
(1) clastic rock, such as conglomerate, sandstone, and shale, formed of fragments of other rock transported from their sources and deposited in water; and
(2) rocks formed by precipitation from solution, as rock salt and gypsum, or from secretions of organisms, such as most limestone. Many sedimentary rocks show distinct layering, which is the result of different types of sediment being deposited in succession.

Sedimentation —
(1) Strictly, the act or process of depositing sediment from suspension in water. Broadly, all the processes whereby particles of rock material are accumulated to form sedimentary deposits. Sedimentation, as commonly used, involves not only aqueous but also glacial, aeolian, and organic agents.
(2) (Water Quality) Letting solids settle out of wastewater by gravity during treatment.

Sedimentation Basin (SB) — A surface water runoff storage facility intended to trap suspended solids, suspended and buoyant debris, and adsorbed or absorbed potential pollutants that are carried by surface water runoff. The sedimentation basin may be part of an overall multipurpose detention and retention facility.

Sedimentation Tanks — Wastewater tanks in which floating wastes are skimmed off and settled solids are removed for disposal.

Sediments — Soil, sand, and minerals washed from the land into water, usually after rain. They pile up in reservoirs, rivers, and harbors, destroying fish and wildlife habitat, and clouding the water so that sunlight cannot reach aquatic plants. Careless farming, mining, and building activities will expose sediment materials, allowing them to wash off the land after rainfall.

Seep —
(1) To pass slowly through small openings or pores; ooze.
(2) Groundwater emerging on the face of a stream bank.
(3) An area which slowly passes water out of the ground to the surface, or where water moves slowly from surface bodies to groundwater bodies, as from canals and ditches into the underlying groundwater table.
(4) An area of minor groundwater outflow onto the land surface or into a stream channel or other water body. Flows are usually too small to be a spring.

Seepage —
(1) The passage of water or other fluid through a porpus medium, such as the passage of water through an earth embankment or masonry wall.
(2) Groundwater emerging on the face of a stream bank.
(3) The slow movement of water through small cracks, pores, Interstices, etc., of a material into or out of a body of surface or subsurface water.
(4) The Interstitial movement of water that may take place through a dam, its foundation, or its Abutments.
(5) The loss of water by infiltration into the soil from a canal, ditches, laterals, watercourse, reservoir, storage facilities, or other body of water, or from a field. Seepage is generally expressed as flow volume per unit of time. During the process of priming (a field during initial irrigation), the loss is called Absorption Loss.

Seepage Bed — A trench or bed more than 36 inches (0.91 meter) wide containing at least 12 inches (30.5 centimeters) of clean, coarse aggregate and a system of distribution piping through which treated sewage may seep into the surrounding soil.

Seepage Lakes — Lakes whose ecology is determined primarily by ground water rather than surface water.

Seiche — An oscillation of the water surface of a lake or other body of water due to variations of atmospheric pressure, wind, or minor earthquakes. The oscillation may be a foot or more in amplitude and may last several hours.

Self-Purification — The ability of a body of water to rid itself of pollutants. The removal of organic material, plant nutrients, or other pollutants from a lake or stream by the activity of the resident biological community. Biodegradable material added to a body of water will gradually be utilized by the microorganisms in the water, lowering the pollution levels. If excessive amounts of additional pollutants are not added downstream, the water will undergo self-cleansing. This process does not apply to pollution by non-biodegradable organic compounds or metals.

Semiaquatic — Adapted for living or growing in or near water; not entirely aquatic.

Semivolatile organic compound (SVOC) — Operationally defined as a group of synthetic organic compounds that are solvent-extractable and that can be determined by gas chromatography/mass spectrometry. SVOCs include phenols, phthalates, and polycyclic aromatic hydrocarbons (PAHs).

Senescence (Senescent) — Describing plants or specific ecosystems that are nearing the end of their normal life span. Sometimes used to describe lakes or other bodies of water in advanced stages of Eutrophication. For example, a lake that is filling with accumulated aquatic vegetation, dead plant material, and sediments can be described as senescent because it is nearing extinction as a productive lake environment.

Sensitive Species —
(1) Those plant or animal species susceptible or vulnerable to activity impacts or habitat alterations. Species not yet officially listed but undergoing status review for listing on the U.S. Fish and Wildlife Service’s (USFWS) official threatened and endangered list; species whose populations are small and widely dispersed or restricted to a few localities; and species whose numbers are declining so rapidly that official listing may be necessary. A
(2) Those plant and animal species identified by a Regional Forester for which population viability is a concern as evidenced by:
.... (a) significant current or predicted downward trends in population numbers or density; or

.... (b) significant current or predicted downward trend in habitat capability that would reduce a ..........species’ existing distribution.

Sequencing Batch Reactors (SBR) — (Water Quality) A wastewater treatment technique consisting of an activated sludge system which operates sequentially in time rather than in space, that is, all steps of the process take place, one after the other, in the same tank instead of moving to a second tank for the continuation of the treatment. The typical SBR operation involves filling a tank with raw wastewater or primary effluent, aerating the wastewater to convert the organics into a microbial mass, providing a period for settling, discharging the treated effluent, and a period identified as “idle” that represents the time after discharging the tank and before refilling. Typically, a multiple tank system is required, allowing incoming flow to be switched to one tank while the other is going
through the aeration, clarification, discharge, and idle functions. A key element of the SBR process is that a tank is never completely emptied, but rather a portion of settled solids is left in the tank for the next cycle. The retention of sludge within the tank establishes a population of microorganisms uniquely suited to treating the waste.

Sequestering Agent — (Water Quality) A chemical compound such as EDTA or certain polymers that chemically tie up (sequester) other compounds or ions so they cannot be involved in chemical reactions.

Sere — A transitional stage in plant succession. Environmental conditions, species, or biotic communities may be described as seral in contrast to Climax.

Serial Distribution — An arrangement of Absorption Trenches, Seepage Pits, or Seepage Beds so that each is forced to pond, utilizing the total effective absorption area, before liquid flows into the succeeding component.

Seston — All material, both organic and inorganic, suspended in a waterway.

Setback — Denotes the positioning of a levee or structure in relationship to a stream bank. A setback levee is placed a substantial distance from a stream to allow it to meander without consequences to the levee and to accommodate a floodplain that can store and convey flood flows. A setback regulation of a certain number of feet can be a requirement for the placement of urban buildings away from a stream-channel bank.

Settling Pond — (Water Quality) An open Lagoon into which wastewater contaminated with solid pollutants is placed and allowed to stand. The solid pollutants suspended in the water sink to the bottom of the lagoon and the liquid is allowed to overflow out of the enclosure.

Settling Velocity — (Water Quality) The rate of downward movement of particles through water. This gravitational settling removes particles naturally and is used also in pollution control devices, for example the Settling Tanks and Ponds in a sewage treatment facility. The settling velocity of a particle is often the same as the Terminal Settling Velocity.

Seven-Day 10-Year Low Flow (7Q10) — The period of lowest stream flow during a seven-day interval that is expected to occur once every 10 years. The 7Q10 has a 10-percent chance of occurring in any given year. During this time of low flow, the amount of Dissolved Oxygen in the water would be expected to be the lowest encountered under normal conditions. Since such conditions are considered to be the worst natural case, the dissolved oxygen levels during such episodes are used to establish Ambient Water Quality Standards for that stream.

Severance (Water Rights) — The act of severing or separating water rights from the land. In most western states, water rights are considered attached to the land on which they are used. When land is sold, water rights automatically are transferred with the land to the new owner, unless such rights are specifically severed from the land by provisions of the deed.

Sewage Lagoon — A shallow pond where natural processes are employed to treat sanitary waste from households or public rest rooms. Solid material settles to the bottom and is degraded by Anaerobic microbial communities. The enclosure is open to the atmosphere, which permits Aerobic mineralization of organic compounds in the upper layers of the water. The decomposition processes are analogous to those in effect in Primary and Secondary Wastewater Treatment processes. The effluent from these ponds is usually allowed to flow into nearby streams without further purification.

Sewage Sludge — Settled sewage solids combined with varying amounts of water and dissolved materials that are removed from sewage by screening, sedimentation, chemical precipitation, or bacterial digestion. The terms Biosolids, Sludge, and sewage sludge can be used interchangeably.

Sewage Treatment Plant — A facility designed to receive the wastewater from domestic sources and to remove materials that damage water quality and threaten public health and safety when discharged into receiving streams or bodies of water. The substances removed are classified into four basic areas:
[1] greases and fats;
[2] solids from human waste and other sources;
[3] dissolved pollutants from human waste and decomposition products; and
[4] dangerous microorganisms.
Most facilities employ a combination of mechanical removal steps and bacterial decomposition to achieve the desired results. Chlorine is often added to discharges from the plants to reduce the danger of spreading disease by the release of pathogenic bacteria.

Sharp-crested Weir — A device for measuring water, featuring a notch cut in a relatively thin plate and having a sharp edge on the upstream side of the crest.

Shock Load — (Water Quality) The arrival at a water treatment plant of raw water containing unusual amounts of algae, colloidal matter, color, suspended solids, turbidity, or other pollutants.

Shoot —
(1) A rush of water down a steep or rapid.
(2) A place where a stream runs or descends swiftly.
(3) To cause
to move suddenly or swiftly forward as motion down rapids.

Short-Circuiting — (Water Quality) A condition that exists when some of the water in water treatment tanks or basins flows faster than the rest; may result in shorter contact, reaction, or settling times than calculated or presumed.

Sideslope — The erosional slope around the sides of an erosional fan remnant, hill, ballena, mountain, etc., that is composed of shoulder, backslope, footslope and perhaps toeslope components. Also, the planimetrically-linear portions of the slopes around a digitately-dissected fan remnant or hill, etc., as compared with the planimetrically convex noseslope and concave headslope portions.

Slash and Burn — (Environmental) An agricultural practice involving the rapid destruction of natural forest for limited farming activity. The natural forest is cleared, the residue from the clearing process is burned, and crops are planted for a few years. When the fertility of the soil is depleted, the process is repeated in a new area, and the initial land is abandoned.

Slip-Off Slope Bank — The bank of a meandering stream which is not eroded by stream action, and which may be built up gradually.

Slope Wash — Soil and rock material that is being or has been moved down a slope predominantly by the action of gravity assisted by running water that is not concentrated into channels. The term applies to the process as well as the materials.

Sloping Gage — A staff gage used to register the elevation of the water surface in a stream channel, conduit, reservoir or tank, with a scale graduated to represent vertical elevation. A sloping gage is usually installed on a flat sloping bank where it is desirable to increase the accuracy of reading the gage.

Slough —
(1) A place of deep mud or mire; a wet or marshy place as a swamp or marshland creek. Also a side channel or inlet as from a river; ordinarily found on or at the edge of the flood plain or a river; a Bayou.
(2) (Localized) In the Mississippi Valley and in California, a tide flat or bottom-land creek.
(3) (Sewage Disposal) Of a filter, to cast off a thin film of scum or a mass of bacterial growth or fungus.
(4) Also Slue. A stagnant swamp, marsh, bog, or pond, especially as part of a bayou, and inlet, or a backwater.

Slow Sand Filter — Essentially, a concrete basin covered with graded gravel and about 3 feet of sand used to pass raw water through at low velocity, resulting in substantial removal of chemical and biological contaminants. Cleaning is accomplished by removing the surface layers of sand at periodic intervals. These are then washed and stored for reuse. The slow sand filtration process is simple, reliable and relatively inexpensive. The slow sand filter is an appropriate technique for removing suspended organic and inorganic matter and also may remove pathogenic organisms. The filter reduces bacteria, cloudiness, and organic levels, thereby reducing the need for disinfection and, consequently, the presence of disinfection byproducts in the finished water. There are limitations, however. Slow sand filters require a large land area, large quantities of filter media, and manual labor for cleaning. Water with high turbidity levels can quickly clog the fine sand used in these filters. Also, slow sand filters are less effective at removing microorganisms from cold water because as temperatures decrease, the biological activity within the filter bed declines. In operation, a sticky mat of biological matter, called a Schmutzdecke, forms on the sand surface, where particles are trapped and organic matter is biologically degraded. Slow sand filters rely on this cake filtration at the surface of the filter for particulate straining. As the surface cake develops during the filtration cycle, the cake assumes the dominant role in filtration rather than the granular media.

Smolt — Salmonid migratory life stage that transforms the freshwater rearing life stage for survival in saltwater.

Soil and Sediment Adsorption Isotherm Test — A test method that measures the Adsorption of a chemical substance to soil or sediment and thus indicates the likely distribution pathways in the environment. If a substance is readily adsorbed, it will increase in concentration in a soil or sediment layer; if it is not adsorbed, it will be free to move through the soil into groundwater or run off into surface water.

Soil Bioengineering — A process involving the use of live and dead woody cuttings and poles or posts collected from native plants to revegetate watershed slopes and stream banks. The cuttings, posts, and vegetative systems composed of bundles, layers, and mats of the cuttings and posts provide structure, drains, and vegetative cover to repair eroding and slumping slopes. Also referred to as Biotechnical Slope Protection.

Soil Classification — The systematic arrangement of soils into groups or categories on the basis of their characteristics. Broad groupings are made on the basis of general characteristics and subdivisions on the basis of more detailed differences in specific properties. Soil Taxonomy is the study of soil classification systems.

Soil Flushing — A treatment technique for cleaning soil contaminated with inorganic or organic hazardous waste. The process involves the flooding of the soil with a flushing solution, which may be acidic, basic, or contain Surfactants, and the subsequent removal of the Leachate via shallow wells or subsurface drains. The recovered leachate is then purified.

Soil Management — The basis of all scientific agriculture, which involves six essential practices:
(1) proper tillage;
(2) maintenance of a proper supply of organic matter in the soil;
(3) maintenance of a proper nutrient supply, including water;
(4) control of soil pollution;
(5) maintenance of the correct soil acidity; and
(6) control of erosion.

Soil Moisture (Soil Water) — Water diffused in the upper part of the Unsaturated Zone (Zone of Aeration) of the soil, from which water is discharged by the Transpiration of plants, by Evaporation, or Interflow. Also referred to as Soil Moisture Content or Available Water Content (AWC).

Soil Morphology — The physical constitution, particularly the structural properties, of a soil profile as exhibited by the kinds, thickness, and arrangement of the horizons in the profile and by the texture, structure, consistency, and porosity of each horizon.

Soil Profile —
(1) The arrangement of soil horizons or layers below the ground surface.
(2) A vertical section of the soil from the surface through all its horizons. The three basic horizontal layers that may be observed in a soil profile are the A, B, and C horizons. The A horizon, or topsoil, is the main source of plant nutrients. Soluble materials leach from the A horizon down to the subsoil, or B horizon. This is the zone of clay accumulation. The deepest layer, the C horizon, is composed of partially weathered bedrock. Also referred to as Soil Horizons. Also see Soil Core.

Soil Sorption Coefficient (Kd) — A parameter relating the partitioning of a chemical between soil and water in a soilwater mixture. The coefficient is computed by: Kd = Cs/Cw where Cs is the amount of a specific chemical bonded to the soil (micrograms chemical per gram of soil) and Cw is the concentration of the same substance dissolved in the water (micrograms of the chemical per gram of water).

Soil Structure — The physical properties of different soils. Sand has little structure because sand particles do not tend to stick together, whereas clay has a firm structure because clay particles tend to pack tightly together to form a solid material.

Soil Taxonomy — The systematic study and arrangement of soils into classifications based on their characteristics.

Soil Thin-Layer Chromatography — A method used to estimate the potential for leaching a chemical from soil by measuring the mobility of the chemical in soil under controlled conditions.

Soil Vapor Survey — A non-invasive method for the detection of volatile or semivolatile organic contaminants in shallow subsurface soil. The technology is especially useful in the analysis of soils with a high sand content and of shallow groundwater. A probe is driven into the ground, and samples of vapors in the soil are drawn to the surface for analysis.

Solar Aquatic Systems (SAS) — A technology which, under controlled conditions, duplicates the natural water purification processes of streams and wetlands. The solar greenhouse wastewater treatment system includes solar tanks which optimize photosynthetic reactions and biological activity, and artificially-controlled ponds and wetlands which replicate a natural environment. Within such a system, wastewater is circulated through ecologically-engineered aquatic environments where the contaminants and nutrients are metabolized or “bound up.” Typically, tertiary treatment of wastewater requires up to four days.

Solar Pond — A pool of salt water heated by the sun and used either as a direct source of heat or to provide power for an electric generator.

Solifluction —
(1) The slow downhill flow or creep of soil and other loose materials that have become saturated.
(2) The slow (normally 0.5-5.0 cm/year), viscous, downslope flow of water logged soil and other unsorted and saturated surficial material.

Solubility — The relative capacity of a substance to serve as a solute. Sugar has a high solubility in water, whereas gold has a low solubility in water.

Solubilize — To make (substances such as fats and lipids) soluble in water by the action of a detergent or similar agent.

Soluble — That which can be dissolved; able to pass into solution.

Soluble Minerals — Naturally occurring substances capable of being dissolved.

Source Water Assessment Program (SWAP) — A requirement established under 1996 amendments to the Safe Drinking Water Act (SDWA) which requires every state with primacy for the Public Water Supply Supervision program to develop and implement a program intended to identify ground and surface waters that supply drinking water for public water systems. Once a source area is identified or delineated, a state must then locate contaminants within the delineated area which could potentially degrade source water.

Source Water Protection — The prevention of pollution of the lakes, reservoirs, rivers, streams, and groundwater that serve as sources of drinking water. Wellhead protection would be an example of a source water protection approach that protects groundwater sources, whereas management of land around a lake or reservoir used for drinking water would be an example for surface water supplies. Source water protection programs typically include: delineating source water protection areas; identifying sources of contamination; implementing measures to manage these changes; and planning for the future.

Source Water Protection Program (SWPP) — A program encouraged and highly recommended under 1996 amendments to the Safe Drinking Water Act (SDWA) intended to develop and implement a program to extend and supplement existing Wellhead Protection Programs (WPP) by allowing communities and local governments to obtain financial assistance to protect both surface and ground public drinking water sources from future contamination.

Special Assessment District — A legally established area for the express purpose of levying a special fee for public improvements that are of a special rather than a general benefit.

Special District — A political subdivision of a state established to provide a single public service (as water supply or sanitation) within a specific geographical area. Also see General Improvement District (GID).

Special Flood Hazard Area (SFHA) —
(1) Areas in a community that have been identified as susceptible to a one percent or greater chance of flooding in any given year. A one-percent-probability flood is also known as the 100- year flood or the base flood.
(2) An area having special flood, mudslide, or flood-related erosion hazards, and shown on a Flood Boundary Floodway Map (FBFM), Floodway Hazard Boundary Map (FHBM), or a Flood Insurance Rate Map (FIRM). SFHA’s are further subdivided into Flood Hazard Zones A, AO, AH, A1-A30, AE, A99, AR, AR/A, AR/AE, AR/A1-A30, AR/AH, AR/AO, V, V1-V30, and VE.

Species of Special Concern [California] — Species which are not federal or state-listed as endangered, threatened, or rare, but are declining or are so few in number in California that extirpation is a possibility.

Specific Capacity (of a Well) — In ground water hydrology, the ratio of the discharge or yield of a well, usually measured in gallons per minute per foot, to drawdown after a period of sustained pumping.

Specific Gravity (SG or SP GR) —
(1) The ratio of the density of a substance to the density of some substance (as pure water) taken as a standard when both densities are obtained by weighing in air.
(2) The ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water at 4EC (39EF) or of a gas to an equal volume of air or hydrogen under prescribed conditions of temperature and pressure. Relative to water, the specific gravity (SG) is given by:
SG = ñ/ñw where ñ is the density (weight per unit volume) of the unknown substance and ñw is the density of water. The parameter has no units and is frequently used to determine the concentration of a Solution.

Specific Heat (SP HT) — (1) The ratio of the amount of heat required to raise the temperature of a unit mass of a substance by one unit of temperature to the amount of heat required to raise the temperature of a similar mass of a reference material, usually water, by the same amount. (2) The amount of heat, measured in calories, required to raise the temperature of one gram of a substance by one Celsius degree (1EC); the specific heat of water is 1 calorie.

Specific Humidity — The mass of water vapor per unit mass of moist air.

Specific Retention (of a Water-Table Aquifer) — Generally, a measure of the water-retaining capacity of a porous medium. The amount of water held in saturated rock or soil after the excess gravitational water has drained away as compared to the total volume of the rock or soil. Specific retention is dependent on both pore characteristics as well as factors affecting the surface tension, such as temperature, viscosity, mineral composition of the water, etc. Also referred to as Field Capacity or Water-Holding Capacity.

Specific Yield (of an Aquifer) — The volume of water available per unit volume of aquifer, if drawn by gravity. Specific yield is expressed as a percent. For example, if 0.2 cubic meter of water will drain from 1 cubic meter of aquifer sand, the specific yield is 20 percent.

Specific Yield (Ground Water) — The ratio of the volume of water that a rock will yield by gravity, after being saturated, to its own volume, expressed as a percentage.

Specification — The process in the construction of Econometric Models during the selection of the Independent or Exogenous Variables by which we attempt to provide the best and most complete explanation of the Dependent or Endogenous Variable. Correct model specification results in a representation of the “true” physical, economic, or behavioral relationships described by the model’s structure. Mis-specified models generally result from the inclusion of irrelevant variables in the model’s structure or relevant variables omitted from the model. Another model mis-specification can occur when the model is represented by a linear relationship when in fact the true relationship is nonlinear, or vice versa.

Spectroscopic Hygrometer — An instrument used to measure the selective absorption by water vapor of light in certain bands of the spectrum.

Spiles — (Irrigation) Small pipes, generally straight, from 1–4 inches (2.5–10 centimeters) in diameter, used to distribute water from a ditch into furrows, borders, or corrugations.

Spill —
(1) To cause or allow to run or fall from a container unintentionally so as to be lost or wasted.
(2) With respect to a dam and reservoir system, the water passed over a spillway without going through turbines to produce electricity. Spill can be forced, when there is no storage capability and flows exceed turbine capacity, or planned, for example, when water is spilled to enhance fish passage or to support other downstream uses (e.g., agriculture, wetland maintenance, etc.).
(3) With reference to reservoir operations, water that is released, either inadvertently or through precautionary releases, in excess of that required to compensate for deliver system losses and to meet irrigation demand.

Spillway — The channel or passageway around or over a dam through which excess water is diverted. If the flow is controlled by gates, it is a controlled spillway; if the elevation of the spillway crest is the only control, it is an uncontrolled spillway. The following lists some typical spillways:
[1] Auxiliary Spillway (Emergency Spillway) — A secondary spillway designed to operate only during exceptionally large floods;
[2] Fuse Plug Spillway — An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during normal inflow and flood flows;
[3] Primary Spillway (Principal Spillway) — The principal or first-used spillway during normal inflow and flood flows;
[4] Shaft Spillway (Morning Glory Spillway) — A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel; if the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a “bellmouth“ or “morning glory” spillway;
[5] Side Channel Spillway — A spillway whose crest is roughly parallel to the channel immediately downstream of the spillway;
[6] Siphon Spillway — A spillway with one or more siphons built at crest level; this type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.

Spring (Water) —
(1) A concentrated discharge of ground water coming out at the surface as flowing water; a place where the water table crops out at the surface of the ground and where water flows out more or less continuously.
(2) A place where ground water flows naturally from a rock or the soil into the land surface or into a body of surface water. Its occurrence depends on the nature and relationship of rocks, especially permeable and impermeable strata, on the position of the water table, and on the topography.

Spring, Cold — A spring whose water has a temperature appreciably below the mean annual atmospheric temperature in the area.

Spring, Hot — A thermal spring whose temperature is above that of the human body.

Spring Melt/Thaw — The process whereby warm temperatures melt winter snow and ice. Because various forms of acid deposition may have been stored in the frozen water, the melt can result in abnormally large amounts of acidity entering streams and rivers, sometimes causing fish kills.

Spring Overturn — A physical phenomenon that may take place in a lake or similar body of water during the early spring, most frequently in lakes located in temperate zones where the winter temperatures are low enough to result in freezing of the lake surface. The sequence of events leading to spring overturn include:
(1) the melting of ice cover;
(2) the warming of surface waters;
(3) density changes in surface waters producing convection currents from top to bottom;
(4) circulation of the total water volume by wind action; and
(5) vertical temperature equality. The overturn results in a uniformity of the physical and chemical properties of the entire water mass.

Spring Runoff — Snow melting in the spring causes water bodies to rise. This in streams and rivers is called “spring runoff”.

Spring Tide — The highest high and the lowest low tides during the lunar month. The exceptionally high and low tides that occur at the time of the new moon or the full moon when the sun, moon, and earth are approximately aligned.

Stability Hazard — A potential or rating for a slope assessing its susceptibility to slope failure.

Stabilization — (Environmental) A broad expression used to denote a process that is intended to lessen the damage that a pollutant or discharge causes in the environment; the process of changing an active substance into inert, harmless material, or physical activities at a site that act to limit the further spread of contamination without actual reduction of toxicity. For example, the stabilization of sewage involves allowing microorganisms to degrade those components that can be decomposed.

Stable Isotope Ratio — A unit expressing the ratio of the abundance of two Radioactive Isotopes. Isotope ratios are used in hydrologic studies to determine the age or source of specific waters, to evaluate mixing of different waters, as an aid in determining reaction rates, and other chemical or hydrologic processes.

Staff Gage — A graduated scale used to indicate the height of the water surface in a stream channel, reservoir, lake, or other water body.

Stage — The height of a water surface above some established reference point or Datum (not the bottom) at a given location. Also referred to as Gage Height.

Stage-Capacity Curve — A graph showing the relation of the surface elevation of the water in a reservoir, usually plotted as the ordinate, to the volume below that elevation, plotted as the abscissa.

Stage-Discharge Relationship — The relationship between the amount of water flowing in a river or stream and Stage at any particular point, usually represented by means of a Rating Curve or Rating Table.

Stagnation — A lack of motion in a mass of water that tends to hold pollutants in place.

Stagnation Point — A place in a ground-water flow field at which the ground water is not moving. The magnitude of vectors of Hydraulic Head at the point are equal but opposite in direction.

Stakeholders — (Planning) Individuals and organizations with an interest in a particular area, issue or project. Stakeholders may include public agencies at all levels (i.e., federal, state and local), non-profit organizations, private landowners, industry, and others.

Stand — A biotic community, particularly of trees, possessing sufficient uniformity of composition, age, and spatial arrangement to be distinguishable from adjacent communities. Stand structure refers to the composition, age, and arrangement of the trees in a delimited biotic community.

Standard Methods — A short form for Standard Methods for the Examination of Water and Wastewater which is prepared and published every five years jointly by the American Public Health Association, American Water Works Association, and the Water Pollution Control Federation. The book serves as the primary reference for analytical methods employed in investigations and monitoring of water purification, sewage treatment and disposal, water pollution, sanitary quality, and other functions.

Standard Permeability — The permeability corresponding to a temperature of 60EF.

Standard Project Flood —
(1) The flood that may be expected from the most severe combination of meteorological and hydrological conditions considered reasonably characteristic of the geographical area in which the drainage basin is located, excluding extremely rare combinations.
(2) (U.S. Army Corps of Engineers) A term used to designate a flood that may be expected from the most severe combination of meteorological and hydrological conditions that is considered reasonably characteristic of the geographical area in which the drainage basin is located, excluding extremely rare combinations. The peak flow for a standard project flood is generally 40 to 60 percent of the probable maximum flood for the same location.

Standard Sample — The part of finished drinking water that is examined for the presence of coliform bacteria.

Standardized Precipitation Index (SPI) – A precipitation index based on the probability of precipitation for any time duration. The SPI was specifically designed to quantify the precipitation deficit for multiple periods of time reflecting the impact of drought on the availability of different water resources. Soil moisture conditions respond to precipitation anomalies on a relatively short scale, while groundwater, stream flow and reservoir storage levels reflect the longer-term precipitation anomalies. For this reason, the SPI is calculated on both a short-term and longterm (up to four years) basis. Positive SPI values indicate greater than median (middle-most) precipitation, while negative values indicate less than median precipitation. Because the index is normalized, wetter and drier climates can be determined in the same way. A drought event occurs any time the SPI is continuously negative and reaches an intensity where the SPI is -1.0 or less. SPI indexes range as follows:
2.0 or greater – extremely wet
1.5 to 1.99 – very wet
1.0 to 1.49 – moderately wet
-0.99 to 0.99 – near normal
-1.0 to -1.49 – moderately dry
-1.5 to -1.99 – severely dry
-2.0 or less – extremely dry

Standing Crop — The quantity of plant Biomass in a given area. For example, the amount of plant material per acre of forest or swamp, or per cubic meter of water. Usually expressed as mass (dry) per unit area, or energy content per unit area.

Standpipe —
(1) A large vertical pipe into which water is pumped in order to produce a desired pressure; a high vertical pipe or reservoir that is used to secure a uniform pressure in a water-supply system.
(2) A pipe or tank connected to a closed conduit and extending to or above the hydraulic grade line of the conduit to afford relief from surges of pressure in pipelines.
(3) A tank used for storage of water in distribution systems.

State Water Project (SWP) [California] — State-owned and operated water project conisting of twenty-two dams and reservoirs which deliver 2.4 million acre-feet of water each year from the Sacramento Valley to the San Francisco Bay Area, Central Valley and Los Angeles.

State Water Resources Control Board (SWRCB) [California] — The water rights and water permitting agency of the State of California. The SWRCB consists of five members (to include a Chairman and Vice Chairman) whose responsibility it is to “protect water quality and allocate water rights” within the State of California. To assist in these functions, the SWRCB is served by a staff to include an Executive Director, a Chief Deputy and nine (9) Regional Board Executive Officers serving the regions of:
[1] North Coast Region;
[2] San Francisco Bay Region;
[3] Central Coast Region;
[4] Los Angeles Region;
[5] Central Valley Region;
[6] Lahontan Region;
[7] Colorado River Basin Region;
[8] Santa Ana Region; and
[9] San Diego Region.

State Wellhead Protection Program — A program established to protect wellhead protection areas within a State’s jurisdiction from contaminants that may have any adverse effects on the health of persons (Safe Drinking Water Act (SDWA), subsection 1428[a]).

Static (Fixed Water) — Pertaining to water stored in a tank but not under pressure.

Static Head — The difference in elevation in feet between the water surface of the body of water being pumped and the centerline of the discharge pipe at the point of release. It is the lift measured in feet.

Static Level (Ground Water) — The level of water in a nonpumping or nonflowing well. For the purpose of computing the drawdown, it generally is the water level immediately before pumping begins.

Static Lift — The vertical distance between source and discharge water levels in a pump installation.

Static Pressure — The pressure exerted by a still liquid or gas, especially water or air.

Static Water Depth — (Hydraulics) For a water well, the vertical distance from the centerline of the pump discharge down to the surface level of the free pool while no water is being drawn from the pool or water table.

Static Water Level —
(1) The elevation or level of the water table in a well when the pump is not operating.
(2) The level or elevation to which water would rise in a tube connected to an Artesian Aquifer or basin in a conduit under pressure.

Statistical Inference — (Statistics) The area of statistics that describes the procedures by which we use the observed data (the sample) to draw conclusions about the population from which the data came or about the process by which the data were generated. Our assumptions is that there is an unknown process that generates the data and that this process can be described by a probability distribution, i.e., a likelihood of occurring. Statistical inference can be classified as Classical Inference and Bayesian Inference.

Steady State —
(1) State of balance in a Hydrologic System where little or no change in hydraulic head occurs through time.
(2) In a system with a flow-through of material (e.g., water) or energy, the equilibrium condition in which the flow in equals the flow out.

Steady-State or Apparent Plateau — (Biology) In testing chemical substances for their Bioconcentration Potential in fish, the situation in which the amount of chemical substance taken into the test fish from the water is equal to the amount being eliminated from the test fish. If the substance bioconcentrates in the fish, the steady-state condition will follow an uptake phase in which the absorbance rate exceeds the elimination rate.

Steam Injection Well — A method of recovering deposits of oil and other minerals which involves injecting steam directly into the deposit to decrease viscosity and facilitate extraction. Also used in deep-buried oil-sand deposits in an “in situ” process to separate the bitumen from the sand so it may be pumped to the surface along with water and a trace of natural gas.

Steam Stripping — The removal of volatile compounds from wastewater by forcing steam through the liquid. The higher the wastewater temperature increases, the higher the evaporation rate of the volatile contaminants.

Stemflow — The rainfall or snowmelt led to the ground down the trunks or stems of plants.

Stenobathic — Limited to or able to live only within a narrow range of water depths. Used of aquatic organisms.

Stenohaline — Pertaining to an aquatic organism unable to withstand wide variation in salinity of the surrounding water.

Step Aeration — (Water Quality) A modification of the activated sludge process wherein the waste enters the aeration tank at a number of different points, while recycled sludge enters at the head of the tank.

Sterile (Water) — A condition in which a quantity of water does not contain viable organisms such as viruses and bacteria. The term, however, is not synonymous with clean.

Sterilization (of Water) — The process of killing, inactivating, or removing microorganisms (bacteria, viruses, etc.) from a quantity of water. The normal methods of sterilization involve agents such as heat, chemicals, or radiation.

Stewardship — (Ecology)
(1) Caring for land and associated resources and maintaining healthy ecosystems for future generations.
(2) Administrative and/or custodial actions taken to preserve and protect the Natural Resources, particularly the plant (Flora) and animal (Fauna) life, of an area or Ecosystem.

Stilling Basin — An open structure or excavation at the foot of an overfall, chute, drop, or Spillway to reduce the energy of the descending stream. A basin constructed to dissipate the energy of fast-flowing water, e.g., from a spillway or Bottom Outlet, and to protect the stream bed below a dam from erosion.

Stilling Well — A device used to allow monitoring of water levels in turbulent flow.

Still Water — A flat or level section of a stream where no flow or motion of the current is discernible and the water is still.

Stochastic Hydrology — That branch of Hydrology involving the manipulation of statistical characteristics of hydrologic variables with the aim of solving hydrologic problems, using the stochastic properties of the events.

Stopcock — A valve that regulates the flow of fluid through a pipe; a faucet.

Stoplogs — Large logs or timber or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit providing a temporary closure versus a permanent bulkhead gate.

Storage —
(1) Water artificially impounded in surface or underground reservoirs for future use.
(2) Water naturally detained in a drainage basin, such as ground water, channel storage, and depression storage. The term Drainage Basin Storage, or simply Basin Storage, is sometimes used to refer collectively to the amount of water in natural storage in a drainage basin.
(3) (Water Quality) The temporary holding of waste pending treatment or disposal, as in containers, tanks, waste piles, and surface impoundments.

Storage Capacity, Active (Usable) — The volume of water normally available for release from a reservoir below the stage of the maximum controllable level; equal to total capacity less inactive and dead capacity.

Storage Capacity, Conservation — Storage capacity available for all useful purposes such as municipal water supply, power, irrigation, recreation, fish and wildlife, etc., excluding joint and exclusive flood control capacity.

Storage Capacity, Dead — The volume of a reservoir below the sill or invert of the lowest outlet.

Storage Capacity, Exclusive Flood Control — The space in reservoirs reserved for the sole purpose of regulating flood inflows to abate flood damage.

Storage Capacity, Inactive — The portion of capacity below which the reservoir is not normally drawn, and which is provided for sedimentation, recreation, fish and wildlife, aesthetic reasons, or for creation of a minimum controlled operational or power head in compliance with operating agreements or restrictions.

Storage Capacity, Joint Use — The volume of a reservoir available to store water jointly for flood control and conservation purposes.

Storage Capacity, Live — The volume of a reservoir exclusive of dead and surcharge capacity.

Storage Capacity, Sediment — The volume of a reservoir planned for the deposition of sediment.

Storage Capacity, Surcharge — The volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation with the crest gates in the normal closed position.

Storage Capacity, Total — The total volume of a reservoir exclusive of surcharge.

Storage Coefficient —
(1) For surface water, the relation of storage capacity in a reservoir to the mean annual flow of a stream above the dam forming the reservoir.
(2) For ground water, primarily in a Confined Aquifer, it is a measure of the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. This definition is similar to that of the Specific Yield for a water-table aquifer. However, confined aquifers remain saturated at all times and therefore water release in not met by drainage of the void spaces as is the case of an Unconfined Aquifer, but due to the change in pore pressure. Also referred to as the Storativity.

Storage Ratio — The net available storage divided by the mean flow for one year.

Storage-Required Frequency Curve — A graph showing the frequency with which storage equal to or greater than selected amounts will be required to maintain selected rates of regulated flow.

Storage Reservoir — A reservoir that has space for retaining water from springtime snowmelt or other hydrologic events. Retained water is released as necessary for multiple uses — power production, fish passage, irrigation, navigation, municipal and industrial, etc. Compare to a Run-of-River Dam.

Storage Right — The authority granted by a responsible state entity to impound water in a reservoir.

Storage, Specific (Ground Water) — The amount of water released from or taken into storage per unit volume of a porous medium per unit change in head.

Storage, Usable — The volume of water normally available for release from a reservoir below the stage of the maximum controllable level.

Storativity — The volume of water that a permeable unit, i.e., aquifer, will absorb or expel from storage per unit surface area per unit change in head. In an unconfined aquifer, the storativity value is equal to the Specific Yield. The specific yield of the aquifer can be used to estimate the time between when pumping begins and equilibrium groundwater conditions are reached.

Stream — A general term for a body of flowing water; natural water course containing water at least part of the year. In Hydrology, the term is generally applied to the water flowing in a natural channel as distinct from a canal. More generally, as in the term Stream Gaging, it is applied to the water flowing in any channel, natural or artificial.
Some classifications of streams include, in relation to time:
[1] Ephemeral Streams — Streams which flow only in direct response to precipitation and whose channel is at all times above the water table.
[2] Intermittent or Seasonal Streams — Streams which flow only at certain times of the year when it receives water from springs, rainfall, or from surface sources such as melting snow.
[3] Perennial Streams — Streams which flow continuously. And, in relation to ground water:
[4] Gaining Streams — Streams or a reach of a stream that receive water from the zone of saturation. Also referred to as an Effluent Stream.
[5] Insulated Streams — Streams or a reach of a stream that neither contribute water to the zone of saturation nor receive water from it. Such streams are separated from the zones of saturation by an impermeable bed.
[6] Losing Streams — Streams or a reach of a stream that contribute water to the zone of saturation.
[7] Perched Streams — Perched streams are either losing streams or insulated streams that are separated from the underlying ground water by a zone of aeration.

Streambank Erosion —
(1) The wearing away of streambanks by flowing water.
(2) The removal of soil particles from a bank slope primarily due to water action. Climatic conditions, ice and debris, chemical reactions, and changes in land and stream use may also lead to bank erosion.

Streambank Erosion Control — Vegetative or mechanical control of erodible stream banks, including measures to prevent stream banks from caving or sloughing, such as jetties, revetments, riprap and plantings necessary for permanent protection.

Streambank Erosion Damage — Value of land areas destroyed, reduced value of land due to threat of future erosion, and the destruction or damage of wildlife habitat, buildings, bridges, utilities, or other structures.

Streambank Protection Works — Structure place on or near a distressed stream bank to control bank erosion or prevent failure.

Streambanks — The usual boundaries, not the flood boundaries, of a stream channel. Right and left banks are named facing downstream (in the direction of flow).

Streambank Stabilization — Natural geological tendency for a stream to mold its banks to conform with the channel of least resistance to flow. Also the lining of streambanks with riprap, matting, etc., to control erosion.

Streambed — The channel through which a natural stream of water runs or used to run, as a dry streambed.

Streambed Erosion — The movement of material, causing a lowering or widening of a stream at a given point or along a given reach.

Stream Capture — The process whereby a stream rapidly eroding headward cuts into the divide separating it from another drainage basin, and provides an outlet for a section of a stream in the adjoining valley. The lower portion of the partially diverted stream is called a Beheaded Stream. Also referred to as Stream Piracy.

Stream Channel — The bed where a natural stream of water runs or may run; the long narrow depression shaped by the concentrated flow of a stream and covered continuously or periodically by water.

Stream Clearance — The removal of natural or man-caused debris from stream channel areas by mechanical means.

Stream, Coldwater — A stream that supports a coldwater fishery, usually including trout, typically with optimum temperatures of 50E–60EF (10E–15.5EC).

Stream, Coolwater — A stream that supports a coolwater fishery, usually including smallmouth bass and/or rock bass, typically with optimum temperatures of 60E–70EF (15.5E–21EC).

Stream, Effluent — A stream or reach of a stream fed by ground water. It is also referred to as a Gaining Stream.

Stream, Ephemeral — A stream that flows only in response to precipitation. See Stream.

Streamflow — The discharge that occurs in a natural channel. Although the term “discharge” can be applied to the flow of a canal, the word streamflow uniquely describes the discharge in a surface stream course. Streamflow is a more general term than “runoff” as streamflow may be applied to discharge whether or not its is affected by diversion or regulation.

Streamflow Depletion — The amount of water that annually flows into a valley or onto a particular land area minus the amount that flows out of the valley or away from the particular land area. It is also the amount of water taken from a stream.

Streamflow Regulation — The artificial manipulation of the flow of a stream.

Streamflow Routing — A technique used to compute the effect of channel storage on the shape and movement of a flood wave.

Stream Gaging — The quantitative determination of stream flow using Gages, Current Meters, Weirs, or other measuring instruments at selected locations.

Stream-Gaging Station — A gaging station where a continuous record of the discharge of a stream is obtained.

Stream Gradient — A general slope or rate of change in vertical elevation per unit of horizontal distance of the water surface of a flowing stream.

Stream, Influent — A stream that contributes water to the Zone of Saturation. Also referred to as a Losing Stream.

Stream, Intermittent — A stream that flows only part of the time or through only part of its reach. See Stream.

Streamlet — A small stream.

Streamline (Flowline) —
(1) A line that is parallel to the direction of flow of a fluid at a given instant.
(2) The path followed by a particle of water as it moves through a saturated soil mass.

Stream Load — All the material transported by a stream or river either as visible sediment (Bed Load and Suspended Load) or in solution (Dissolved Load).

Stream Order —
(1) Designation of stream segments within a drainage basin; a system of numbering streams according to sequence of tributary size. The smallest perennial tributary is designated as order 1, the junction of two first-order streams produces a stream segment of order 2, etc.
(2) A method of numbering streams as part of a drainage basin network as adopted by the U.S. Geological Survey (USGS). The smallest unbranched mapped tributary is a first-order stream, the stream receiving the tributary is a second-order stream, and so on, with the main stream always of the highest order. It is usually necessary to specify the scale of the map used, as a first-order stream on a 1:62,500 map may be a third-order stream on a 1:12,000 map. Tributaries which have no branches are designated as of the first order, streams which receive only first-order tributaries are of the second order, larger branches which receive only first-order and second-order tributaries are designated third order, and so on, the main stream being always of the highest order.

Stream, Perennial — A stream that flows continuously. See Stream.

Stream Piracy — The process whereby a stream rapidly eroding headward cuts into the divide separating it from another drainage basin, and provides an outlet for a section of a stream in the adjoining valley. The lower portion of the partially diverted stream is called a Beheaded Stream. Also referred to as Stream Capture.

Stream Power — Measure of energy available to move sediment, or any other particle in a stream channel. It is affected by discharge and slope.

Stream Reach — The continuous portion of a stream channel and adjoining floodplain from one selected point to another, usually measured along the Thalweg of the channel.

Stream Segment — (Water Planning) Surface waters of an approved planning area exhibiting common biological, chemical, hydrological, natural, and physical characteristics and processes. Segments will normally exhibit common reactions to external stresses, for example, discharge or pollutants.

Streamside Management Zone (SMZ) — A designated zone that consists of the stream and an adjacent area of varying width where management practices that might affect water quality, fish, or other aquatic resources are modified. It is a zone which acts as an effective filter and adsorptive zone for sediment; maintains shade; protects aquatic and terrestrial riparian habitats; protects channel and streambanks; and promotes floodplain stability. The zone may be wider than just the riparian area.

Stream Terrace —
(1) A surface representing remnants of a stream’s channel or flood plain when the stream was flowing at a higher level. Subsequent downward cutting by the stream leaves remnants of the old channel or flood plain standing as a terrace above the present level of the stream.
(2) A transversely level erosional remnant of a former axial stream or major desert stream floodplain that slopes in the same direction as the adjacent, incised stream, and is underlain by well sorted and stratified sand and gravel or by loamy or clayey sediments.

Stream, Underground — A subsurface stream which has all the characteristics of a water-course on the surface — a definite channel with bed and banks, a definite stream of water, and a definite source(s) of supply.

Stream, Warmwater — A stream that supports a warmwater fishery, usually including largemouth bass and sunfish, typically with spawning temperatures in excess of 70EF (21EC).

Streeter-Phelps Equation — (Water Quality) A means of predicting the effects upon dissolved oxygen levels of organic loading to a stream based on the deaeration and reaeration rates, and resulting in an Oxygen Sag Curve.

Stress — Growing conditions that endanger a plant’s health. Examples include lack of water, too much heat, wind or moisture, or low temperatures. The stressful condition varies according to the particular plant and its needs.

Stressed Waters — A portion of an aquatic environment with poor species diversity due to human actions. If a facility applying for a water permit will discharge into an aquatic system that is stressed by the actions of others, then it will not be held responsible for the existing poor conditions but must demonstrate to the environmental agency issuing the permit that further degradation will not occur as a result of its effluent.

Striation — One of a number of parallel lines or scratches on the surface of a rock that were inscribed by rock fragments embedded in the base of a glacier as it moved across the rock.

Stringer — A long, narrow line of vegetation, usually following a stream channel or bottom of a draw.

Strip Cropping — Growing crops in a systematic arrangement of strips or bands that serve as barriers to wind and water erosion.

Strip Mining — The process of removing mineral deposits that are found close enough to the surface so that the construction of tunnels (underground mining) is not necessary. The soil and strata that cover the deposit are removed to gain access to the mineral deposit. The primary environmental concerns related to this technique are the disposition of spoils removed to gain access to the deposit and the scoring of the landscape that remains following the complete removal of the mineral deposit. Water pollution is also a concern because runoff from the mining area is frequently rich in sediments and minerals. Furthermore, such operations sometimes necessitate the removal of groundwater that infiltrates the mining pit, consequently altering the groundwater flow with potential implications on the water table and aquifer characteristics. Also referred to as Open-Pit Mining or Surface Mining.

Stripping — Methods for the removal of unwanted dissolved gases from water. Stripping techniques involve increasing the surface area of the water to be stripped and maintaining the atmospheric partial pressure of the gas(es) to be removed at a low level relative to the partial pressure of the gas dissolved in the water. Oxygen, ammonia, hydrogen sulfide, volatile organic compounds, and carbon dioxide are commonly stripped from water. Also referred to as Air Stripping.

Structural Floodplain Management Measures — Those physical or engineering measures employed to modify the way floods behave, e.g., dams, dikes, levees, channel enlargements and diversions.

Structural Measures — Measures that delay, reduce, or control flood flows. These measures include reservoirs, channel improvements, levees, and diversion channels.

Stubble Mulching — The management of plant residues by harvesting, tilling, planting, and cultivating in such a way so as to keep protective amounts of vegetation on the soil surface.

Subaqueous — Existing, formed, or taking place in or under water.

Sub-Basin —
(1) A portion of a subregion or basin drained by a single stream or group of minor streams.
(2) The smallest unit into which the land surface is subdivided for hydrologic study purposes.

Subclimax — (Ecology) A stage in the ecological succession of a plant or animal community immediately preceding a Climax, and often persisting because of the effects of fire, flood, or other conditions.

Subduction Zone — According to the theory of Plate Tectonics, a region at the boundary of two of the plates that form the crust of the earth where one of the plates is forced downward into the mantle. The deep ocean trenches off the Pacific coasts of Mexico and South America are examples of these regions. These zones are also referred to as Convergent Plate Boundaries.

Subirrigation (Subirrigate) —
(1) Irrigation below the surface (as by a periodic rise of the water table or by a system of underground porous pipes).
(2) Irrigation of crops from water table(s) that are in turn supplied by seepage from above-lying canals, laterals, reservoirs, or irrigated fields.

Subirrigated Land — Land with a high water table condition, either natural or artificially controlled, that normally supplies a crop irrigation requirement.

Sublimation —
(1) The transformation of a solid to the gaseous phase without passing through the normally intermediate liquid phase.
(2) The change of a solid to a vapor (or the reverse) without the appearance of a liquid state, as in the changing of snow directly into water vapor without melting.

Sublittoral —
(1) Situated, occurring, or formed on the aquatic side of a shoreline or Littoral Zone.
(2) The deeper part of the Littoral portion of a body of water.
(3) The region in a lake between the deepest-growing rooted vegetation and the part of the lake below the Thermocline.
(4) The region in an ocean between the lowest point exposed by a low tide and the margin of the Continental Shelf.

Sublittoral Zone — The part of the shore from the lowest water level to the lower boundary of plant growth; the transition zone from the Littoral to Profundal bottom.

Submeander — A small meander contained within the banks of a main channel, associated with relatively low discharges.

Submergence — Condition of a Weir when the elevation of the water surface on the downstream side is equal to or higher than that of the weir crest.

Submergent Plant — A vascular or nonvascular Hydrophyte, either rooted or nonrooted, which lies entirely beneath the water surface, except for flowering parts in some species; e.g., wild celery (Vallisneria americana) or the stoneworts (Chara spp.).

Subsequent Stream — A tributary stream flowing along beds of less erosional resistance, parallel to beds of greater resistance. Its course is determined subsequent to the uplift that brought the more resistant beds within its sphere of erosion.

Subsidence —
(1) The sinking of the land surface due to a number of factors, of which groundwater extraction is one.
(2) A sinking of a large area of the earth’s crust. Typically this may result from the over-pumping of a basin’s water table and the inability of the soils to re-absorb water from natural or artificial injection. Also frequently results from overdrafts of the aquifer and its inability to fully recharge, a process termed Aquifer Compaction.

Subsoil — Soil material underlying the surface soil.

Substrate —
(1) The substances used for food by microorganisms in liquid suspension, as in wastewater treatment.
(2) The physical surface upon which an organism lives; the surface, natural or artificial, upon which an organism grows or to which it is attached.
(3) The layer of material beneath the surface soil.

Substrate, Artificial —
(1) A device placed in the water for a specified period of time that provides living spaces for a multiplicity of organisms; for example, glass slides, concrete blocks, multi-plate samplers, or rock baskets; used primarily to collect organisms in areas where the physical habitat is limiting or cannot be adequately sampled using conventional methods. (2) A device which is purposely placed in a stream or lake for colonization of organisms. The artificial substrate simplifies the community structure by standardizing the substrate from which each sample is taken. Examples of artificial substrates are basket samplers (made of wire cages filled with clean streamside rocks) and multiplate samplers (made of hardboard) for benthic organism collection, and plexiglass strips for periphyton collection.

Substrate, Natural — Any naturally occurring immersed or submersed solid surface, such as a rock or tree, upon which an organism lives.

Subsurface Flow (SF) Constructed Wetland — A type of constructed wetland, a man-made marsh-like area used to treat wastewater. In the wetland, the effluent flows through a bed of rock or gravel, with the water level remaining below the surface of the gravel bed. Among the advantages of this type of system are its lack of odors, low potential for insect breeding, and minimal risk of public exposure and contact with the water in the system. Contrast with Free Water Surface (FWS) Constructed Wetland in which the water is exposed to the air.

Succession — (Biology)
(1) The ecological process of sequential replacement by plant communities on a given site as a result of differential reproduction and competition.
(2) Directional, orderly process of change in a living community in which the community modifies the physical environment to eventually establish an ecosystem which is as stable as possible at the site in question.

Suction Lysimeter — A sampling device for the collection of groundwater from the unsaturated zone. A sample is drawn by applying a negative pressure to a porous ceramic cup embedded in the soil layer.

Suction Pump — A pump for drawing up a liquid by means of suction produced by a piston drawn through a cylinder.

Suitability — (Environmental) The appropriateness of applying certain resource management practices to a particular area of land, as determined by an analysis of the economic and environmental consequences.

Sulfur Dioxide (SO2) — A colorless, irritating gas that is a primary cause of Acid Rain. It is a by-product of coal combustion.

Sullage —
(1) Silt deposited by a current of water.
(2) Waste materials or sewage; refuse.

Summer Kill — The complete or partial kill of a fish population in ponds, lakes, or reservoirs during the warm months caused by a combination of factors to include excessively warm water, a depletion of dissolved oxygen, and the release of toxic substances from a decaying algal bloom.

Sump —
(1a) A low-lying place, such as a pit, that receives drainage; (1b) A cesspool.
(2) A hole at the lowest point of a mine shaft into which water is drained in order to be pumped out.

Sump Pump —
(1) A pump designed and so placed in a Sump to remove the water or other liquids collected there.
(2) Device used to remove water from seepage or rainfall that collects in areas protected by a levee, floodwall, or dry floodproofing. In addition, a sump pump is often part of a standard house drainage system that removes water that collects below a basement slab floor.

Sunbow — A rainbow-like display of colors resulting from refraction of sunlight through a spray of water. Also see Rainbow.

Sunspot Activity — (Climate) A theory which posits that weather patterns may be caused by the level and timing of sunspot activity. Sunspots represent increased solar (irradiance) activity on the sun’s surface that occurs with periodic frequency of, on the average, once every 11.13 years. They last often a month, sometimes for several months, and their appearance is frequently accompanied by magnetic storms on earth. In research by the National Center for Atmospheric Research in Boulder, Colorado and the U.S. Geological Survey (USGS) in Lawrence, Kansas, researchers have shown a very high probability that the interior of the United States will be dominated by a low pressure system when there is little solar flare (sunspot) activity and the winds high above the equator are blowing from the west. This system results in cold weather in much of the country but warm weather in the Northeast. These researchers have also shown a much higher probability of wet weather in the West four years after a peak of solar irradiance, a peak that occurs, more or less, in conjunction with a peak in sunspot activity. The last recorded period of high sunspot activity was unusual as it was marked by a double peak, one in 1989 and one in 1991.

Supercritical Water — A type of thermal treatment using moderate temperatures and high pressures to enhance the ability of water to break down large organic molecules into smaller, less toxic ones. Oxygen injected during this process combines with simple organic compounds to form carbon dioxide and water.

(Hazardous Substances) Superfund — A federal trust fund for use in the cleanup of spills or sites containing hazardous waste that pose a significant threat to the public health or the environment. The fund, originally called the Hazardous Substances Response Trust Fund, was established by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) in 1980 to carry out U.S. Environmental Protection Agency (EPA) solid waste emergency and long-term removal and remedial activities. Beginning in 1980, $1.5 billion was to be collected over five years, mainly from taxes on crude oil, petroleum products, petrochemicals, and certain inorganic chemicals. The 1986 re-authorization of the law, which changed the fund’s name to the Hazardous Substances Superfund (or just “Superfund”), increased the fund to $8.5 billion and broadened the tax base to include a general
corporate Superfund tax. Another one-half billion dollars was included to clean up leaks from underground storage tanks. Also see U.S. Environmental Protection Agency (EPA) and the appendix related to EPA activities.

Superfund Law (Comprehensive Environmental Response, Compensation, and Liability Act — CERCLA) — This statute, originally enacted in 1980 and substantially modified in 1986, establishes the U.S. Environmental Protection Agency’s (EPA) authority for emergency response and cleanup of hazardous substances that have been spilled, improperly disposed of, or released into the environment. The primary responsibility for response and cleanup is on the generators or disposers of the hazardous substances, with a backup federal response using a trust fund provision.

Superfund List — A list of the hazardous waste disposal sites most in need of cleanup. The list is updated annually by the U.S. Environmental Protection Agency (EPA) based primarily on how a site scores using the Hazard Ranking System. Also referred to as the National Priorities List (NPL).

Superfund Site — A hazardous waste landfill on the National Priorities List (NPL) (also referred to as the Superfund List) being cleaned up by the responsible parties or using proceeds from the Hazardous Substances Superfund.

Superimposed Stream — A stream whose present course was established on young rocks burying an old surface. With uplift, this course was maintained as the stream cut down through the young rocks to and into the old surface.

Supernatant —
(1) Floating on the surface.
(2) (Water Quality) The clear fluid that is removed from the top of tanks or ponds used to allow solids to settle from suspension. Also referred to as Overflow.

Supersaturate —
(1) To cause (a chemical solution) to be more highly concentrated than is normally possible under given conditions of temperature and pressure.
(2) To cause (a vapor) to exceed the normal saturation vapor pressure at a given temperature.

Supplemental Irrigation — When irrigation water supplies are obtained from more than one source, the source furnishing the initial supply is commonly designated the primary source, and the source(s) furnishing the additional supplies, the Supplemental Sources.

Supplemental Sources — When irrigation water supplies are obtained from more than one source, the source furnishing the principal supply is commonly designated the primary source, and the sources furnishing the additional supplies, the supplemental sources.

Supplier of Water — Any person who owns or operates a Public Water Supply System (PWSS).

Surface Mining — The process of removing mineral deposits that are found close enough to the surface so that the construction of tunnels (underground mining) is not necessary. The soil and strata that cover the deposit are removed to gain access to the mineral deposit. The primary environmental concerns related to this technique are the disposition of spoils removed to gain access to the deposit and the scoring of the landscape that remains following the complete removal of the mineral deposit. Water pollution is also a concern because runoff from the mining area is frequently rich in sediments and minerals. Furthermore, such operations sometimes necessitate the removal of groundwater that infiltrates the mining pit, consequently altering the groundwater flow with potential implications on the water table and aquifer characteristics. Also referred to as Open-Pit Mining or Strip Mining.
Also see Dewatering.

Surface Mining Control and Reclamation Act — An act passed in 1977 requiring that mine operators take measures to avoid acid or other toxic mine drainage. To correct existing acid drainage problems, the section of the law dealing with abandoned mine land states that land and water affected by mining that took place before 1977 can be cleaned up with fees paid by coal operators into the Abandoned Mine Reclamation Fund. About 90 percent of existing stream damage in the United States is from underground coal mining that took place before 1977. The federal enforcement agency is the U.S. Department of the Interior, Bureau of Mines, Office of Surface Mining
Reclamation and Enforcement (OSM).

Surface Runoff — That part of the runoff which travels over the soil surface to the nearest stream channel. It is also defined as that part of the runoff of a drainage basin that has not passed beneath the surface since precipitation. Also applies to snowmelt or irrigation water in excess of what can infiltrate the soil surface and be stored in small surface depressions. In terms of surface water quality, surface runoff may constitute a major transporter of Non-Point Source (NPS) Pollution. The term is misused when applied in the sense of Direct Runoff. Also see Runoff, Direct Runoff, Overland Flow, Ground-Water Runoff, and Surface Water.

Surface Tension — A phenomenon caused by a strong attraction towards the interior of the liquid action on liquid molecules in or near the surface in such a way to reduce the surface area. The tension that results is usually expressed in dynes per centimeter (cm) or ergs per square centimeter (cm2).

Surface Water —
(1) An open body of water such as a stream, lake, or reservoir.
(2) Water that remains on the earth’s surface; all waters whose surface is naturally exposed to the atmosphere, for example, rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc., and all springs, wells, or other collectors directly influenced by surface water.
(3) A source of drinking water that originates in rivers, lakes and run-off from melting snow. It is either drawn directly from a river or captured behind dams and stored in reservoirs.

Surface Water Disposal — Refers to the release of reclaimed water or treated effluent directly into a surface body of water (including marshes and wetlands). This does not include water discharged into ponds for holding or Percolation purposes.

Surface Water Supply Index (SWSI) (Drought Index) — Originally developed to complement the Palmer Drought Severity Index (PDSI), the SWSI, unlike the PDSI which is basically a soil moisture index, was designed to be an indicator of surface water conditions in which mountain snowpack is a major component. The intent of the SWSI is to incorporate both hydrological and climatological features into a single index value resembling the PDSI. Four inputs are required for the SWSI: snowpack, streamflow, precipitation and reservoir storage. Because the SWSI calculations are unique to each watershed or defined region, it is difficult to compare SWSI values among these geographic areas.

Surface Water Treatment Rule (SWTR) — Water quality treatment standards as prescribed under the Safe Drinking Water Act (SDWA) and amendments thereto. The rule is a set of treatment technique requirements which apply to all water systems using surface water and those using ground water which is under the influence of surface water. Surface water systems are those using water exposed to the atmosphere, such as rivers, lakes, reservoirs, or streams. Ground water systems that are under the influence of surface water may include shallow wells, infiltration galleries,and springs which may contain the same disease-causing microorganisms of concern in surface water. The rule requires that these systems properly filter the water, unless they can meet certain strict criteria. The rule also requires that these systems disinfect the water. There are no exceptions from the disinfection requirement.

Surface Water Withdrawals — Includes all waters taken from streams, rivers, ponds, lakes, reservoirs, springs and all effluent and other waste water.

Surfactant — An agent that is used to decrease the surface tension of water, useful for removing or dispersing oils or oily residues. Most detergents are surfactants. The term is derived from surface active agent.

Suspended Particulate Matter — A sample drawn from natural water or from a wastewater stream consists of a mixture of both dissolved and suspended matter. Those solid materials that are retained on a filter prescribed by the specific technique being followed are referred to as particulate matter. The suspended particulate matter can be subdivided into two fractions: volatile and fixed. The volatile particulates are those that are lost when the filter is heated to about 550EC (1,022EF), and the fixed particulates are those that are not lost upon being so heated. The volatile substances are generally considered to be of biological origin, and the fixed solids are considered to be minerals.

Suspended, Recoverable — The amount of a given constituent that is in solution after the part of a representative suspended-sediment sample that is retained on a 0.45-micrometer membrane filter has been digested by a method (usually using a dilute acid solution) that results in dissolution of only readily soluble substances. Complete dissolution of all the particulate matter is not achieved by the digestion treatment and thus the determination represents something less than the “total” amount (that is, less than 95 percent) of the constituent present in the sample. To achieve comparability of analytical data, equivalent digestion procedures are required of all laboratories performing such analyses because different digestion procedures are likely to produce different analytical results.
Determinations of “suspended, recoverable” constituents are made either by analyzing portions of the material collected on the filter or, more commonly, by the difference, based on determinations of (1) dissolved and (2) total recoverable concentrations of the constituent.

Suspended sediment — Particles of rock, sand, soil, and organic detritus carried in suspension in the water column, in contrast to sediment that moves on or near the streambed.

Sustainability — (Ecosystem) Management practices that do not take more from an ecosystem than it can provide. Theoretically, sustainable management practices can continue in perpetuity, since they do not lead to exhaustion of natural resources.

Sustainable Development — Describes those efforts to guide economic growth, especially in less-developed countries, in an environmentally sound manner, with an emphasis on natural resource conservation.

Sustainable Management — A method of exploiting a resource that can be carried on indefinitely. For example, the removal of water from an aquifer in excess of recharge is, in the long term, not a sustainable management method.

Sustained Overdraft — A long-term withdrawal from the aquifer of more water than is being recharged. Also see Ground Water Mining.

Sustained Yield —
(1) (General) Achievement and maintenance, in perpetuity, of a high-level annual or regular periodic output or harvest of the various renewable land and water resources.
(2) (Hydrology) The amount of water that may be removed (say, through groundwater pumping) from an hydrographic area during a period of time without affecting future yields. Under such conditions, sustained yield is approximately equal to annual recharge. Contrast with Groundwater Mining.
(3) (Ecology) The perpetual output of a renewable resource, achieved and maintained at a given management intensity, without impairment of the productivity of the land.

Swale —
(1) A slight depression, sometimes swampy, in the midst of generally level land.
(2) A shallow depression in an undulating ground moraine due to uneven glacial deposition.
(3) A long, narrow, generally shallow, troughlike depression between tow beach ridges, and aligned roughly parallel to the coastline.
(4) A piece of meadow, often a slight depression or valley, as in a plain or moor, marshy and rank with vegetation. Swales usually carry flows only during or immediately after rainfall or snowmelt events. Swales vary in size from small conveyances providing drainage along roadways and behind or between buildings to larger waterways.

Swamp — A term frequently associated with Wetlands. Wet, spongy land; low saturated ground, and ground that is covered intermittently with standing water, sometimes inundated and characteristically dominated by trees or shrubs, but without appreciable peat deposits. Swamps may be fresh or salt water and tidal or non-tidal. It differs from a Bog in not having an acid substratum.

Swash —
(1a) A splash of water or other liquid hitting a solid surface; (1b) The sound made by such a splash.
(2a) A narrow channel through which tides flow; (2b) A bar over which waves wash freely.

Swash Marks — The wavy lines of fine sand or bits of debris left on the beach at the upward limit of the rush of water following the breaking of a wave.

Sweet (Water) — Water that is pleasing to the senses; agreeable and not saline or polluted; drinkable; Potable.

Swell —
(1) To rise or extend above the surrounding level, as clouds. To rise in swells, as the sea.
(2) A long wave on water that moves continuously without breaking.

Swill — (1) To flood with water, as for washing. (2) Semiliquid waste material consisting of food scraps and free liquids.

Synecology — The study of different natural communities or Ecosystems.

Synoptic Studies — Short-term investigations of specific water-quality conditions during selected seasonal or hydrologic periods to provide improved spatial resolution for critical water-quality conditions. For the period and conditions sampled, they assess the spatial distribution of selected water-quality conditions in relation to causative factors, such as land use and contaminant sources.

Synthetic Organic Chemicals (SOCs) — Man-made chemicals containing carbon including pesticides, herbicides, and polychlorinated biphenyls (PCBs). The U.S. Environmental Protection Agency (EPA) maintains a listing of SOCs that are regulated with respect to Maximum Contaminant Levels (MCLs) as part of the Safe Drinking Water Act (SDWA).

Synthetic Seawater — An artificial product of the approximate ionic composition of seawater.

System Design Capacity — (Water Quality) Plant design capacity plus any blending water added to the desalting plant product.

System Head Curve — A curve of system head comprising total static head and head loss in the system versus flow through the system.

System with a Single Service Connection — A system that supplies drinking water to consumers via a single service line.

Systemic — Any chemical that is absorbed into a plant’s system, either to kill organisms that feed on the plant or to kill the plant itself. There are systemic insecticides, fungicides, and herbicides.