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Debouchure — An opening or mouth, as of a river or stream.

Declared Underground Water Basin — An area of a state designated in some states by their respective State Engineers to be underlain by a ground water source having reasonably ascertainable boundaries. By such a designation, the State Engineer assumes jurisdiction over the appropriation and use of ground water from the source. May not be applicable in states which already claim regulatory rights over both surface and ground waters.

Deflocculate — To cause the particles of the disperse phase of a colloidal system to become suspended in the dispersion medium.

Deflocculating Agent — A material added to a suspension to prevent settling.

Degradation (River Beds or Stream Channels) — The general lowering of the streambed by erosive processes, such as scouring by flowing water. The removal of channel bed materials and downcutting of natural stream channels. Such erosion may initiate degradation of tributary channels, causing damage similar to that due to gully erosion and valley trenching.

Dehydratase — (Biochemistry) An Enzyme that catalyzes the removal of oxygen and hydrogen from organic compounds in the form of water.

Demersal — (1) Dwelling at or near the bottom of a body of water, such as demersal fish. (2) Sinking to or deposited near the bottom of a body of water, such as demersal fish eggs.

Dendritic — (1) A drainage pattern in which tributaries branch irregularly in all directions from and at almost any angle to a larger stream. (2) A tree-like pattern, typical of most drainage networks. From an aerial view, it resembles the branching pattern of trees.

Denitrification — The removal of nitrate ions (NO3–) from soil or water; involves the Anaerobic biological reduction of nitrate to nitrogen gas. The process reduces desirable fertility of an agricultural field or the extent of undesirable aquatic weed production in aquatic environments. Also see Denitrifying Bacteria.

Denitrifying Bacteria — Bacteria in soil or water that are capable of anaerobic respiration, using the nitrate ion as a substitute for molecular oxygen during their metabolism. The nitrate is reduced to nitrogen gas (N2), which is lost to the atmosphere during the process.

Density Current — (1) A flow of water maintained by gravity through a large body of water, such as a reservoir or lake, which retains its identity because of a difference in density. (2) Submerged gravity-driven flows which occur when inflows to a water body are denser than the ambient water. The inflow subsequently plunges and continues as a distinct flow which can be envisioned as a submerged stream. Density currents, also called Underflows, are known to form intermittently on coastal continental shelves, in reservoirs and at effluent discharge sites.

Density Stratification — The arrangement of water masses into separate, distinct horizontal layers as a result of differences in density. Such differences may be caused by differences in temperature or dissolved and suspended solids. Also see Thermal Stratification.

Depauperate — (Biology) An area poor in species quantities and/or diversity; an aquatic sample showing few life forms. Impoverished habitat.

Depositing Substrates — Bottom areas where solids are being actively deposited; often occurring in the vicinity of effluent discharges.

Deposition — Settlement of materials out of the water column and onto the stream bottom

Depth of Runoff — The total runoff from a drainage basin divided by its area. For convenience in comparing runoff with precipitation, depth of runoff is usually expressed in inches during a given period of time over the drainage area expressed in inches per square mile.

Depuration — A process during which an organism, such as an oyster or clam, eliminates dangerous chemicals or microorganisms when placed in uncontaminated water.

De-Seasonalization — (Statistics) A process which removes the seasonal effects from time series data. One way to determine if a de-seasonalization transformation of the data is necessary is to examine the autocorrelations. If, for monthly data, the twelfth autocorrelation is abnormally high, or for quarterly data, the fourth autocorrelation abnormally high, then the data is seasonal in nature and requires de-seasonalization before attempting to fit a model to its behavior. More frequently referred to as Seasonal Adjustment (S.A.). Also see Seasonal Adjustment, Seasonal
Adjustment Factors, Seasonal Factors, and Seasonality.

Designated Watersheds — Watershed areas that have been set aside as sources of municipal water or other similar purposes would be included in this category. Other uses are either modified or excluded.

Designer Bugs — A popular term for Microbes developed through Biotechnology that can degrade specific toxic chemicals at their source in toxic waste dumps, in ground water, or on the land surface. May also be useful in cleaning (decomposing) oil spills.

Desilting Area — An area of grass, shrubs, or other vegetation used for inducing the deposition of silt and other debris from flowing water. Typically located above a stock tank, pond, field, or other area needing protection from sediment accumulation.

Diadromous — Relating to a fish that migrates between salt and fresh waters.

Diatomaceous Earth — A yellow, white or light-gray material composed of the siliceous shells of Diatoms (fossilized diatoms) and used in water filtration to filter out solid waste in wastewater treatment plants; also used as an active ingredient in some powdered pesticides.

Diffusivity, Soil Water — The hydraulic conductivity divided by the differential water capacity, or the flux of water per unit gradient of moisture content in the absence of other force fields.

Dioxin — Any of several carcinogenic or teratogenic heterocyclic hydrocarbons that occur as impurities in petroleum derived herbicides and through over-use or runoff may threaten both surface and groundwater supplies. Dioxin has been linked to cancer, damage to the immune system, and other serious health conditions. It is also produced in paper mills when chlorine is mixed with wood pulp to brighten paper. Dioxin ends up in the mills’ wastewater, which is then discharged into rivers.

Direct Discharger — A municipal or industrial facility which introduces pollution through a defined conveyance or system such as outlet pipes; a point source.

Discharge Area —
(1) An area in which ground water is discharged to the land surface, surface water, or atmosphere.
(2) An area in which there are upward components of hydraulic head in the aquifer. Ground water is flowing toward the surface in a discharge area and may escape as a spring, seep, or base flow, or by evaporation and transpiration.

Discharge, Average — The arithmetic average of the annual discharges for all complete water years of record whether or not they are consecutive. The term average is generally reserved for average of record and mean is used for averages of shorter periods; namely, daily mean discharge.

Discharge Coefficient — (Hydraulics) The ratio of actual rate of flow to the theoretical rate of flow through orifices, weirs, or other hydraulic structures.

Discharge Curve — A curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location. Also called Rating Curve and Discharge Rating Curve.

Discharge Formula — (Hydraulics) A formula used to calculate the rate of flow of fluid in a conduit or through an opening. For a steady flow discharge, Q = A V where Q is the rate of flow, A is the cross-sectional area, and V is the mean velocity. Common units are cubic feet per second.

Discharge Measurement — Total discharge is equal to the cross-sectional area of the water in a channel or pipe times its average velocity.

Discharge, Sediment — The rate at which sediment passes a section of a stream or the quantity of sediment, as measured by dry weight or by volume, that is discharged in a given time.

Discharge Velocity — An apparent velocity, calculated by Darcy’s Law, which represents the flow rate at which water would move through an aquifer if the aquifer were an open conduit. Also referred to as Specific Discharge.

Disembogue — To discharge or pour fourth; to flow out or empty, as water from a channel.

Dissolved Oxygen (DO) —
(1) Concentration of oxygen dissolved in water and readily available to fish and other aquatic organisms.
(2) The amount of free (not chemically combined) oxygen dissolved in water, wastewater, or
other liquid, usually expressed in milligrams per liter, parts per million, or percent of saturation. The content of water in equilibrium with air is a function of atmospheric pressure, temperature, and dissolved-solids concentration of the water. The ability of water to retain oxygen decreases with increasing temperature or dissolved solids, with small temperature changes having the more significant offset. Photosynthesis and respiration may cause diurnal variations in dissolved-oxygen concentration in water from some streams. Adequate concentrations of dissolved oxygen are necessary for the life of fish and other aquatic organisms and the prevention of offensive odors.
Dissolved oxygen levels are considered the most important and commonly employed measurement of water quality and indicator of a water body’s ability to support desirable aquatic life. The ideal dissolved oxygen level for fish is between 7 and 9 milligrams per liter (mg/l); most fish cannot survive at levels below 3 mg/l of dissolved oxygen. Secondary and advanced wastewater treatment techniques are generally designed to ensure adequate dissolved
oxygen in waste-receiving waters.

Dissolved solids — Amount of minerals, such as salt, that are dissolved in water; amount of dissolved solids is an indicator of salinity or hardness.

Distributary — A diverging stream which does not return to the main stream, but discharges into another stream or the ocean. Also refers to conduits that take water from a main canal for delivery to a farm. See Distributary Channel or Stream.

Diversion —
(1) A structure in a river or canal that divers water from the river or canal to another watercourse.
(2) The transfer of water from a stream, lake, aquifer, or other source of water by a canal, pipe, well, or other conduit to another watercourse or to the land, as in the case of an irrigation system. Also, a turning aside or alteration of the natural course of a flow of water, normally considered physically to leave the natural channel. In some states, this can be a consumptive use direct from a stream, such as by livestock watering. In other states, a diversion must consist of such actions as taking water through a canal or conduit.

Diversity Index — A numerical expression of the evenness of distribution of aquatic organisms.

Diving Reflex — A reflexive response to diving in many aquatic mammals and birds, characterized by physiological changes that decrease oxygen consumption, such as slowed heart rate and decreased blood flow to the abdominal organs and muscles, until breathing resumes. Though less pronounced, the reflex also occurs in certain non-aquatic animals, including human beings, upon submersion in water.

Dominant Discharge — The channel-forming discharge, which is equivalent to the bankfull discharge, responsible for the active channel that erodes and deposits, creates pools, riffles, and meanders. The discharge, in terms of flood frequency, usually has a return period or recurrence interval of 1.5 to 2 years in natural channels. This represents a flow condition where the stream flow completely fills the stream channel up to the top of the bank before overflowing onto the floodplain.

Drainage basin — The portion of the surface of the earth that contributes water to a stream through overland runoff, including tributaries and impoundments.

Drinking-water standard or guideline — A threshold concentration in a public drinking-water supply, designed to protect human health. As defined here, standards are U.S. Environmental Protection Agency regulations that specify the maximum contamination levels for public water systems required to protect the public welfare; guidelines have no regulatory status and are issued in an advisory capacity.