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Intensive Fixed Sites — Basic Fixed Sites with increased sampling frequency during selected seasonal periods and analysis of dissolved pesticides for 1 year.

Ichthyosaurus (Ichthyosaur) [Nevada] — The chief genus of Ichthyosauria, also known by a prior name, Proteosaurus. An ancient, extinct marine reptile whose name means “fish-lizard” and who ruled the world’s oceans during the Mesozoic era some 200 million years ago. Fossil remains of the Ichthyosaur have been found on every continent except Africa. Possessing a fish-like body, porpoise-like snout, short neck, dorsal and caudal fins with limbs flattened into paddles, the Ichthyosaur ruled the seas for some 135 million years as the pre-eminent marine predator. The Ichthyosaur attained a maximum length of approximately 60–70 feet. The Ichthyosaurus shonisaurus popularis was the name given to a species discovered in Nevada in 1928. Some 40 of these reptiles became stranded in mud flats from a receding equatorial sea which once covered the state. The longest specimen found at this site, located at an elevation of 7,000 feet in the Shoshone Mountain Range near the town of Berlin in northwestern Nye County, Nevada, was 55 feet long and represented the only complete fossilized skeleton of the species ever found in the United States. In 1977 the Nevada State Legislature named the Ichthyosaurus Shonisaurus popularis as Nevada’s official state fossil.

Igneous Rock — (Geology) A rock formed by the solidification of molten materials (magma). The rock is extrusive (or volcanic) if it solidifies on the surface and intrusive (or plutonic) if it solidifies beneath the surface.

Immiscible — Applied to liquids which are insoluble in each other. The chemical property where two or more liquids or phases do not readily dissolve in one another, such as oil and water.

Impact Loads — (Flooding) Loads induced by the collision of sold objects on a structure carried by floodwater. Debris can include trees, lumber, displaced sections of structures, tanks, runaway boats, and chunks of ice. Debris impact loads are difficult to predict accurately, yet reasonable allowances must be made for them in the design of potentially affected structures.

Impact Zone — The spot on a wave where the water is just about to collapse and explode, the spot of greatest danger to and opportunity for a surfer.

Impaired — Water bodies that cannot reasonably be expected to attain or maintain applicable water quality standards, and at least one beneficial use shows some degree of degradation.

Importation (of Water) — The act or process whereby water is brought into an area or region which would not naturally receive such waters. Typically, it refers to the artificial transport of water through aqueducts, canals, or pipelines from one water basin, drainage area, or Hydrographic Area to another, thereby affecting the natural surface and groundwater drainage and flow patterns in both the water exporting and importing areas. In terms of a Water Banking or Water Marketing concept, such actions to move water from areas of low use to areas of high use place a more realistic monetary value on water as a scarce economic commodity and result in enhanced economic efficiency by putting existing water resources, wherever located, to more productive economic use.
However, considerable public concern and controversy surround this practice. These concerns deal primarily with issues relating to altering the natural flows of both surface and ground waters, adverse environmental and habitat impacts on water exporting areas, the limitations placed on the long-term growth and development of the water exporting region or hydrographic area, the potentially adverse hydrologic effects on groundwater (water table and aquifer) conditions in the exporting area as well as the generally unknown effects on surrounding hydrographic areas and aquifer conditions, and the dependency acquired by the water importing area to continued diversions and water importations. The concept of a public policy limiting an area’s development to its natural ability to support population growth only through existing and readily available natural resources, particularly water, is referred to
as an Antediluvian Policy.

Impoundment —
(1) Generally, an artificial collection or storage of water, as a reservoir, pit, gugout, or sump.
(2) A body of water such as a pond, confined by a dam, dike, floodgate or other barrier. It is used to collect and store water for future use.
(3) (Water Quality) Generally an artificial collection and storage area for water or wastewater confined by a dam, dike, floodgate, or other barrier.

Incised Channel (River) —
(1) A river which cuts its channel through the bed of the valley floor, as opposed to one flowing on a floodplain; its channel formed by the process of degradation.
(2) A stream that has degraded and cut its bed into the valley bottom. Indicates accelerated and often destructive erosion.

Indicator Bacteria — (Water Quality) Nonpathogenic bacteria whose presence in water indicate the possibility of pathogenic species in the water.

Indigenous — Existing, growing, or produced naturally in a region.

Influent Seepage — The movement of gravity water in the Zone of Aeration from the ground surface toward the water table.

Influent Stream — A stream that contributes water to the Zone of Saturation and to Bank Storage. This term has generally been replaced by the term Losing Stream.

Injection — Generally refers to a system of artificially introducing surface water into the ground water system as a means of storage or recharge. Most typically, this includes the use of Recharge Wells which work directly opposite of pumping wells to inject surface water into underlying formations. Depending on the water-bearing formation, these methods may have limited usefulness and are generally better used for pumping water into deep, confined aquifers. (Water Quality) Refers to a system of subsurface disposal of brine effluent into an acceptable formation.

Instream Flow —
(1) The amount of water remaining in a stream, without diversions, that is required to satisfy a particular aquatic environment or water use.
(2) Nonconsumptive water requirements which do not reduce the water supply; water flows for uses within a defined stream channel. Examples of instream flows include:
[1] Aesthetics — Water required for maintaining flowing steams, lakes, and bodies of water for visual enjoyment;
[2] Fish and Wildlife — Water required for fish and wildlife;
[3] Navigation — Water required to maintain minimum flow for waterborne commerce;
[4] Quality Dilution — Water required for diluting salt and pollution loading to acceptable concentrations; and
[5] Recreation — Water required for outdoor water recreation such as fishing, boating, water skiing, and swimming.

Instream Flow Needs — Those habitat requirements within the running water Ecosystem related to current velocity and depth which present the optimum conditions of density (or diversity) or physiological stability to the aquatic organisms being examined at various life cycle stages.

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.

Intermontane Basin — A generic term for wide structural depressions between mountain ranges that are partly filled with alluvium and are called “valleys” in the vernacular. Intermontane basins may be drained internally (Bolsons) or externally (Semi-Bolsons).

Interstices — The openings or pore spaces in a rock, soil, and other such material. In the Zone of Saturation they are filled with water. Synonymous with Void or Pore.

Inundation Map — A map delineating the area that would be inundated in the event of a dam failure.

Invasive Plant — A plant that moves in and takes over an Ecosystem to the detriment of other species; often the result of Environmental Manipulation.

Ion Exchange — The substitution of one Ion for another in certain substances. Either Anion Exchange or Cation Exchange is possible. The most common cation exchange involves the conversion of Hard Water to Soft Water by means of a Water Softening process. Hard water contains the divalent ions of calcium (Ca+2) and magnesium (Mg+2), which cause soap and detergents to form precipitates in water. A Water Softener consists of a resin that is saturated with sodium ions (Na+). As hard water percolates through the resin, the ions of calcium or magnesium are removed as they attach to the resin, thus releasing (being exchanged for) sodium ions.

Isobath — An imaginary line on the earth’s surface or a line on a map connecting all points which are the same vertical distance above the upper or lower surface of a water-bearing formation or aquifer.

Isochrone — Plotted line graphically connecting all points having the same time of travel for contaminants to move through the saturated zone and reach a well.

Isothermy — In Limnology, a state in which a lake is at the same temperature throughout and is well-mixed.