Glossary of Terms

Abatement - Reducing the degree or intensity of, or eliminating, pollution. 

Ablation - The process by which ice and snow waste away as a result of melting and/or evaporation. 

Abrasion - Removal of stream-bank soil as a result of sediment-laden water, ice, or debris rubbing against the bank. 

Acid rain - Rainfall with a pH of less than 5.7. Long-term deposition of these acids is linked to adverse effects on aquatic organisms and plant life in areas with poor neutralizing (buffering) capacity. 

Acidic - The condition of water or soil in which the amount of acid substances are sufficient to lower the pH below 7.0. 

Acre - A measure of area equal to 43,560 ft2 (4,046.87 m2). One square mile equals 640 acres. 

Acre-foot (af) - A quantity or volume of water covering 1 acre to a depth of 1 ft; equal to 43,560 ft3 or 325, 851 gal. 

Active storage capacity - The total usable storage capacity available for seasonal or cyclic water storage. It is gross reservoir capacity minus inactive storage capacity. 

Aeration - Any active or passive process by which intimate contact between air and liquid are assured, generally by spraying liquid in the air, bubbling air through water, or mechanical agitation of the liquid to promote surface absorption of air. 

Aerobic - Characterizing organisms able to live only in the presence of air or free oxygen, and conditions that exist only in the presence of air or free oxygen. Contrast with anaerobic. 

Affluent (stream) - A stream or river that flows into a larger one; a tributary. 

Afterbay - A reservoir that regulates fluctuating discharges from a hydroelectric power plant or a pumping plant. 

Aggradation (Aggrade) - A progressive buildup or raising of the channel bed and floodplain due to sediment deposition. The geologic process by which streambeds are raised in elevation and flood plains are formed. Aggradation indicates that stream discharge and/or bed-load characteristics are changing. Opposite of degradation. 

Algae - Microscopic plants that grow in sunlit water containing phosphates, nitrates, and other nutrients. Algae, like all aquatic plants, add oxygen to the water and are important in the food chain. 

Alluvial - Deposited by running water. 

Alluvium - A general term for detrital deposits made by stream processes on riverbeds, floodplains, and alluvial fans; esp. a deposit of silt or silty clay laid down during times of flood. The term applies to stream deposits of recent time. It does not include subaqueous sediments of seas or lakes. 

Anabranch - A diverging branch of a river that re-enters the main stream. 

Anadromous - Pertaining to fish that spend a part of their life cycle in the sea and return to freshwater streams to spawn. 

Angler-day - The time spent fishing for any part of a day by one person. 

Aquaduct - A pipe or conduit made to bring water from a source. 

Aquatic ecosystem - Any body of water, such as a stream, lake, or estuary, and all organisms and nonliving components within it, functioning as a natural system. 

Aquatic habitat - Habitat that occurs in free water. 

Aquifer - A body of rock that is sufficiently permeable to conduct groundwater and to yield economically significant quantities of water to wells and springs. 

Arid - A term describing a climate or region in which precipitation is so deficient in quantity or occurs so infrequently that intensive agricultural production is not possible without irrigation. 

Armoring - A natural or artificial process where an erosion-resistant layer of relatively large particles is established on the surface of the streambed through the removal of finer particles by stream flow. A properly armored streambed generally resists movement of the bed material at discharges up to approximately "three fourths" bank-full depth. 

Artificial discharge - Addition of surface water to a groundwater reservoir by human activity, such as putting surface water into spreading basins. See also groundwater recharge, recharge basin. 

Augmentation (of stream flow) - Increasing stream flow under normal conditions, by releasing storage water from reservoirs. 

Average annual runoff - For a specified area, it is the average value of annual runoff amounts calculated for a whole hydrologic cycle of record that represents average hydrologic conditions. 

Average yearly supply - The average annual supply of a water development system over a whole hydrologic system. 

Average year water demand - Demand for water under average hydrologic conditions for a defined level of development. 

Avulsion - A change in channel course that occurs when a stream suddenly breaks through its banks. 

Backwater - (1) A small, generally shallow body of water attached to the main channel, with little or no current of its own, or (2) A condition in subcritical flow where the water surface elevation is raised by downstream flow impediments. 

Backwater pool - A pool that formed as a result of an obstruction like a large tree, weir, dam, or boulder. 

Bank stability - The ability of a streambank to counteract erosion or gravitational forces. 

Bank-full channel depth - the maximum depth of a channel within a rifle segment when flowing at a bank-full discharge. 

Bank-full channel width - The top surface width of a stream channel when flowing at a bank-full discharge.

Bank-full discharge - The stream discharge corresponding to the water stage that first overtops the natural banks. This flow occurs, on average, about once every 1 or 2 years. 

Bank-full width - the width of a river or stream channel between the highest banks on either side of a stream. 

Bar - an accumulation of alluvium (usually gravel or sand) caused by a decrease in sediment transport capacity on the inside of meander bends or in the center of an overwide channel.

Barrier - A physical block or impediment to the movement or migration of fish, such as a waterfall (natural barrier) or a dam (man-made barrier). 

Base flow - The sustained portion of stream discharge that is drawn from natural storage sources, and not affected by human activity or regulation. 

Base level - The elevation to which a stream-channel profile has developed. 

Bed - The bottom of a channel. 

Bed load - Sediment moving on or near the streambed and transported by jumping, rolling, or sliding on the bed layer of a stream. See also suspended load. 

Bed material - The sediment mixture that a streambed is composed of. 

Bed material load - That portion of the total sediment load with sediments of a size found in the streambed. 

Bed roughness - A measure of the irregularity of the streambed as it contributes to flow resistance. Commonly expressed as a Manning "n" value. 

Bed scarp (or nick point) - An abrupt change of grade in the bottom of a stream channel that moves progressively upstream; the change in grade forms a waterfall. Also, the location where a streambed is actively eroding downward to a new base level. 

Bed slope - The inclination of the channel bottom, measured as the elevation drop per unit length of channel. 

Benthic invertebrates - Aquatic animals without backbones that dwell on or in the bottom sediments of fresh or salt water. Examples: clams, crayfish, and a wide variety of worms. 

Benthos - All plants and animals living on or closely associated with the bottom of a body of water. 

Best management practice (BMP) - Conservation measures intended to minimize or mitigate impacts from a variety of land-use activities. 

Biota - All living organisms of a region, as in a stream or other body of water. 

Blowdown - Trees felled at the base by high winds.

Bog - Freshwater wetlands that are poorly drained and characterized by a buildup of peat. 

Boulder - A large substrate particle that is larger than cobble, 256 mm in diameter.

Brackish water - Generally, water containing dissolved minerals in amounts that exceed normally acceptable standards for municipal, domestic, and irrigation uses. Considerably less saline than seawater. Also, marine and estuarine waters with mixohaline salinity (0.5-17.0 ppt due to ocean salts). Water containing between 500-17,000 parts per million (PPM) total dissolved solids (TDS). The term brackish water is frequently interchangeable with saline water. The term should not be applied to inland waters. 

Braided channel - A stream characterized by flow within several channels, which successively meet and divide. Braiding often occurs when sediment loading is too large to be carried by a single channel. 

Braiding (of river channels) - Successive division and rejoining of riverflow with accompanying islands. 

Buffer strip - A barrier of permanent vegetation, either forest or other vegetation, between waterways and land uses such as agriculture or urban development, designed to interrupt and filter out pollution before it reaches the surface water resource. 

Butt end - The bottom end of a cutting taken from a riparian plant that will root if planted in soil (it is opposite the budding tip's end of the cutting). 

Canal - A constructed open channel for transporting water. 

Canopy - A layer of continuous foliage in a forest stand. This most often refers to the upmost layer of foliage, but it can be used to describe lower layers in a multistoried stand. Leaves, branches and/or water that provide shade and cover for fish and wildlife.

Cascade - A short, steep drop in streambed elevation often marked by boulders and agitated white water. 

Catchment - (1) the catching or collecting of water, especially rainfall. (2) A reservoir or other basin for catching water. (3) The water thus caught. (4) A watershed. 

Caving - The collapse of a stream bank by undercutting due to wearing away of the toe or an erodible soil layer above the toe. 

Channel - An area that contains continuously or periodically flowing water that is confined by banks and a streambed. 

Channel-forming discharge - See dominant discharge. 

Channelization - The process of changing (usually straightening) the natural path of a waterway. 

Check dam - A structure placed bank-to-bank downhill from a headcut on a hillslope to help revegetate a gully.

Clay - (1) Substrate particles that are smaller than silt and generally less than 0.003 mm in diameter. (2) A soil textural class containing > 40% clay, < 45% sand, and < 40 % silt.

Closed basin - A basin whose topography prevents surface outflow of water. It is considered to be hydrologically closed if neither surface nor underground outflow of water can occur. 

Coarse woody debris (CWD) - Portion of a tree that has fallen or been cut and left in the woods. Usually refers to pieces at least 20 inches in diameter. 

Cobble - Substrate particles that are smaller than boulders and larger than gravels, and are generally 64-256 mm in diameter. Can be further classified as small and large cobble. 

Confined aquifer - A water-bearing subsurface stratum that is bounded above and below by formations of impermeable soil or rock. 

Confluence - (1) The act of flowing together; the meeting or junction of two or more streams; also, the place where these streams meet. (2) The stream or body of water formed by the junction of two or more streams; a combined flood. 

Conifer - A tree belonging to the Gymnospermae, comprising a wide range of trees that are mostly evergreens. Conifers bear cones (hence, coniferous) and have needle-shaped or scale-like leaves. 

Conjunctive use - The operation of a groundwater basin in combination with a surface water storage and conveyance system. Water is stored in the groundwater basin for later use by intentionally recharging the basin during years of above-average water supply. 

Conservation - The process or means of achieving recovery of variable populations.

Conservation area - Designated land where conservation strategies are applied for the purpose of attaining a viable plant or animal population. 

Conservation recommendations - Suggestions by conservation agencies regarding discretionary measures to minimize or avoid adverse effects on a proposed action of federally listed threatened or endangered species or designated critical habitat.

Conservation strategy - A management plan for a species, group of species, or ecosystem that prescribes standards and guidelines that, if implemented, provide a high likelihood that the species, groups of species or ecosystem, with its full complement of species and processes, will continue to exist well-distributed throughout a planning area, i.e. a viable population. 

Contaminate - To make impure or unclean by contact or mixture.

Contiguous habitat - Habitat suitable to support the life needs of a species that is distributed continuously across the landscape.

Core area - The area of habitat essential in the breeding, nesting, and rearing of young, up to the point of dispersal of the young. 

Creel census survey - The collection of data concerning the number of fish caught by sport fishers on a particular stream or in a particular area. 

Critical habitat - Under the Endangered Species Act, critical habitat is defined as(1) the specific areas within a geographic area occupied by a federally listed species on which are found physical and biological features essential to the conservation of the species, and that may require special management considerations or protections; and (2) specific areas outside the geographic area occupied by a listed species, when it is determined that such areas are essential for the conservation of the species. 

Critical shear stress - The minimum amount of shear stress exerted by stream currents required to initiate soil particle motion. Because gravity also contributes to streambank particle movement but not on streambeds, critical shear stress along streambanks is less than for streambeds.

Crown - the upper part of a tree or other woody plant that carries the main system of branches and the foliage. 

Crown cover - The degree to which the crowns of trees are nearing general contact with one another. 

Cubic feet per second (cfs) - A unit used to measure water flow. One cubic foot per second is equal to 449 gallons per minute. 

Culvert - A buried pipe that allows flows to pass under a road.

Cut bank - The outside bank of a bend, often eroding opposite a point bar. 

Cut off - A channel cut across the neck of a bend. 

Daylight - In the restoration field, a verb that denotes the excavation and restoration of a stream channel from an underground culvert, covering, or pipe. 

Deadman - A log, block of concrete, rebar, or other object buried in a stream bank that is used to tie in revetment with cable or chain. 

Debris flow - A rapidly moving mass of rock fragments, soil, and mud, with more than half of the particles being larger than sand size. 

Debris torrent - Rapid movement of a large quantity of materials (wood and sediment) down a stream channel during storms or floods. This generally occurs in smaller streams and results in scouring of the streambed.

Deciduous - Trees and plants that shed their leaves at the end of a growing season. 

Decomposer - Any of various organisms (as many bacteria and fungi) that feed on and break down organic substances (such as dead plants and animals). 

Decomposition - The breakdown of matter by bacteria and fungi, changing the chemical makeup and physical appearance of materials. 

Deep percolation - the percolation of water through the ground and beyond the lower limit of the root zone of plants into a groundwater aquifer. 

Degradation (degrade) - (1) A progressive lowering of the channel bed due to scour. Degradation is an indicator that the stream's discharge and/or sediment load is changing. The opposite of aggradation. (2) A decrease in value for a designated use. 

Dependable supply - The annual average quantity of water that can be delivered during a drought period. 

Depletion - A water use term. The water consumed within a service area and no longer available as a source of supply. For agriculture and wetlands, it is evapotranspiration of applied water (ETAW) and evapotranspiration (ET) of flooded wetlands, plus irrecoverable losses. For urban water use, it is ETAW (water applied to landscaping or home gardens), sewage effluent that flows to a salt link, and incidental ET losses. For instream use, it is the amount of dedicated flow that becomes groundwater and is not available for reuse. 

Dike (groin, spur, jetty, deflector, boom) - (1) (Engineering) An embankment to confine or control water, especially one built along the banks of a river to prevent overflow of lowlands; a levee. (2) A low wall that can act as a barrier to prevent a spill from spreading. (3) (Geology) A tabular body of igneous (formed by volcanic action) rock that cuts across the structure of adjacent rocks or cuts adjacent rocks. 

Discharge - The volume of water passing through a channel during a given time, usually measured in cubic feet per second.

Discount rate - The interest rate used in evaluating water (and other) projects to calculate the present value of future benefits and future costs or to convert benefits and costs to a common time basis. 

Dissolved gas concentrations - The amount of chemicals normally occurring as gases, such as nitrogen and oxygen, that are held in solution in water, expressed in units such as milligrams of the gas per liter of liquid. Supersaturation occurs when these solutions exceed the saturation level of the water (beyond 100 percent). 

Dissolved organic compounds - Carbon substances dissolved in water. 

Dissolved oxygen (DO) - 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. 

Ditch - A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

Diversion - The transfer of water from a stream, lake, aquifer, or other conduit to another watercourse or to the land, as in the case of an irrigation system.

Diversion channel - (1) An artificial channel constructed around a town or other point of high potential flood damages to divert floodwater from the main channel to minimize flood damage. (2) A channel carrying water from a diversion dam. 

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 area - The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. 

Drainage basin - The total area of land from which water drains into a specific river. 

Dredging - Removing material (usually sediments) from wetlands or waterways, usually to make them deeper and wider. 

Drought - Generally, the term is applied to periods of less than average or normal precipitation over a certain period of time sufficiently prolonged to cause a serious hydrological imbalance resulting in biological losses (impact flora and fauna ecosystems) and/or economic losses (affecting man). In a less precise sense, it can also signify nature's failure to fulfill the water wants and needs of man. 

Dry wash - A streambed that carries water only during and immediately following rainstorms. 

Ecology - The study of interrelationships of living organisms to one another and to their surroundings. 

Economic demand - The consumer's willingness and ability to purchase some quantity of a commodity based on the price of that commodity. 

Ecosystem - (1) Recognizable, relatively homogeneous units, including the organisms they contain, their environment, and all the interactions among them. (2) Any complex of organisms in an environment considered as a unit for the purpose of study.

Ecosystem management - A strategy or plan to manage ecosystems to provide for all associated organisms, as opposed to a strategy or plan for managing individual species. 

Eddy current - A circular current of water, usually resulting from an obstruction, that develops when the main flow becomes separated from the bank.

Effluent - (1) Something that flows out or forth, especially a stream flowing out of a body of water. (2) (Water Quality) Discharged wastewater such as the treated wastes from municipal sewage plants, brine wastewater from desalting operations, and coolant waters from a nuclear power plant. 

Embankment - An artificial deposit of material that is raised above the natural surface of the land and used to contain, divert, or store water, support roads and railways, or for other similar purposes. 

Energy dissipation - The loss of kinetic energy of moving water due to internal turbulence, bottom friction, large rocks, debris, or other obstacles that impede flow. 

Enhancement - Emphasis on improving the value of particular aspects of water and related land resources. 

Environment - The sum of all external influences and conditions affecting the life and development of an organism or ecological community; the total social and cultural conditions. 

Environmental analysis - An analysis of alternative actions and their predictable short-term and long-term environmental effects, incorporating physical, biological, economic, and social considerations. 

Environmental assessment (EA) - A systematic analysis of site-specific activities used to determine whether such activities have a significant effect on the quality of the human environment and whether a formal environmental impact statement is required; and to aid an agency's compliance with the National Environmental Policy Act when no environmental impact statement is necessary. 

Environmental impact - the positive or negative effect of any action upon a given area or source. 

Environmental impact statement (EIS) - A formal document to be filed with the Environmental Protection Agency that considers significant environmental impacts expected from implementation of a major federal action. 

Ephemeral streams - Streams that flow only in direct response to precipitation and whose channel is at all times above the water table. 

Erosion - Wearing away of rock or soil by the gradual detachment of soil or rock fragments by water, wind, ice, and other mechanical, chemical, or biological forces. 

Estuary - A coastal body of water that is semi-enclosed, openly connected with the ocean, and mixes with freshwater drainage from land. 

Eutrophic - Usually refers to a nutrient-enriched, highly productive body of water. 

Eutrophication - The process of enrichment of water bodies by nutrients and the subsequent depletion of dissolved oxygen it produces.. 

Evaporation - The physical process by which a liquid (or a solid) is transformed to the gaseous state. In hydrology, evaporation is vaporization that takes place at a temperature below the boiling point. 

Evapotranspiration (ET) - The quantity of water transpired (given off), retained in plant tissues, and evaporated from plant tissues and surrounding plant surfaces. Quantitatively, it is usually expressed in terms of depth of water per unit area during a specified period of time. 

Evapotranspiration of applied water (ETAW) - the portion of the total evapotranspiration provided by irrigation. 

Failure - Collapse or slippage of a large mass of bank material into a stream. 

Fill - (1) (Geology) Any sediment deposited by any agent such as water so as to fill or partly fill a channel, valley, sink, or other depression. (2) (Engineering) Soil or other material placed as part of a construction activity. 

Filter fabric - A polypropylene textile used to keep soil separate from water. Comes in many different forms and is used for constructing roads, lining ponds, and in many erosion control projects. 

Final environmental impact statement (FEIS) - The final report of environmental effects of proposed action on an area of land. This is required for major federal actions under Section 102 of the National Environmental Policy Act. It is a revision of the draft environmental impact statement to include public and agency responses to the draft. 

Flash flood - A sudden flood of great volume, usually caused by a heavy rain. Also, a flood that crests in a short length of time and is often characterized by high velocity flows. 

Flood insurance rate map (FIRM) - A map used to establish the insurance rates for structures under the National Flood Insurance Plan. 

Floodplain - Land built of sediment that is regularly covered with water as a result of the flooding of a adjacent stream. 

Floodplain (100-year) - The area adjacent to a stream that is on average inundated once a century. 

Flood stage - An above average elevation for the water level at high flows.

Floodway - A regulatory floodplain under the National Flood Insurance Program that includes the channel and that portion of the adjacent floodplain that is required to pass flood flows (normally the one-in-100-year flood) without increasing the water surface elevation more than a designated height (1 foot in most areas). 

Flow - The amount of water passing a particular point in a stream or river, usually expressed in cubic feet per second (cfs). 

Flow augmentation - Increased flow from release of water from storage dams. 

Fluvial - Migrating between main rivers and tributaries. Of or pertaining to streams or rivers. 

Ford - A shallow place in a body of water, such as a river, where one can cross by walking or riding on an animal or in a vehicle. 

Forebay - A reservoir or pond situated at the intake of a pumping plant or power plant to stabilize water levels; also, a storage basin for regulating water for percolation into groundwater basins. 

Fry - A recently hatched fish. 

Gabion - A wire basket or cage that is filled with gravel or cobble and generally used to stabilize streambanks. 

Gaging station - A particular site in a stream, lake, reservoir, etc., where hydrologic data are obtained. 

Gallons per minute (gpm) - A unit used to measure water flow. 

Geographic information system (GIS) - A computer system capable of storing and manipulating spatial data. 

Geomorphology - A branch of both physiography and geology that deals with the form of the earth, the general configuration of its surface, and the changes that take place due to erosion of the primary elements and the buildup of erosional debris. 

Glide - A section of stream that has little or no turbulence.

Grade control structure - A weir, dam, sill, drop structure, or other structure used to control erosion in stream channels with steep grades or where the slope has been destabilized. 

Gradient - Vertical drop per unit of horizontal distance. 

Grass/forb - Herbaceous vegetation. 

Gravel - An unconsolidated natural accumulation of rounded rock fragments, mostly of particles larger than sand (diameter greater than 2 mm), such as boulders, cobbles, pebbles, granules, or any combination of these. 

Gray water - Wastewater from a household or small commercial establishment that specifically excludes water from a toilet, kitchen sink, dishwater, or water used for washing diapers. 

Greenbelt - Strip of natural vegetation growing parallel to a stream that provides wildlife habitat and an erosion and flood buffer zone. This strip of vegetation also retards rainfall runoff down the bank slope and provides a root system that binds soil particles together. 

Groundwater - Subsurface water and underground streams that can be collected with wells, or that flow naturally to the earth's surface through springs. 

Groundwater basin - A groundwater reservoir, defined by an overlying land surface and the underlying aquifers that contain water stored in the reservoir. In some cases, the boundaries of successively deeper aquifers may differ and make it difficult to define the limits of the basin. 

Groundwater flow - Water that moves through the subsurface soil and rocks. 

Groundwater overdraft - The condition of a groundwater basin in which the amount of water withdrawn by pumping exceeds the amount of water that recharges the basin over a period of years during which water supply conditions approximate average.

Groundwater prime supply - Long-term average annual percolation into major groundwater basins from percolation into major groundwater basins from precipitation falling on the land and from flows in rivers and streams. 

Groundwater recharge - Increases in groundwater storage by natural conditions or by human activity. See also artificial recharge. 

Groundwater storage capacity - The space or voids contained in a given volume of soil and rock deposits. 

Groundwater table - The upper surface of the zone of saturation, except where the surface is formed by an impermeable body. 

Habitat - The local environment in which organisms normally live and grow. 

Habitat conservation plan (HCP) - An agreement between the Secretary of the Interior and either a private entity or a state that specifies conservation measures that will be implemented in exchange for a permit that would allow taking of a threatened or endangered species.

Habitat diversity - The number of different types of habitat within a given area. 

Habitat fragmentation - The breaking up of habitat into discrete islands through modification or conversion of habitat by management activities. 

Hardpan - A layer of nearly impermeable soil beneath a more permeable soil, formed by natural chemical cementing of the soil particles. 

Hard water - Water high in multivalent cations, such as calcium and magnesium. This type of water does not lather easily when used with soap and forms a scale in containers when allowed to evaporate. 

Hatch box - A device used to incubate relatively small numbers of eggs. The hatch box is usually located adjacent to a stream, which supplies the box with water. 

Hazardous materials - Anything that poses a substantive present or potential hazard to human health or the environment when improperly treated, stored, transported, disposed of, or otherwise managed. 

Headcut - A break in the slope at the top of a gully or section of a gully that forms a "waterfall" which in turn causes the underlying soil to erode and the gully to expand uphill. 

Headcutting - The action of a bedscarp or headward erosion of a locally steep channel or gully. 

Headwater - Referring to the source of a stream or river.

Heavy metals - Metallic elements with high atomic weights, e.g., mercury, chromium, cadmium, arsenic, and lead. They can damage living things at low concentrations, and tend to accumulate in the food chain. 

Hungry water - Clear water minus its expected suspended sediment load, usually released from an impoundment that has excess energy, which erodes sediment from the downstream channel. 

Hydraulic gradient - The slope or the water surface. See also streambed gradient. 

Hydraulic radius - The cross-sectional area of a stream divided by the wetted perimeter. 

Hydric - Wet. 

Hydrograph - A curve showing stream discharge over time. 

Hydrologic balance - An accounting of all water inflow to, water outflow from, and changes in water storage within a hydrologic unit over a specified period of time. 

Hydrologic region - A study area, consisting of one or more planning subareas, that has a common hydrologic character. 

Hydrologic unit - A distinct watershed or river basin defined by an eight-digit code. 

Hydrology - The scientific study of water of the earth, its occurrence, circulation, and distribution, its chemical and physical properties, and its interaction with the environment, including its relationship to living things.

Hyporheic zone - The area under the stream channel and floodplain where groundwater and the surface waters of a stream are exchanged freely. 

Impermeable channel - A material that has properties preventing movement of water through it. Nonporous. 

Incised river (channel) - A river that erodes its channel by a process of degradation to a lower base level than existed previously or is consistent with the current hydrology. 

Infiltration (soil) - The movement of water through the soil surface into the soil. 

Inflow - Water that flows into a stream, lake, reservoir, or Forebay during a specified period.

Instream cover - The layers of vegetation, like trees, shrubs, and overhanging vegetation, that are in the stream or immediately adjacent to the wetted channel. 

Instream flows - (1) Portion of a flood flow that is contained by the channel. (2) A flow or flow regime needed to maintain ecological health in a river/stream. 

Instream use - Use of water that does not require diversion from its natural watercourse. For example, the use of water for navigation, recreation, fish and wildlife, aesthetics, and scenic enjoyment. 

Intermittent stream - Any nonpermanent flowing drainage feature having a definable channel and evidence of scour or deposition. This includes what are sometimes referred to as ephemeral streams if they meet these two criteria. 

Invertebrate drift - Stream and terrestrial invertebrates that float with the current. 

Irrigation diversion - Generally, a ditch or channel that deflects water from a stream channel for irrigation purposes. 

Irrigation efficiency - The efficiency of water application and use. Computed by dividing evapotranspiration of applied water by applied water and converting the result to a percentage. Efficiency can be computed at three levels: farm, district, or basin. 

Irrigation return flow - applied water that is not transpired, evaporated, or deep-percolated into a groundwater basin but returns to a surface water supply. 

Key watershed - As defined by National Forest and Bureau of Land Management District fish biologists, a watershed containing (1) habitat for potentially threatened species or stocks of anadromus salmonids or other potentially threatened fish, or (2) greater than six square miles with high-quality water and fish habitat. 

Keyway (key) - The notch excavated into the side of a gully or stream to anchor a check dam or other structure. 

Lake - An inland body of standing water deeper than a pond, an expanded part of a river, a reservoir behind a dam. 

Landscape - A heterogenous land area with interacting ecosystems that are repeated in similar form throughout. 

Landscape diversity - The size, shape, and connectivity of different ecosystems across a large area. 

Landscape features - The land, water, vegetation, and structures that compose the characteristic landscape. 

Landslide - a movement of earth mass down a steep slope. 

Large woody debris (LWD) - Pieces of naturally occurring wood larger than 10 ft long and 6 in. in diameter, in a stream channel. 

Leaching - The flushing of minerals or pollutants from soil or other material by the percolation of applied water. 

Leaf area index - A measure of the total area of leaves, twigs, stems, etc. relative to the area of the canopy in a forest. 

Levee - An embankment constructed to prevent a river from overflowing (flooding). 

Lifts - Layers of loose soil. Used to specify how much loose soil should be laid down at a time before it must be compacted or wrapped in geotextile fabric. 

Limiting factor - A requirement such as food, cover, or another, physical, chemical, or biological factor that is in shortest supply with respect to all resources necessary to sustain life and thus "limits" the size or retards production of a population. 

Limnology - The study of life in lakes, ponds, and streams. 

Loading - The influx of pollutants to a selected water body. 

Longitudinal profile - A graphic presentation of elevation vs. distance; in channel hydraulics it is a plot of water surface elevation against upstream to downstream distance. 

Lotic - Meaning or regarding things in running water. 

Macroinvertebrate - Invertebrates visible to the naked eye, such as insect larvae and crayfish.

Macrophytes - Aquatic plants that are large enough to be seen with the naked eye. 

Main stem - The principal channel of a drainage system into which other smaller streams or rivers flow. 

Mass movement - The downslope movement of the earth caused by gravity. Includes but is not limited to landslides, rock falls, debris avalanches, and creep. It does not however, include surface erosion by running water. It may be caused by natural disturbances (e.g., earthquakes or fire events) or human disturbances (e.g., mining or road construction).

Maximum contaminant level (MCL) - The highest concentration of a constituent in drinking water permitted under federal and State Safe Drinking Water Act regulations.

Mean annual discharge - Daily mean discharge averaged over a period of years. Mean annual discharge generally fills a channel to about one-third of its bank-full depth. 

Mean velocity - The average cross-sectional velocity of water in a stream channel. Surface values typically are much higher than bottom velocities. May be approximated in the field by multiplying the surface velocity, as determined with a float, times 0.8.

Meander - The winding of a stream channel, usually in an erodible alluvial valley. A series of sine-generated curves characterized by curved flow and alternating banks and shoals. 

Meander amplitude - The distance between points of maximum curvature of successive meanders of opposite phase in a direction normal to the general course of the meander belt, measured between centerlines of channels. 

Meander belt width - The distance between lines drawn tangential to the extreme limits of fully developed meanders. Not to be confused with meander amplitude. 

Meander length - The lineal distance downvalley between two corresponding points of successive meanders of the same phase. 

Mesic - Moderately wet. 

Milligrams per liter (mg/l) - the weight in milligrams of any substance dissolved in one liter of liquid; nearly the same as parts per million by weight. 

Mineralization - The process whereby concentrations of minerals, such as salts, increase in water, often a natural process resulting from water dissolving minerals found in rocks and soils through which it flows. 

Moisture stress - A condition of physiological stress in a plant caused by lack of water. 

Morphology - the form, shape, or structure of a stream or organism. 

Multipurpose project - A project designed to serve more than one purpose. For example, one that provides water for irrigation, recreation, fish and wildlife, and, at the same time, controls floods or generates electric power. 

National Pollutant Discharge Elimination System (NPDES) - A provision of Section 402 of the Federal Clean Water Act of 1972 that established a permitting system for discharges of waste materials to watercourses. 

Natural flow - The flow past a specified point on a natural stream that is unaffected by stream diversion, storage, import, export, return flow, or change in use caused by modifications in land use. 

Net water demand (net water use) - The amount of water needed in a water service area to meet all requirements. It is the sum of evapotranspiration of applied water (ETAW) in an area, the irrecoverable losses from the distribution system, and the outflow leaving the service area; does not include reuse of water within a service area (such as reuse of deep-percolated applied water or use of tailwater). 

Nick point - The point at which a stream is actively eroding the streambed to a new base level. 

Non-cohesive soil - Soil particles that have no natural resistance to being pulled apart at their point of contact, for example, silt, sand, gravel. 

Non-point source pollution (NPS) - Pollution that does not originate from a clear or discrete source. 

Normalization - The mathematical manipulation of a variable to allow comparisons with an otherwise different variable. 

Normalized demand - The process of adjusting actual water use in a given year to account for unusual events such as dry weather conditions, government interventions for agriculture, rationing programs, or other irregularities. 

Nutrient depletion - Detrimental changes at a site in the total amount of nutrients and/or their rates of input, uptake, release, movement, transformation, or export. 

Off-channel area - Any relatively calm portion of a stream outside of the main flow. 

Off-site enhancement - The improvement in conditions for fish and wildlife species away from the site or development activities that may have detrimental effects on fish and/or wildlife, as part or total compensation for those effects. 

Overbank flow - Water flow over the top of the bankfull channel onto the floodplain. 

Outfall - The mouth or outlet of a river, stream, lake, drain or sewer. 

Oxbow - An abandoned meander in a river or stream, caused by cutoff. Used to describe the U-shaped bend in the river or the land within such a bend of a river.