USGS Water Resources Information
National Water Summary on Wetland Resources
United States Geological Survey Water Supply Paper 2425

Supplemental Information-


Absorption - the process by which substances in gaseous, liquid, or solid form are assimilated or taken up by other substances.

Acid - pH of water less than 5.5; pH modifier used in the U.S. Fish and Wildlife Service wetland classification system.

Acidic - has a pH of less than 7.

Acidic deposition - the transfer of acidic or acidifying substances from the atmosphere to the surface of the Earth or to objects on its surface. Transfer can be either by wet-deposition processes (rain, snow, dew, fog, frost, hail) or by dry deposition (gases, aerosols, or fine to coarse particles).

Acre-foot (acre-ft.) - the volume of water needed to cover an acre of land to a depth of one foot; equivalent to 43,560 cubic feet or 325,851 gallons.

Adsorption - the adherence of gas molecules, ions, or molecules in solution to the surface of solids.

Aerate - to supply air to water, soil, or other media.

Aerobic - pertaining to or caused by the presence of oxygen.

Algal bloom - the rapid proliferation of passively floating, simple plant life, such as blue-green algae, in and on a body of water.

Alkaline - has a pH greater than 7; pH modifier in the U.S. Fish and Wildlife Service wetland classification system; in common usage, a pH of water greater than 7.4.

Alluvium - general term for sediments of gravel, sand, silt, clay, or other particulate rock material deposited by flowing water, usually in the beds of rivers and streams, on a flood plain, on a delta, or at the base of a mountain.

Alpine snow glade - a marshy clearing between slopes above the timberline in mountains.

Anadromous fish - migratory species that are born in freshwater, live mostly in estuaries and ocean water, and return to freshwater to spawn.

Anaerobic - pertaining to or caused by the absence of oxygen.

Anthropogenic - having to do with or caused by humans.

Anticline - a fold in the Earth's crust, convex upward, whose core contains stratigraphically older rocks.

Aquatic - living or growing in or on water.

Aquaculture - the science of farming organisms that live in water, such as fish, shellfish, and algae.

Aquifer - a geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to springs and wells.

Arroyo - a small, deep, flat-floored channel or gully of an ephemeral or intermittent stream, usually with nearly vertical banks cut into unconsolidated material-term commonly used in the arid and semiarid regions of the Southwestern United States.

Atmospheric pressure - the pressure exerted by the atmosphere on any surface beneath or within it; equal to 14.7 pounds per square inch at sea level.

Backwater - a body of water in which the flow is slowed or turned back by an obstruction such as a bridge or dam, an opposing current, or the movement of the tide.

Bacteria - single-celled microscopic organisms.

Bank storage - the change in the amount of water stored in an aquifer resulting from a change in stage of an adjacent surface-water body.

Barrier bar - an elongate offshore ridge submerged at least at high tide, built up by the action of waves or currents.

Barrier beach - a narrow, elongate sandy ridge rising slightly above the high-tide level and extending generally parallel with the mainland shore, but separated from it by a lagoon.

Base flow - the sustained low flow of a stream, usually ground-water inflow to the stream channel.

Basic - the opposite of acidic; has a pH of greater than 7.

Bed material - sediment composing the streambed.

Bedrock - a general term used for solid rock that underlies soils or other unconsolidated material.

Benthic organism - a form of aquatic life that lives on the bottom or near the bottom of streams, lakes, or oceans.

Bind - to exert a strong chemical attraction.

Biochemical-oxygen demand (BOD) - the amount of oxygen, in milligrams per liter, that is removed from aquatic environments by the life processes of micro-organisms.

Biochemical process - a process characterized by, produced by, or involving chemical reactions in living organisms.

Biomass - the amount of living matter, in the form of organisms, present in a particular habitat, usually expressed as weight- per-unit area.

Biota - all living organisms of an area.

Blowout - a small saucer or trough-shaped hollow or depression formed by wind erosion on a pre - existing dune or other sand deposit.

Bog - a nutrient-poor, acidic wetland dominated by a waterlogged spongy mat of sphagum moss that ultimately forms a thick layer of acidic peat; generally has no inflow or outflow; fed primarily by rain water.

Bolson - an extensive, flat, saucer-shaped, alluvium-floored basin or depression, almost or completely surrounded by mountains and from which drainage has no surface outlet; a term used in the desert regions of the Southwestern United States.

Boreal - a climatic zone having a definite winter with snow and a short summer that is generally hot, and which is characterized by a large annual range of temperature.

Bosque - a dense growth of trees and underbrush.

Bottom land - See flood plain.

Bottom-land forest - low-lying forested wetland found along streams and rivers, usually on alluvial flood plains.

Brackish water - water with a salinity intermediate between seawater and freshwater (containing from 1,000 to 10,000 milligrams per liter of dissolved solids) interlacing or tangled network of several small branching and reuniting shallow channels.

Brine - water that contains more than 35,000 milligrams per liter of dissolved solids.

Calcareous - formed of calcium carbonate or magnesium carbonate by biological deposition or inorganic precipitation in sufficient quantities to effervesce when treated with cold hydrochloric acid.

Caldera - a large, more or less circular, basin-shaped volcanic depression whose diameter is many times greater than the volcanic vent.

Channel scour - erosion by flowing water and sediment on a stream channel; results in removal of mud, silt, and sand on the outside curve of a stream bend and the bed material of a stream channel.

Channelization - the straightening and deepening of a stream channel to permit the water to move faster or to drain a wet area for farming.

Cienaga - a marshy area where the ground is wet due to the presence of seepage or springs.

Circumneutral - pH of water between 5.5 and 7.4; pH modifier used in the U.S. Fish and Wildlife Service wetland classification system.

Cirque - a deep, steep-walled half-bowllike recess or hollow situated high on the side of a mountain and commonly at the head of a glacial valley and produced by the erosive activity of mountain glaciers.

Concentration - the ratio of the quantity of any substance present in a sample of a given volume or a given weight compared to the volume or weight of the sample.

Confining layer - a body of impermeable or distinctly less permeable material stratigraphically adjacent to one or more aquifers that restricts the movement of water into and out of the aquifers.

Conglomerate - a coarse-grained sedimentary rock composed of fragments larger than 2 millimeters in diameter.

Contact recreation - recreational activities where there is prolonged or intimate contact with water and in which there is a likelihood of ingesting water.

Contributing area - the area in a drainage basin that contributes water to streamflow or recharge to an aquifer.

Coral reef - a ridge of limestone, composed chiefly of coral, coral sands, and solid limestone resulting from organic secretion of calcium carbonate; occur along continents and islands where the temperature is generally above 18° C.

Core sample - a sample of rock, soil, or other material obtained by driving a hollow tube into the undisturbed medium and withdrawing it with its contained sample.

Cypress dome - small, isolated, circular, depressional, forested wetlands, in which cypress predominates, that have convex silhouettes when viewed from a distance.

Deciduous - shedding foliage at the end of the growing season.

Deepwater habitat - permanently flooded lands lying below the deepwater boundary of wetlands.

Degraded - condition of the quality of water that has been made unfit for some specified purpose.

Delta - the low, nearly flat tract of land at or near the mouth of a river, resulting from the accumulation of sediment supplied by the river in such quantities that it is not removed by tides, waves, or currents. Commonly a triangular or fan-shaped plain.

Direct runoff - the runoff entering stream channels promptly after rainfall or snowmelt.

Discharge - the volume of fluid passing a point per unit of time, commonly expressed in cubic feet per second, million gallons per day, gallons per minute, or seconds per minute per day.

Discharge area (ground water) - area where subsurface water is discharged to the land surface, to surface water, or to the atmosphere.

Dissolved oxygen - oxygen dissolved in water; one of the most important indicators of the condition of a water body. Dissolved oxygen is necessary for the life of fish and most other aquatic organisms.

Dissolved solids - minerals and organic matter dissolved in water.

Dolomite - a sedimentary rock consisting chiefly of magnesium carbonate.

Dominant plant - the plant species controlling the environment.

Drainage basin - the land area drained by a river or stream.

Drought - a prolonged period of less-than-normal precipitation such that the lack of water causes a serious hydrologic imbalance.

Ecosystem - a community of organisms considered together with the nonliving factors of its environment.

Emergent plants - erect, rooted, herbaceous plants that may be temporarily to permanently flooded at the base but do not tolerate prolonged inundation of the entire plant.

Endangered species - a species that is in imminent danger of becoming extinct.

Environment - the sum of all conditions and influences affecting the life of organisms.

Ephemeral stream - a stream or part of a stream that flows only in direct response to precipitation; it receives little or no water from springs, melting snow, or other sources; its channel is at all times above the water table.

Erosion - the process whereby materials of the Earth's crust are loosened, dissolved, or worn away and simultaneously moved from one place to another.

Estuarine wetlands - tidal wetlands in low-wave-energy environments where the salinity of the water is greater than 0.5 part per thousand and is variable owing to evaporation and the mixing of seawater and freshwater; tidal wetlands of coastal rivers and embayments, salty tidal marshes, mangrove swamps, and tidal flats.

Estuary - area where the current of a stream meets the ocean and where tidal effects are evident; an arm of the ocean at the lower end of a river.

Eutrophication - the process by which water becomes enriched with plant nutrients, most commonly phosphorus and nitrogen.

Evaporation - the process by which water is changed to gas or vapor; occurs directly from water surfaces and from the soil.

Evaporites - a class of sedimentary rocks composed primarily of minerals precipitated from a saline solution as a result of extensive or total evaporation of water.

Evapotranspiration - a term that includes water discharged to the atmosphere as a result of evaporation from the soil and surface-water bodies and by plant transpiration.

Exotic species - plants or animals not native to the area.

Fall line - imaginary line marking the boundary between the ancient, resistant crystalline rocks of the Piedmont province of the Appalachian Mountains, and the younger, softer sediments of the Atlantic Coastal Plain province in the Eastern United States. Along rivers, this line commonly is reflected by waterfalls.

Fallow - cropland, tilled or untilled, allowed to lie idle during the whole or greater part of the growing season.

Fen - peat - accumulating wetland that generally receives water from surface runoff and (or) seepage from mineral soils in addition to direct precipitation; generally alkaline; or slightly acid.

Filtrate - liquid that has been passed through a filter.

Flood - any relatively high streamflow that overflows the natural or artificial banks of a stream.

Flood attenuation - a weakening or reduction in the force or intensity of a flood.

Flood plain - a strip of relatively flat land bordering a stream channel that is overflowed at times of high water.

Fluvial - pertaining to a river or stream.

Flyway - a specific air route taken by birds during migration.

Freshwater - water with less than 1,000 milligrams per liter of dissolved solids.

Friable - descriptive of a rock or mineral that crumbles naturally or is easily broken, pulverized, or reduced to powder.

Geomorphic - pertaining to the form of the Earth or of its surface features.

Geomorphology - the science that treats the general configuration of the Earth's surface; the description of landforms.

Glacial - of or relating to the presence and activities of ice or glaciers.

Glacial drift - a general term for rock material transported by glaciers or icebergs and deposited directly on land or in the sea.

Glacial lake - a lake that derives its water, or much of its water, from the melting of glacial ice; also a lake that occupies a basin produced by glacial erosion.

Glacial outwash - stratified detritus (chiefly sand and gravel) "washed out" from a glacier by meltwater streams and deposited in front of or beyond the end moraine or the margin of an active glacier.

Ground water - in the broadest sense, all subsurface water; more commonly that part of the subsurface water in the saturated zone.

Ground-water flow system - the underground pathway by which ground water moves from areas of recharge to areas of discharge.

Growing season - the frost-free period of the year.

Habitat - the part of the physical environment in which a plant or animal lives.

Hardpan - a relatively hard, impervious, and usually clayey layer of soil lying at or just below land surface-produced as a result of cementation by precipitation of insoluble minerals.

Herbaceous - with characteristics of an herb; a plant with no persistent woody stem above ground.

Herbicide - a type of pesticide designed to kill plants.

Hydraulic head - the height of the free surface of a body of water above a given point beneath the surface.

Hydraulic gradient - the change of hydraulic head per unit of distance in a given direction.

Hydric soil - soil that is wet long enough to periodically produce anaerobic conditions, thereby influencing the growth of plants.

Hydrologic cycle - the circulation of water from the sea, through the atmosphere, to the land, and thence back to the sea by overland and subterranean routes.

Hydrology - the science that deals with water as it occurs in the atmosphere, on the surface of the ground, and underground.

Hydrophyte - any plant growing in water or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content.

Hydrostatic pressure - the pressure exerted by the water at any given point in a body of water at rest. Hydrologic regime-the characteristic behavior and total quantity of water involved in a drainage basin.

Igneous rocks - rocks that have solidified from molten or partly molten material.

Immobilize - to hold by a strong chemical attraction.

Impaired - condition of the quality of water that has been adversely affected for a specific use by contamination or pollution.

Indurated - cemented, hardened, or a rocklike condition.

Infiltration - the downward movement of water from the atmosphere into soil or porous rock.

Inorganic - containing no carbon; matter other than plant or animal.

Inorganic soil - soil with less than 20 percent organic matter in the upper 16 inches.

Integrated drainage - drainage developed during maturity in an arid region, characterized by coalescence of drainage basins as a result of headward erosion in the lower basins or spilling over from the upper basins.

Interface - in hydrology, the contact zone between two fluids of different chemical or physical makeup.

Intermittent stream - a stream that flows only when it receives water from rainfall runoff or springs, or from some surface source such as melting snow.

Intermontane - situated between or surrounded by mountains, mountain ranges, or mountainous regions.

Internal drainage - surface drainage whereby the water does not reach the ocean, such as drainage toward the lowermost or central part of an interior basin.

Intertidal - alternately flooded and exposed by tides.

Ion - a positively or negatively charged atom or group of atoms.

Irrigation - controlled application of water to arable land to supply requirements of crops not satisfied by rainfall.

Karst - a type of topography that results from dissolution and collapse of carbonate rocks such as limestone, dolomite, and gypsum, and that is characterized by closed depressions or sinkholes, caves, and underground drainage.

Kettle - a steep-sided hole or depression, commonly without surface drainage, formed by the melting of a large detached block of stagnant ice that had been buried in the glacial drift.

Kettle lake - a body of water occupying a kettle, as in a pitted outwash plain or in a kettle moraine.

Lacustrine - pertaining to, produced by, or formed in a lake.

Lacustrine wetlands - wetlands within a lake or reservoir greater than 20 acres or within a lake or reservoir less than 20 acres if the water is greater than 2 meters deep in the deepest part of the basin; ocean-derived salinity is less than 0.5 part per thousand.

Lagoon - a shallow stretch of seawater (or lakewater) near or communicating with the sea (or lake) and partly or completely separated from it by a low, narrow, elongate strip of land.

Latent heat - the amount of heat given up or absorbed when a substance changes from one state to another, such as from a liquid to a solid.

Lateral moraine - a low ridgelike moraine carried on, or deposited near, the side margin of a mountain glacier.

Leachate - a liquid that has percolated through soil containing soluble substances and that contains certain amounts of these substances in solution.

Life zone - major area of plant and animal life; region characterized by particular plants and animals and distinguished by temperature differences.

Limestone - a sedimentary rock consisting chiefly of calcium carbonate, primarily in the form of the mineral calcite.

Limnetic - the deepwater zone (greater than 2 meters deep); a subsystem of the Lacustrine System of the U.S. Fish and Wildlife Service wetland classification system.

Littoral - the shallow-water zone (less than 2 meters deep); a subsystem of the Lacustrine System of the U.S. Fish and Wildlife Service wetland classification system.

Load - material that is moved or carried by streams, reported as weight of material transported during a specified time period, such as tons per year.

Loess - a widespread, homogeneous, commonly nonstratified, porous, friable, slightly coherent, fine-grained blanket deposit of wind-blown and wind-deposited silt and fine sand.

Main stem - the principal trunk of a river or a stream.

Marine wetland - wetlands that are exposed to waves and currents of the open ocean and to water having a salinity greater than 30 parts per thousand; present along the coastlines of the open ocean.

Marsh - a water-saturated, poorly drained area, intermittently or permanently water covered, having aquatic and grasslike vegetation.

Maturity - a stage in the evolutionary erosion of land areas where the flat uplands have been widely dissected by deep river valleys.

Maturity (stream) - the stage in the development of a stream at which it has reached its maximum efficiency, when velocity is just sufficient to carry the sediment delivered to it by tributaries; characterized by a broad, open, flat-floored valley having a moderate gradient and gentle slope.

Mean low tide - the average altitude of all low tides recorded at a given place over a 19-year period.

Mean high tide - the average altitude of all high tides recorded at a given place over a 19-year period.

Mesophyte - any plant growing where moisture and aeration conditions lie between the extremes of "wet" and "dry."

Metamorphic rocks - rocks derived from preexisting rocks by mineralogical, chemical, or structural changes (essentially in a solid state) in response to marked changes in temperature, pressure, shearing stress, and chemical environment at depth in the Earth's crust.

Mineral soil - soil composed predominantly of mineral rather than organic materials; less than 20 percent organic material.

Mitigation - actions taken to avoid, reduce, or compensate for the effects of human-induced environmental damage.

Montane - of, pertaining to, or inhabiting cool upland slopes below the timber line; characterized by the dominance of evergreen trees.

Moraine - a mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift, predominantly till, deposited chiefly by direct action of glacier ice.

Muck - dark, finely divided, well-decomposed, organic matter forming a surface deposit in some poorly drained areas.

Muskeg - large expanses of peatlands or bogs that occur in subarctic zones.

National Geodetic Vertical Datum of 1929 - geodetic datum derived from a general adjustment of first - order level nets of the United States and Canada-formerly called "Sea Level Datum of 1929."

Natural levee - a long, broad, low ridge built by a stream on its flood plain along one or both banks of its channel in time of flood.

Navigable water - in the context of the Clean Water Act, all surface water.

Noncontact water recreation - recreational activities, such as fishing or boating, that do not include direct contact with the water.

Nonpersistent emergent plants - emergent plants whose leaves and stems break down at the end of the growing season from decay or by the physical forces of waves and ice; at certain seasons, there are no visible traces of the plants above the surface of the water.

Nonpoint source - a source (of any water-carried material) from a broad area, rather than from discrete points.

Nuisance species - undesirable plants and animals, commonly exotic species.

Nutrient - any inorganic or organic compound needed to sustain plant life.

Organic - containing carbon, but possibly also containing hydrogen, oxygen, chlorine, nitrogen, and other elements.

Organic soil - soil that contains more than 20 percent organic matter in the upper 16 inches.

Orographic - pertaining to mountains, in regard to their location and distribution; said of the precipitation caused by the lifting of moisture-laden air over mountains.

Overland flow - the flow of rainwater or snowmelt over the land surface toward stream channels.

Oxbow - a bow-shaped lake formed in an abandoned meander of a river.

Palustrine wetlands - freshwater wetlands including open water bodies of less than 20 acres in which water is less than 2 meters deep; includes marshes, wet meadows, fens, playas, potholes, pocosins, bogs, swamps, and shallow ponds; most wetlands are in the Palustrine system.

Pathogen - any living organism that causes disease.

Peat - a highly organic soil, composed of partially decomposed vegetable matter.

Perched ground water - unconfined ground water separated from an underlying main body of ground water by an unsaturated zone.

Percolation - the movement, under hydrostatic pressure, of water through interstices of a rock or soil (except the movement through large openings such as caves).

Periphyton - micro-organisms that coat rocks, plants, and other surfaces on lake bottoms.

Perennial stream - a stream that normally has water in its channel at all times.

Permafrost - any frozen soil, subsoil, surficial deposit, or bedrock in arctic or subarctic regions where below-freezing temperatures have existed continuously from two to tens of thousands of years.

Permeability - the capacity of a rock for transmitting a fluid; a measure of the relative ease with which a porous medium can transmit a liquid.

Pesticide - any substance used to kill plant or animal pests; major categories of pesticides include herbicides and insecticides.

pH - a measure of the acidity (less than 7) or alkalinity (greater than 7) of a solution; a pH of 7 is considered neutral.

Photosynthesis - the synthesis of compounds with the aid of light.

Physiographic province - a region in which the landforms differ significantly from those of adjacent regions.

Physiography - a description of the surface features of the Earth, with an emphasis on the mode or origin.

Pioneer plant - herbaceous annual and perennial seedling plants that colonize bare areas as a first stage in secondary succession.

Piping - erosion by percolating water in a layer of subsoil, resulting in caving and in the formation of narrow conduits, tunnels, or "pipes" through which soluble or granular soil material is removed.

Placer - a surficial mineral deposit formed by mechanical concentration of mineral particles from weathered debris.

Playa - a dry, flat area at the lowest part of an undrained desert basin in which water accumulates and is quickly evaporated; underlain by stratified clay, silt, or sand and commonly by soluble salts; term used in Southwestern United States.

Playa lake - a shallow, temporary lake in an arid or semiarid region, covering or occupying a playa in the wet season but drying up in summer; temporary lake that upon evaporation leaves or forms a playa.

Pocosin - a local term along the Atlantic coastal plain, from Virginia south, for a shrub-scrub wetland located on a relatively flat terrain, often between streams.

Point source - originating at any discrete source.

Population - a collection of individuals of one species or mixed species making up the residents of a prescribed area.

Porosity - the ratio of the volume of voids in a rock or soil to the total volume.

Potential evapotranspiration - the amount of moisture which, if available, would be removed from a given land area by evapotranspiration, expressed in units of water depth.

Prairie pothole - a shallow depression, generally containing wetlands, occurring in an outwash plain, a recessional moraine, or a till plain; usually the result of melted blocks of covered glacial ice; occur most commonly in the North-Central United States and in States west of the Great Lakes from Wisconsin to eastern Montana.

Precipitation - any or all forms of water particles that fall from the atmosphere, such as rain, snow, hail, and sleet. The act or process of producing a solid phase within a liquid medium.

Pristine - the earliest condition of the quality of a water body; unaffected by human activities.

Rain shadow - a dry region on the lee side of a topographic obstacle, usually a mountain range, where rainfall is noticeably less than on the windward side.

Reach - a continuous part of a stream between two specified points.

Reaeration - the replenishment of oxygen in water from which oxygen has been removed.

Recessional moraine - an end moraine built during a temporary but significant pause in the final re treat of a glacier.

Recharge (ground water) - the process involved in the absorption and addition of water to the zone of saturation; also, the amount of water added.

Recharge area (ground water) - an area in which water infiltrates the ground and reaches the zone of saturation.

Recurrence interval - the average interval of time within which the magnitude of a given event, such as a storm or flood, will be equaled or exceeded once.

Regolith - the layer or mantle of fragmented and unconsolidated rock material, residual or transported, that nearly everywhere forms the surface of the land and overlies or covers the bedrock.

Regulation (of a stream) - artificial manipulation of the flow of a stream.

Return flow - that part of irrigation water that is not consumed by evapotranspiration and that returns to its source or another body of water.

Riparian - pertaining to or situated on the bank of a natural body of flowing water.

Riverine wetlands - wetlands within river and stream channels; ocean-derived salinity is less than 0.5 part per thousand.

Runoff - that part of precipitation or snowmelt that appears in streams or surface-water bodies.

Salina - an area where deposits of crystalline salt are formed, such as a salt flat; a body of saline water, such as a saline playa or salt marsh.

Saline water - water that is considered generally unsuitable for human consumption or for irrigation because of its high content of dissolved solids; generally expressed as milligrams per liter (mg/L) of dissolved solids; seawater is generally considered to contain more than 35,000 mg/L of dissolved solids. A general salinity scale is -

Description Dissolved solids,
in milligrams per liter
Slightly 1,000-3,000
Moderately 3,000-10,000
Very 10,000-35,000
Brine More than 35,000

In the U.S. Fish and Wildlife Service wetland classification system, a general term for waters containing various dissolved salts; applied specifically to inland waters where the ratio of salts often vary; the term haline is applied to coastal waters where the salts are roughly in the same proportion as found in sea water.

Salinity - the concentration of dissolved salts in a body of water; commonly expressed as parts per thousand.

Salt flat - the level, salt-encrusted bottom of a dried up lake or pond.

Salt meadow - a meadow subject to overflow by salt water.

Saltwater - water with a high concentration of salt; sometimes used synonymously with seawater or saline water.

Sandstone - a medium-grained sedimentary rock composed of abundant fragments of sand that are more or less firmly united by a cementing material.

Saturated zone - generally the zone within sediment and rock formations where all voids are filled with water under pressure greater than atmospheric.

Savanna - a plain characterized by coarse grasses and scattered tree growth.

Scrub-shrub wetland - wetlands dominated by woody vegetation less than 6 meters tall.

Sea level - the long-term average position of the sea surface; in this volume, it refers to the National Geodetic Vertical Datum of 1929.

Secondary succession - an association of plants that develops after the destruction of all or part of the original plant community.

Sediment - particles derived from rocks or biological materials that have been transported by, suspended in, or deposited by air, water, or ice or that are accumulated by other natural agents, such as chemical precipitation from solution or secretion by organisms.

Sedimentary rocks - rocks resulting from the consolidation of loose sediment that has accumulated in layers.

Sedimentation - the act or process of forming or accumulating sediment in layers; the process of deposition of sediment.

Seep - a small area where water percolates slowly to the land surface.

Seiche - a sudden oscillation of the water in a moderate-size body of water, caused by wind.

Shale - a fine-grained sedimentary rock formed by the consolidation of clay, silt, or mud.

Shallows - a term applied to a shallow place or area in a body of water; a shoal.

Shoal - a relatively shallow place in a stream, lake, or sea.

Shrubland - land covered predominantly with shrubs.

Siltation - the deposition or accumulation of silt (or small-grained material) in a body of water.

Siltstone - an indurated silt having the texture and composition of shale but lacking its fine lamination.

Silviculture - the cultivation of forest trees.

Sinkhole - a depression in an area underlain by limestone. Its drainage is subterranean.

Slough - a small marshy tract lying in a swale or other local shallow undrained depression; a sluggish creek or channel in a wetland.

Soil horizon - a layer of soil that is distinguishable from adjacent layers by characteristic physical and chemical properties.

Soil moisture - water occurring in the pore spaces between the soil particles in the unsaturated zone from which water is discharged by the transpiration of plants or by evaporation from the soil.

Spit - a small point or low tongue or narrow embankment of land having one end attached to the mainland and the other terminating in open water.

Specific conductance - a measure of the ability of a substance to conduct an electrical current.

Spoil - overburden or other waste material removed in mining, quarrying, dredging, or excavating.

Spring - area where there is a concentrated discharge of ground water that flows at the ground surface.

Stage - height of the water surface above an established datum plane, such as in a river above a predetermined point that may (or may not) be near the channel floor.

Storm surge - an abnormal and sudden rise of the sea along a shore as a result of the winds of a storm.

Streamflow - the discharge of water in a natural channel.

Submersed plant - a plant which lies entirely beneath the water surface, except for flowering parts in some species.

Subsidence - the gradual downward settling or sinking of the Earth's surface with little or no horizontal motion.

Substrate - the surface beneath a wetland in which organisms grow or to which organisms are attached.

Subtidal - continuously submerged; an area affected by ocean tides.

Surface runoff - runoff which travels over the land surface to the nearest stream channel.

Surface water - an open body of water such as a lake, river, or stream.

Suspended sediment - sediment that is transported in suspension by a stream.

Swale - a slight depression, sometimes filled with water, in the midst of generally level land.

Swamp - an area intermittently or permanently covered with water, and having trees and shrubs.

Tarn - a relatively small and deep, steep-sided lake or pool occupying an ice-gouged basin amid glaciated mountains.

Tectonic activity - movement of the Earth's crust resulting in the formation of ocean basins, continents, plateaus, and mountain ranges.

Terrestrial - pertaining to, consisting of, or representing the Earth.

Terminal moraine - the end moraine extending across a glacial plain or valley as an arcuate or crescent ridge that marks the farthest advance or maximum extent of a glacier.

Terrain - physical features of a tract of land.

Terrane - area or surface over which a particular rock type or group of rock types is prevalent.

Thermokarst - an irregular land surface formed in a permafrost region by melting ground ice and a subsequent settling of the ground.

Tidal flat - an extensive, nearly horizontal, tract of land that is alternately covered and uncovered by the tide and consists of unconsolidated sediment.

Tide - the rhythmic, alternate rise and fall of the surface (or water level) of the ocean, and connected bodies of water, occurring twice a day over most of the Earth, resulting from the gravitational attraction of the Moon, and to a lesser degree, the Sun.

Till - predominantly unsorted and unstratified drift, deposited directly by and underneath a glacier without subsequent reworking by meltwater, and consisting of a heterogeneous mixture of clay, silt, sand, gravel, and boulders.

Tinaja - a pocket of water developed below a waterfall; a term used in the Southwestern United States; used loosely to mean a temporary pool.

Topography - the general configuration of a land surface or any part of the Earth's surface, including its relief and the position of its natural and man-made features.

Trace element - a chemical element that occurs in minute quantities in a substance.

Trade winds - a system of easterly winds that dominate most of the tropics. A major component of the general circulation of the atmosphere.

Transpiration - the process by which water passes through living organisms, primarily plants, into the atmosphere.

Tundra - a vast, nearly level, treeless plain of the arctic and subarctic regions. It usually has a marshy surface which supports mosses, lichens, and low shrubs, underlain by mucky soils and permafrost.

Turbidity - the state, condition, or quality of opaqueness or reduced clarity of a fluid due to the presence of suspended matter.

Unconfined aquifer - an aquifer whose upper surface is a water table free to fluctuate under atmospheric pressure.

Understory - a foliage layer lying beneath and shaded by the main canopy of a forest.

Unsaturated zone - a subsurface zone above the water table where the pore spaces may contain a combination of air and water.

Upland - a general term for nonwetland; elevated land above low areas along streams or between hills; any elevated region from which rivers gather drainage.

Vascular plant - a plant composed of or provided with vessels or ducts that convey water or sap. A fern is an example of this type of plant.

Vernal pool - a small lake or pond that is filled with water for only a short time during the spring.

Water budget - an accounting of the inflow to, outflow from, and storage changes of water in a hydrologic unit.

Water column - an imaginary column extending through a water body from its floor to its surface.

Water gap - a deep, narrow pass in a mountain ridge, through which a stream flows.

Watershed - same as drainage basin.

Water table - the top water surface of an unconfined aquifer at atmospheric pressure.

Weathering - process whereby earthy or rocky materials are changed in color, texture, composition, or form (with little or no transportation) by exposure to atmospheric agents.

Wetland function - a process or series of processes that take place within a wetland that are beneficial to the wetland itself, the surrounding ecosystems, and people.

Willow carr - a pool, or wetland dominated by willow trees or shrubs.

Xerophyte - a plant adapted for growth under dry conditions.

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