F (?) (Self-purification Factor) The self-purification factor is an indication of the ability of a stream to assimilate a waste discharge.  It is defined as the ratio of the re-aeration (r) and the rate of deoxygenation (k), or ? = r/k, where ? is called the self-purification factor.
Face (of a Dam) The external surface of a structure, such as the surface of an appurtenance or a dam.
Facilities Plans Plans and studies related to the construction of water treatment works necessary to comply with the Clean Water Act (CWA).  A facilities plan investigates needs and provides information on the cost effectiveness of alternatives, a recommended plan, an Environmental Assessment (EA) of the recommendation, and descriptions of the treatment works, costs, and a completion schedule.
Facultative Bacteria Bacteria that can live under Aerobic or Anaerobic conditions.
Facultative Phreatophyte Plants that utilize moisture from groundwater for a portion of their water requirements.
Fahrenheit (F) (1) A unit of temperature.  (2) Of or relating to a temperature scale that registers the freezing point of water as 32?F and the boiling point as 212?F at one atmosphere of pressure.  See Fahrenheit Temperature Scale.
Fahrenheit Temperature Scale A thermometric scale on which the freezing point of water is at 32?F (Fahrenheit) above the 0?(F) mark on the scale, and the boiling point of water is at 212?F.  The Fahrenheit temperature scale was designed by German physicist Daniel Fahrenheit and is commonly used in the United States.  Contrast with the Centigrade Temperature Scale, using degrees Celsius (?C), in which 0?(C) marks the freezing point of water and 100?C indicates the boiling point of water (at sea level).  The formula for converting a Fahrenheit temperature to Celsius is C?=5/9 x (F? - 32).
Failure The collapse or slippage of a large mass of bank material into a stream.
Fairfield-Hardy Digester (Water Quality) A machine that decomposes garbage, sewage sludge, industrial and other organic wastes by a controlled continuous Aerobic-Thermophilic Process.
Fallon National Wildlife Refuge (NWR) (Nevada) One of the nine National Wildlife Refuges (NWR) located in the State of Nevada, the Fallon NWR was established in 1931 and encompasses approximately 17,900 acres (28 square miles) where the Carson River terminates in the Carson Sink and is situated within the northwest portion of the Stillwater Wildlife Management Area near the town of Fallon in Churchill County, Nevada.  Due to typically limited and uncertain flows of the Carson River at its terminus, generally not enough water enters this refuge to maintain it as a viable wetland area.  The Fallon NWR is currently managed by the U.S. Fish and Wildlife Service (USFWS) along with the Stillwater National Wildlife Refuge and is included as part of the Stillwater Wildlife Management Area.  Also see National Wildlife Refuge (NWR) System and National Wildlife Refuges (NWR) (Nevada).
Fall Overturn A physical phenomenon that may take place in a body of water during early autumn.  The sequence of events leading to fall overturn include:
(1)  The cooling of surface waters;
(2)  A density change in surface waters producing convection currents from top to bottom;
(3)  The circulation of the total water volume by wind action; and
(4)  Eventual vertical temperature equality.
The overturn results in a uniformity of the physical and chemical properties of the entire water body.  Also referred to as Fall Turnover.  Also see Spring Overturn.
Falling Limb The portion of the Hydrograph trace immediately following the peak and reflecting the decreasing production of storm flow.
Fallow (1) Allowing cropland, either tilled or untilled, to lie idle during the whole or greater portion of the growing season.  (2) Land plowed and tilled and left unplanted.
Falls A waterfall or other precipitous descent of water.
Fan A generic term for constructional landforms that are built of more-or-less stratified alluvium and that occur on the piedmont slope, downslope from their source of alluvium.
Fan Apron A component landform comprised of a sheet-like mantle of relatively young alluvium covering part of an older fan piedmont (and occasionally alluvial fan) surface.  It somewhere buries a pedogenic soil which can be traced to the edge of the fan apron where the soil emerges as the land surface, or relict soil.  No buried soils shold occur within a fan-apron mantle, rather, they separate mantles.
Fan Collar A component landform comprised of a thin, short, relatively young mantle of alluvium along the very upper margin of a major alluvial fan at a mountain front.  The mantle somewhere buries a pedogenic soil that can be traced to the edge of the fan collar where it emerges as the land surface, or relict soil.
Fan-Head Trench A relatively deep drainageway originating in a mountain valley and cut into the apex of, and commonly across an alluvial fan.  It may empty into an interfan-valley drainage, debouch onto the fan piedmont, or cross the fan piedmont.
Fan Piedmont The most extensive major landform of most piedmont slopes, formed by the lateral coalescence of mountain-front alluvial fans downslope into one generally smooth slope without the transverse undulations of the semi-conical alluvial fans and by accretion of fan aprons.  Fan piedmonts commonly are complexes of many component landforms.
Fan Remnant A generic term for component landforms that are the remaining parts of various older fan landforms that either have been dissected (erosional fan remnants) or partially buried (non-buried fan remnants).  Erosional fan remnants must have a flattish summit of relict fan surface; non-buried fan remnants are all relict fan surfaces.  Fan remnants may be specifically identified as fan-piedmont remnants, inset-fan remnants, etc.
Fan-Remnant Sideslope A landform element comprised of the relatively young erosional slope around the sides of an erosional fan remnant.  It is composed of should, backslope, and footslope slope components.
Fan Skirt A major landform comprised of laterally coalescing, small alluvial fans that issue from gullies cut into, or are extensions of inset fans of the fan piedmont and that merge along their toeslopes with the basin floor.  Fan skirts are smooth or only slightly dissected and ordinarily do not comprise component landforms.
Fanglomerate Heterogeneous materials that were originally deposited in an Alluvial Fan but since deposition have been cemented into solid rock.
Fanlette A very small, normally undissected alluvial fan, something less than a few tenths of a square mile in area that may occur below a gully, inset fan, or ravine in a variety of positions on the piedmont slope or within mountain valleys.
Farm Delivery Requirement The Crop Irrigation Requirement plus farm losses due to evaporation, deep percolation, surface waste, and nonproductive consumption.  The losses are measured by the Farm Irrigation Efficiency, which is the percent of farm-delivered water that remains in the root zone and is available for crop growth.
Farm Efficiency The consumptive Crop Irrigation Requirement (CIR) divided by the farm water delivery.
Farm Headgate Delivery (Diversion) That amount of water in acre feet (AF) delivered through a farm headgate.
Farm Irrigation Efficiency An expression comparing the amount of water actually required for growing a crop to the amount of irrigation water that is diverted at the farm headgate.  Expressed as a percentage on an annual basis.
Farm Pond A water impoundment made by constructing a dam or embankment or by excavating a pit or "dug out".
Farm Surface Runoff (Tailwater) A portion of the Farm Headgate Delivery that flows off the lower portion of the farm or field surface (drain ditch) flow.  This is one loss component considered in Farm Irrigation Efficiency.
Farm Waste and Deep Percolation The amount of irrigation water delivered to the crop area from a canal turnout or ground water pump that is not consumptively used on the crop area.  Includes water moving through the root zone to the water table, water intercepted by drainage systems, and surface waste to natural or constructed drainage systems, and non-cropped areas.
Farmland, Prime As defined in the Farmland Protection Policy Act of 1981:  Land that has the best combination of physical and chemical characteristics for producing food, feed, forage, fiber, and oilseed crops, and is available for these uses (urban areas are not included).  It has the soil quality, growing season, and moisture supply needed for the economic production of sustained high yields of crops when treated and managed (including water management) according to acceptable farming methods.  Prime farmland includes land that is being used currently to produce livestock and timber, but it excludes land committed to urban development or water storage.
Fata Morgana See Mirage.
Fathom (1) A unit of length equal to 6 feet (1.83 meters), used principally in the measurement and specification of marine depths.  (2) To measure the depth of a body of water as with a Lead Line.
Faucet A device for regulating the flow of a liquid from a reservoir such as a pipe or drum.
Fault (Geology) A fracture in rock along which movement can be demonstrated.  A fracture in the earth's crust forming a boundary between rock masses that have shifted.  Faults may be classified as follows:
(1)  Active Fault - A fault that has moved recently and which is likely to move again, usually defined as one that has shown movement within the last 11,000 years and can be expected to move again within the next 100 years;
(2)  Potentially Active Fault - A fault that moved within the Quaternary Period (i.e., within the last 2 million years) or a fault which, because it is judged to be capable of ground rupture or shaking, poses an unacceptable risk for a proposed project or structure;
(3)  Historically Active Fault - A fault active within the last 200 years;
(4)  Inactive Fault - A fault which has shown no evidence of movement in recent geologic time and no potential for movement in the relatively near future.
Fault Creep A very slow movement along a fault which is unaccompanied by perceptible earthquakes.
Fault Escarpment (Geology) A fracture or fracture zone along which there has been displacement of one side with respect to the other.
Fault-Line Scarp A steep slope produced along an old fault line by differential weathering and erosion, rather than by fault movement.
Fault, Rupture A break in the ground along the fault line during an earthquake.
Fault Sag Ponds A small, enclosed depression along an active or recent fault.  It is caused by differential movement between slices and blocks within the fault zone or by warping and tilting associated with differential displacement along the fault, and it forms the site of a sag pond.
Fault Scarp A cliff formed by a fault, usually modified by erosion unless the fault is very recent.
Fault Trace The intersection of a fault and the earth's surface as often revealed by dislocation of fences and roads and/or by ridges and furrows in the ground.
Fauna (1) A term used to describe the animal species of a specific region or time.  (2) All animal life associated with a given habitat, country, area, or period.
Feasibility Study (FS) (1) A complete assessment of alternative courses of action to solve one or more problems, to meet needs, and to recommend the most practical course of action consistent with state and local planning objectives.  (2) (Environmental) Analysis of the practicability of a proposal, e.g., a description and analysis of potential cleanup alternatives for a site such as one on the National Priorities List (NPL).  The feasibility study usually recommends selection of a cost-effective alternative.  It usually starts as soon as the Remedial Investigation (RI) is underway; together, they are commonly referred to as the "RI/FS".
Fecal Bacteria Any type of bacteria whose normal habitat is the colon of warm-blooded mammals, such as man.  These organisms are usually divided into groups, such as Fecal Coliform or Fecal Streptococci (Streptococcus).
Fecal Coliform Bacteria A group of bacteria normally present in large numbers in the intestinal tracts of humans and other warm-blooded animals.  Specifically, the group includes all of the rod-shaped bacteria that are non-sporeforming, Gram-Negative, lactose-fermenting in 24 hours at 44.5?C, and which can grow with or without oxygen.  In the laboratory, they are defined as all organisms that produce produce blue colonies with specified time frames.  The presence of this type of bacteria in water, beverages, or food is usually taken to indicate that the material is contaminated with solid human waste.  Bacteria included in this classification represent a subgroup of the larger group termed Coliform.  Their concentrations are expressed as number of colonies per 100 mL of sample.
Fecal Material (Water Quality) Solid waste produced by humans and other animals and discharged from the gastrointestinal tract.  Also referred to as feces or solid excrement, it is a component of domestic sewage and must be treated to avoid the transmission of fecal bacteria and other organisms or disease.
Fecal Streptococcal Bacteria Bacteria found in the intestine of warm-blooded animals.  Their presence in water is considered to verify fecal pollution.  They are characterized as gram-positive, cocci bacteria that are capable of growth in brain-heart broth.  In the laboratory, they are defined as all organisms that produce red or pink colonies with 48 hours at 35 degrees centigrade plus or minus 1 degree centigrade on KF-streptococcus medium (nutrient medium for bacterial growth).  Their concentrations are expressed as number of colonies per 100 mL of sample.  Also see Fecal Streptococcus.
Fecal Streptococcus A group of bacteria normally present in large numbers in the intestinal tracts of warm-blooded animals other than humans.  By assessing the ratio of coliforms to streptococci in a water sample, a rough estimate can be made of the relative contribution of fecal contamination from the two mentioned possible sources.
Federal Emergency Management Agency (FEMA) An independent agency of the federal government founded in 1979 and reporting to the President of the United States and headquartered in Washington D.C.  FEMA's mission is to reduce loss of life and property and protect our nation's critical infrastructur
(1)  Region I (Boston, Massachusetts) - Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island, Vermont;
(2)  Region II (New York, N.Y., Caribbean Division - San Juan, Puerto Rico) - New York, New Jersey, the Commonwealth of Puerto Rico and the Territory of the U.S. Virgin Islands;
(3)  Region III (Philadelphia, Pennsylvania) - District of Columbia, Delaware, Maryland, Pennsylvania, Virginia, West Virginia;
(4)  Region IV (Atlanta, Georgia) - Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina and Tennessee;
(5)  Region V (Chicago, Illinois) - Illinois, Indiana, Michigan, Minnesota, Ohio, Wisconsin;
(6)  Region VI (Denton, Texas) - Arkansas, Louisiana, New Mexico, Oklahoma and Texas;
(7)  Region VII (Kansas City, Missouri) - Iowa, Kansas, Missouri, Nebraska;
(8)  Region VIII (Denver, Colorado) - Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming;
(9)  Region IX (San Francisco, California) - Arizona, California, Hawaii and Nevada; and the Territory of American Samoa, the Territory of Guam, the Commonwealth of the Northern Mariana Islands, the Republic of the Marshall Islands, the Federated States of Micronesia, and the Republic of Palau;
(10)  Region X (Bothell, Washington) - Alaska, Idaho, Oregon and Washington.
Federal Insurance Administration (FIA) A component of the Federal Emergency Management Agency (FEMA) directly responsible for administering the flood insurance aspects of the National Flood Insurance Program (NFIP).
Federal Power Act An act of Congress creating a federal licensing system administered by the Federal Energy Regulatory Commission (FERC) and requiring that a license be obtained for nonfederal hydroelectric projects proposing to use Navigable waters or federal lands.  The
Federal Reserved Water Rights A category of federal water rights, created by federal law.  These rights are created when the federal government withdraws land from the public domain to establish a federal reservation such as a national park, forest, or Indian reservation.  By this action, the government is held to have reserved water rights sufficient for the primary purpose for which the land was withdrawn.  Also see Winters Rights (Decision), Reservation Doctrine, Reserved Rights Doctrine, and Winters Doctrine, and Water Law (Federal).
Federal Water Pollution Control Act (Public Law 92-500) More commonly referred to as the Clean Water Act (CWA), constitutes the basic federal water pollution control statute for the United States.  Originally based on the Water Quality Act of 1965 which began setting water quality standards.  The 1966 amendments to this act increased federal government funding for sewage treatment plants.  Additional 1972 amendments established a goal of zero toxic discharges and "fishable" and "swimmable" surface waters.  Enforceable provisions of the CWA include technology-based effluent standards for point sources of pollution, a state-run control program for nonpoint pollution sources, a construction grants program to build or upgrade municipal sewage treatment plants, a regulatory system for spills of oil and other hazardous wastes, and a wetlands preservation program.
Feedlot A confined area for the controlled feeding of animals.  Tends to concentrate large amounts of animal waste that cannot be absorbed by the soil and, therefore, may be carried to nearby streams or lakes by rainfall runoff.
Feedwater (Water Quality) Water input into a desalting or water treatment plant.
Feet Per Second (ft./sec.) A measure of the velocity of moving water.
FEMA See Federal Emergency Management Agency (FEMA).
Fen Low land covered wholly or partly with water; a Moor or Marsh.  A type of Wetland that accumulates peat deposits.  Fens are less acidic than Bogs, deriving most of their water from groundwater rich in calcium and magnesium.  Also see Calcareous Fens.
Fermentation, Anaerobic (Water Quality) The process in which carbohydrates are converted in the absence of oxygen to hydrocarbons (such as methane gas).
Ferrous Sulfate A greenish crystalline compound, FeSO4·H2O, used as a pigment, fertilizer, and feed additive, in sewage and water treatment, and as a medicine in the treatment of iron deficiency.  Also called Copperas.
Fertigation The use of irrigation water as a vehicle for spreading fertilizer on the land.
Fertilizer Any organic or inorganic material of natural or synthetic origin that is added to a soil to supply elements essential to plant growth.  Various types of fertilizers include acid-forming, blended, bulk-blended, chemical, coated, conditioned, granular, liquid, non-granular, prilled, solution, straight, and suspension.
Fetch (1) The distance traveled by waves in open water, from their point of origin to the point where they break.  (2) The distance the wind blows over water or another homogeneous surface without appreciable change in direction.
Field (1) A broad, level, open expanse of land; a meadow.  (2) A cultivated expanse of land, especially one devoted to a particular crop.  (3) A portion of land or a geologic formation containing a specified natural resource.  (4) A wide, unbroken expanse, as of ice.
Field (Moisture) Capacity The capacity of soil to hold water.  It is measured by the soil scientist as the ratio of the weight of water retained by the soil to the weight of the dry soil.
Field Diversion An interception channel near the contour to carry runoff to a waterway.  Intervals vary with the precipitation, slope, and cropping.
Field-Moisture Capacity The quantity of water which can be permanently retained in the soil in opposition to the downward pull of gravity.
Field-Moisture Deficiency The quantity of water which would be required to restore the soil moisture to Field-Moisture Capacity.
Field Permeability Permeability corresponding to the temperature which occurs under field conditions.
Field Sprinkler System A system of closed conduits carrying irrigation water under pressure to orifices designed to distribute the water over a given area.
Filamentous Algae Aggregations of one-celled plants that grow in long strings or mats in water and are either attached or free floating and tend to plug canals, weirs, and other structures, but also provide habitat of invertebrate animals.
Fill (Geology) Any sediment deposited by any agent such as water so as to fill or partly fill a channel, valley, sink, or other depression.
Fill Material Soil that is placed at a specified location to bring the ground surface up to a desired elevation or angle of slope.
Filling Depositing dirt, mud or other materials into aquatic areas to create more dry land, usually for agricultural or commercial development purposes, and frequently with ruinous ecological consequences.  Also see Wetland Banking, Wetland "Clumping" (Aggregation), and Wetland Mitigation.
Filter A device used to remove solids from a mixture or to separate materials.  A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particular matter.  Suspended materials are frequently separated from water using filters.
Filter Bed A layer of sand or gravel on the bottom of a reservoir or tank, used to filter water or sewage.
Filter Cake (1) The solids or semisolids deposited on a filter as a fluid is moved through it.  (2) The remaining solids or semisolids on a filter after the fluid in a material is extracted by a negative pressure.
Filter Fabric A polypropylene textile used to keep soil separate from water.  Comes in many different forms and is used for construction roads, lining ponds, and in many erosion control projects.
Filter Feeder An aquatic animal, such as a clam, barnacle, or sponge, that feeds by filtering particulate organic material from water.
Filter Strip A strip or area of vegetation used for removing sediment, organic matter, and other pollutants from runoff and waste water.
Filter Zone (of a Dam) A band or zone of granular material that is incorporated into a dam and is graded (either naturally or by selection) so as to allow seepage to flow across or down the filter without causing the migration of material from zones adjacent to the filter zone.
Filterable Of particles that are sufficiently small to allow their passage through filters capable of retaining most particles.  For example, a filterable virus is one that will pass through a filter that will normally retain bacteria.
Filtrate Liquid that has been passed through a filter.
Filtration (1) The process in which suspended matter is removed from a liquid through a medium which is permeable to the liquid but not to the suspended material.  (2) (Water Quality) A treatment process, under the control of qualified operators, for removing solid (particulate) matter from water by means of porous media such as sand or a man-made filter; often used to remove particles that contain Pathogens.
Final Clarifier (Water Quality) A gravitational settling tank installed as part of some wastewater treatment plants and placed after the biological treatment step.  The tank functions to remove suspended solids.  Also referred to as Secondary Clarifier.
Finding of No Significant Impact (FONSI) A document prepared by a federal agency showing why a proposed action would not have a significant impact on the environment and thus would not require the preparation of an Environmental Impact Statement (EIS).  A FONSI is based on the results of an Environmental Assessment (EA).
Finished Water (Water Quality) Water that has completed a purification or treatment process; water that has passed through all the processes in a water treatment plant and is ready to be delivered to consumers.  Contrast with Raw Water.
Firm Capacity For public drinking water supplies, the system delivery capacity with the largest single water well or production unit out of service.
FIRM Map Flood insurance rate map used to establish the insurance rates for structures under the National Flood Insurance Program.
Firm Yield (1) The maximum annual supply of a given water development that is expected to be available on demand, with the understanding that lower yields will occur in accordance with a predetermined schedule or probability.  (2) The dependable annual water supply that could be available in all years, without exceeding specified shortages in agricultural deliveries during droughts.  Sometimes referred to as Dependable Yield.
Firn (Firn Snow) Old snow on the top of glaciers that has become granular and compact through temperature changes, forming the transition stage to glacial ice.  Also referred to as Neve.
Firn Line The highest level to which the fresh snow on a glacier's surface retreats during the melting season; the line separating the accumulation area from the ablation area.
First Draw The water that comes out when the tap is first opened, likely to contain the highest level of lead contamination from plumbing fixtures and materials.
First in Time, First in Right A phrase indicating that older water rights have priority over more recent rights if there is not enough water to satisfy all rights.  See (Prior) Appropriation Doctrine and Appropriative Water Rights.
Firth A narrow inlet or arm of the sea; an Estuary.
(United States) Fish and Wildlife Service (USFWS) Part of the U.S. Department of the Interior, the early beginnings of the Fish and Wildlife Service go back to 1871 when the federal government established the Commissioner of Fisheries.  In 1896, the Division of Biological Survey was established within th
(1)  Acquires, protects and manages unique ecosystems necessary to sustain fish and wildlife, such as migratory birds and endangered species;
(2)  As specified in the Endangered Species Act (ESA) (1973), as amended, and in conjunction with the National Marine Fisheries Service (NMFS), determines critical habitat and develops recovery plans for protected endangered and threatened species of plants and animals;
(3)  Operates fish hatcheries to support research, develop new techniques and fulfill the public demand for recreational fishing;
(4)  Operates wildlife refuges to provide, restore, and manage a national network of lands and waters sufficient in size, diversity and location to meet society's needs for areas where the widest possible spectrum of benefits associated with wildlife and wildlands is enhanced and made available;
(5)  Conducts fundamental research on fish, wildlife and their habitats to provide better management and produce healthier and more vigorous animals; also protects fish and wildlife from dislocation or destruction of their habitats;
(6)  Renders financial and professional assistance to states, through federal aid programs, for the enhancement and restoration of fish and wildlife resources;
(7)  Establishes and enforces regulations for the protection of migratory birds, marine mammals, fish and other non-endangered wildlife from illegal taking, transportation or sale within the United States or from foreign countries; and
(8)  Communicates information essential for public awareness and understanding of the importance of fish and wildlife resources, and changes reflecting environmental degradation that ultimately will affect the welfare of human beings.
Also see National Wildlife Refuge System, Endangered Species Act (ESA), Endangered Species, Threaten Species, and National Oceanic and Atmospheric Administration (NOAA).
Fish Credit Water Generally, water reserved in upstream reservoirs for release for downstream fisheries purposes.  Often provisions will be made such that other forms of water credits, e.g., Drought Reserve Water, will convert to fish credit water if snowpack water content or runoff is deemed sufficient by a stipulated date.
Fishing Waters Waters used for angling or for commercial fishing.
Fish Ladder (1) A series of small pools arranged in an ascending fashion to allow the migration of fish upstream past construction obstacles, such as dams.  (2) An inclined trough which carries water from above to below a dam so that fish can easily swim upstream.  There are various types, some with baffles to reduce the velocity of the water and some consisting of a series of boxes with water spilling down from one box to the next.  Also see Fishway.
Fishpond A small body of water managed for fish.
Fish Screen A porous barrier placed across the inlet our outlet of a pond to prevent the passage of fish.
Fishway A passageway designed to enable fish to ascend a dam, cataract, or velocity barrier.  Also referred to as a Fish Ladder.
Fissure A surface of a fracture or crack in a rock along which there is a distinct separation.
Five-Hundred Year Flood (500-Year Flood) The flood that a 0.2 percent chance of being equaled or exceeded in any year.  Areas subject to the 500-year flood have a moderate to low risk of flooding.  Includes flood Zones B, C, and X.  One- to four-unit buildings in these zones are eligible for Preferred Risk Policies.  A 500-year flood would be deeper than a 2100-year flood and would cover a greater area.
Fix a Sample A sample is "fixed" in the field by adding chemicals that prevent water quality indicators of interest in the sample from changing before laboratory measurements are made.
Fixed Ground Water Water held in saturated material within pore spaces so small that it is permanently attached to the walls, or moves so slowly that it is usually not available as a source of water for pumping.
Fjord, or Fiord A long, narrow, deep inlet of the sea between steep slopes.
Flap Valve See Check Valve.
Flash To fill suddenly with water.
Flashboard A temporary barrier, relatively low in height and usually constructed of wood, placed along the crest of the spillway of a dam to allow the water surface in the reservoir to be raised above spillway level in order to increase the storage capacity.  It is designed to be readily removed, lowered or carried away by high flow or floods.
Flash Flood, also Flashflood A sudden flood of great volume, usually caused by a heavy rain.  Also, a flood that crests in a short length of time (hours or minutes) and is often characterized by high velocity flows.  It is often the result of heavy rainfall in a localized area.
Flat A level landform composed of Unconsolidated Sediments usually mud or sand.  Flats may be irregularly shaped or elongate and continuous with the shore, whereas bars are generally elongate, parallel to the shore, and separated from the shore by water.
Flatboat A boat with a flat bottom and square ends used for transportation of bulky freight, especially used in shallow waters.
Flat-Water Of or on a level or slow-moving watercourse.
Float (1) To remain suspended within or on the surface of a fluid without sinking.  To cause to remain suspended without sinking or falling.  (2) To put into water; launch.  (3) To flood (land), as for irrigation.
Floater A Wetland plant that floats on the surface of the water.
Floating Dock (1) A structure that can be submerged to permit the entry and docking of a ship and then raised to lift the ship from the water for repairs.  Also referred to as a Floating Drydock.  (2) A dock that is supported by metal pipes on which it can move up and down with the rise and fall of the water level.
Floating Plant A non-anchored plant that floats freely in the water or on the surface; e.g., water hyacinth (Eichhornia crassipes) or common duckweed (Lemna minor).
Floating-Leaved Plant A rooted, Herbaceous Hydrophyte with some leaves floating on the water surface; e.g., white water lily (Nymphae odorata), floating-leaved pondweed (Potamogeton natans).  Plants such as yellow water lily (Nuphar luteum), which sometimes have leaves raised above the surface, are considered floating-leaved plants or emergents, depending on their growth habit at a particular site.
Floc Generally, a very fine, fluffy mass formed by the aggregation of fine suspended particles, as in a precipitate.  In terms of water quality, clumped solids or precipitates formed in sewage by biological or chemical activity.
Flocculate To aggregate or clump together individual, tiny particles into small clumps or clusters.
Flocculation (Water Quality) In water and wastewater treatment, the agglomeration or clustering of colloidal and finely divided suspended matter after coagulation by gentle stirring by either mechanical or hydraulic means such that they can be separated from water or sewage.
Floe An ice flow.  Also a segment that has separated from such an ice mass.
Floe Ice Ice usually several feet thick, which has formed on the surface of a body of water and then has broken into pieces and is floating on the water's surface.
(The) Flood (Biblical) The universal deluge recorded in the Old Testament as having occurred during the life of Noah.
Flood, or Flood Waters (1) Temporary inundation of normally dry land areas from the overflow of inland or tidal waters, or from the unusual and rapid accumulation or runoff of surface waters from any source.  The rise in water may be caused by excessive rainfall, snowmelt, natu
Flood (FEMA) A general and temporary condition of partial or complete inundation or normally dry land areas from:  (1) overflow of inland or tidal waters; (2) the unusual and rapid accumulation or runoff of surface waters from any source; (3) mudslides (i.e., mudflows) which are proximately caused by flood, as defined above, and are akin to a river of liquid and flowing mud on the surface of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current; and (4) the collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding the cyclical levels which result in flood, as defined above.
Flood, 100-Year A 100-year flood does not refer to a flood that occurs once every 100 years, but to a flood level with a 1 percent or greater chance of being equaled or exceeded in any given year.  Areas below the 100-year flood level are termed Special Flood Hazard Areas.  Areas between the 100-year and the 500-year flood boundaries are termed Moderate Flood Hazard Areas.  The remaining areas are above the 500-year flood level and are termed Minimal Flood Hazard Areas.
Flood, Annual The highest peak discharge in a water year.
Flood Abatement See Flood Control.
Flood-Base Discharge A value of high flow usually computed during the first 5 years of station operation that, on the average, is exceeded about three times per year.
Flood Boundary and Floodway Map (FBFM) Official map of a community on which the Federal Emergency Management Agency (FEMA) or Federal Insurance Administration (FIA) have delineated the boundaries of the flood, mudslide and related erosion areas having special hazards have been designated as Flood Zones A, M, and E.  Now superseded by the Floodway Hazard Boundary Map (FHBM).
Flood Capacity The flow carried by a stream or floodway at bankfull water level.  Also, the storage capacity of the flood pool at a reservoir.
Flood Control (Storage) The control of flood waters by the construction of flood storage reservoirs, flood water retaining structures, channel improvements, levees, bypass channels, other engineering works, or vegetative changes.
Flood Control Districts A district organized to manage flooding hazards through land use controls and construction and maintenance of flood control structures.
Flood Control Pool (1) Reservoir volume reserved for flood runoff and then evacuated as soon as possible to keep that volume in readiness for the next flood.  (2) Reservoir volume above the active conservation and joint-use pool that is reserved for flood runoff and then released as soon as possible to keep that space available for the next flood.
Flood Control Storage Storage above the active storage and joint-use storage that is reserved for flood releases.
Flood Crest The maximum stage or elevation reached or expected to be reached by the waters of a flood at a given location.
Flood Damage The direct and indirect economic loss caused by floods including damage by inundation, erosion, or sediment deposition.  Indirect damages may also include emergency costs and business or financial losses.  Evaluation may be based on the cost of replacing, repairing, or rehabilitating; or the comparative change in market or sales value; or on the change in income or production caused by flooding.
Flood Depth Height of flood waters above the surface of the ground at a given point.
Flood Duration (1) The length of time a stream is above flood stage or overflowing its banks.  (2) The amount of time between the initial rise of flood, including freeboard, waters and their recession.
Flood Duration Curve A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.
Flood Elevation The height of flood waters above an Elevation Datum Plane.
Flood Fighting Actions taken immediately before or during a flood to protect human life and to reduce flood damages such as evacuation, emergency sandbagging and diking, and provision of assistance to flood victims.
Flood Forecasting The process of predicting the occurrence, magnitude and duration of an imminent flood through meteorological and hydrological observations and analysis.  Flood forecasts are primarily the responsibility of the National Weather Service, National Oceanic and Atmospheric Administration (NOAA), and are used to predict flood stages and times and indicate areas subject to flooding.
Flood Frequency (1) Probability, expressed as a percentage, that a flood of a given size will be equaled or exceeded in any given year.  (2) A statistical expression or measure of the average time period between floods equaling or exceeding a given magnitude.  For example, a 100-year flood has a magnitude expected to be equaled or exceeded on the average of once every hundred years; such a flood has a one-percent chance of being equaled or exceeded in any given year.  Similarly, the floods that have a 2-percent probability (1 in 50) and a 0.2-percent probability (1 in 500) of being equaled or exceeded in any year are referred to as the 50-year flood and the 500-year flood, respectively.  The term is oftentimes used interchangeably with Recurrence Interval.  Also see Hundred-Year Flood, One-Percent Duration Flood, X-Year Flood, and X-Year Flood, Y-Duration Rain.
Flood Frequency Curve (1) A graph showing the average interval of time within which a flood of a given magnitude will be equaled or exceeded once.  (2) A similar graph but plotted with the Recurrence Intervals of floods plotted instead.
Flood Fringe The portion of the floodplain outside of the floodway but still subject to flooding.  Sometimes referred to as Floodway Fringe.
Floodgate (1) A gate used to control the flow of a body of water.  Also referred to as a Water Gate.  (2) Something that restrains a flood or an outpouring.
Flood Hazard Boundary Map (FHBM) An official map of a community, issued or approved by the Federal Emergency Management Agency (FEMA) or Federal Insurance Administration (FIA), on which the boundaries of the floodplain and special flood hazard areas have been designated.  This map is prepared according to the best flood data available at the time of its preparation, and is superseded by te Flood Insurance Rate Map (FIRM) after more detailed studies have been completed.
Flood Hazard Zones (Defined) Zones on the Flood Insurance Rate Map (FIRM) in which the risk premium insurance rates have been established by a Flood Insurance Study (FIS).  The following flood hazard zone designations apply:
(1)  Flood Zone V - Areas along coasts subject o inundation by the 100-year flood event with additional hazards associated with storm-induced waves.  Mandatory flood insurance purchase requirements apply;
(2)  Flood Zones EV and V1-V30 - Areas along coasts subject to inundation by the 100-year flood event with additional hazards due to storm-induced velocity wave action.  Mandatory flood insurance purchase requirements apply.  (Zone VE is used on new and revised flood maps in place of Zones V1-V30.);
(3)  Flood Zone A - Areas subject to inundation by the 100-year flood event.  Mandatory flood insurance purchase requirements apply;
(4)  Flood Zones AE and A1-30 - Areas subject to inundation by the 100-year flood event determined by detailed methods.  Mandatory flood insurance purchase requirements apply.  (Zone AE is used on new and revised maps in place of Zones A1-A30.);
(5)  Flood Zone AH - Areas subject to inundation by 100-year shallow flooding (usually areas of ponding) where average depths are between one and three feet.  Mandatory flood insurance purchase requirements apply:
(6)  Flood Zone AO - Areas subject to inundation by 100-year shallow flooding (usually sheet flow on sloping terrain) where average depths are between one and three feet.  Mandatory flood insurance purchase requirements apply;
(7)  Flood Zone A99 - Areas subject to inundation by the 100-year flood event, but which will ultimately be protected upon completion of an under construction Federal flood protection system.  These are areas of special flood hazard where enough progress has been made on the construction of a protective system, such as dikes, dams, and levees, to consider it complete for insurance rating purposes.  Zone A99 may only be used when the flood protection system has reached specified statutory progress toward completion.  Mandatory flood insurance purchase requirements apply;
(8)  Flood Zones B, C and X - Areas identified in the community FIS as areas of moderate or minimal hazard from the principal source of flood in the area.  However, buildings in these zones could be flooded by severe, concentrated rainfall coupled with inadequate local drainage systems.  Local stormwater drainage systems are not normally considered in the community's FIS.  The failure of a local drainage system creates areas of high flood risk within these rate zones.  Flood insurance is available in participating communities but is not required by regulation in these zones.  (Zone X is used on new and revised maps in place of Zones B and C.); and
(9)  Flood Zone D - Unstudied areas where flood hazards are undetermined, but flooding is possible.  No mandatory flood insurance purchase requirements apply, but coverage is available in participating communities.
Flooding Temporary inundation of all or part of the floodplain along a well-defined channel or temporary localized inundation occurring when surface water runoff moves via surface flow, swales, channels, and sewers toward well-defined channels.  Flooding is not necessarily synonymous with Flooding Problem.
Flooding Problem The disruption to community affairs, damage to property and facilities, and the danger to human life and health that occurs when land use is incompatible with the hydrologic-hydraulic system.
Flood Insurance A means of spreading the cost of flood losses.  It enables property owners in communities participating in the National Flood Insurance Program (NFIP) to purchase insurance against loss resulting from floods.
Flood Insurance Rate Map (FIRM) Official map on which the Federal Emergency Management Agency (FEMA) or Federal Insurance Administration (FIA) has delineated both the areas of special flood hazards and the risk premium zones applicable to the community.
Flood Insurance Rate Zone A zone identified on a Flood Insurance Rate Map (FIRM) as subject to a specified degree of flood, mudslide (i.e., mudflow) or flood-related erosion hazards, to which a particular set of actuarial rates and floodplain management requirements apply.
Flood Insurance Study (FIS) (1) A study, funded by the Federal Emergency Management Agency (FEMA) or Federal Insurance Administration (FIA), and carried out by any of a variety of agencies and consultants, to delineate the special flood hazard areas, base flood elevations, and National Flood Insurance Program (NFIP) actuarial insurance rate zones.  The study is based on detailed site surveys and analysis of site-specific hydrologic characteristics and includes flood profiles, the Flood Insurance Rate Map (FIRM), the Flood Boundary and Floodway Map (FBFM), and the water surface elevation of the base flood.  (2) A document containing the results of an examination, evaluation, and determination of flood hazards and, if appropriate, corresponding water surface elevations, mudslides and erosion hazards.
Flood, Intermediate Regional A flood having a one percent probability, or an average frequency of occurrence on the order of once in 100 years, although the flood may occur in any year.  The intermediate regional flood is based on statistical analyses of streamflow records available for the watershed and analyses of rainfall and runoff characteristics in the "general region of the watershed."
Flood Irrigation The application of irrigation water where the entire surface of the soil is covered by a sheet of water, called Controlled Flooding when water is impounded or the flow directed by border dikes, ridges, or ditches.
Flood, Maximum Probable The greatest flood that may be expected at a place, taking into account all pertinent factors of location, meteorology, hydrology, and terrain.
Flood Mitigation Assistance Program (FMA) A grant program funded by the Federal Emergency Management Agency (FEMA) with the objective of providing funding to assist states and communities in implementing measures to reduce or eliminate the long-term risk of flood damage to buildings, manufactured homes and other structures insurable under the National Flood Insurance Program (NFIP).
Flood of Record The highest observed river stage or discharge at a given site during the period of record keeping.  May not necessarily be the highest known stage.
Flood Peak The maximum instantaneous discharge of a flood at a given location.  It usually occurs at or near the time of the flood crest, i.e., the maximum stage or elevation reached by the flood flow.
Flood Plain, also Floodplain (1) (FEMA) Any normally dry land area that is susceptible to being inundated by water from any natural source.  This area is usually low land adjacent to a river, stream, watercourse, ocean or lake.  (2) A strip of relatively smooth land bordering a strea
Floodplain Fringe The portion of the flood plain outside the floodway which is covered by floodwaters during the 100-year recurrence interval flood.  It is generally associated with shallow, standing or slowly moving water rather than deep, rapidly flowing water.
Floodplain Information Reports Reports prepared to provide local governmental agencies with basic technical data to assist in planning for wise use and development of their flood plains.
Floodplain Management (1) (FEMA) The operation of an overall program of corrective and preventive measures for reducing flood damage, including but not limited to flood control projects, floodplain land use regulations, floodproofing of buildings, and emergency preparedness plans.  (2) Comprehensive flood damage prevention programs which require the integration of all alternative measures (structural and nonstructural) in investigation of flood problems and planning for wise use of the floodplain.  Includes corrective and preventive measures for reducing flood damage and preserving and enhancing, where possible, natural resources in the floodplain, including but not limited to emergency preparedness plans, flood control works and floodplain management regulations and ordinances, and open space plans.
Floodplain Management Measures Refers to an overall community program of corrective and preventive measures for reducing future flood damage.  The measures take a variety of forms and generally include zoning, subdivision, or building requirements and special-purpose floodplain ordinances.  Also see National Flood Insurance Program (NFIP) and Federal Emergency Management Agency (FEMA).
Floodplain Management Regulations Any federal, state, or local government regulations and zoning ordinances, subdivision regulations, building codes, health regulations, special purpose ordinances (such as a grading permit and erosion control requirement) and other applications of regulatory power which control development in flood-prone areas specifically for the purpose of preventing and reducing flood loss and damage.
Floodplain of Aggradation A flood plain formed by the building up of the valley floor by sedimentation.
Floodplain Playa A component landform consisting of very low gradient, broad, barren, axial-stream channel segments in an inter-montane basin.  It floods broadly and shallowly and is veneered with barren fine textured sediments that crusts.  Commonly, a floodplain playa is segmented by transverse, narrow bands of vegetation, and it may alternate with ordinary, narrow or braided channel segments.
Floodplain Regulations (FEMA) General term applied to the full range of codes, ordinances and other regulations relating to the use of land and construction within floodplain limits.  The term encompasses zoning ordinances, subdivision regulations, building and housing codes, encroachment laws and open area (space) regulations.
Flood Plane The position occupied by the water surface of a stream during a particular flood.  Also, loosely, the elevation of the water surface at various points along the stream during a particular flood.  More commonly spelled Floodplain.
Flood Prevention Methods or structural measures used to prevent floods.
Flood Probability The statistical probability that a flood of a given size will be equaled or exceeded in a given period of time.
Flood Profile (FEMA) A graph showing the relationship of water surface elevation to a specific location, the latter generally expressed as distance above mouth for a stream of water flowing in an open channel.  It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage.
Flood Proofing (Floodproofing) (1) Any combination of structural and nonstructural additions, changes, or adjustments to structures and properties subject to flooding primarily for the reduction or elimination of flood damage to real estate or improved property, water and sanitary facilities, structures and their contents.  (2) Structural or nonstructural changes or adjustments included in the design, construction, or alternation of a building that reduce damage to the building and its contents from flooding and erosion.  See Dry Floodproofing and Wet Floodproofing.
Flood Protection Elevation (FPE) Elevation of the highest flood, including freeboard, that a retrofitting method is intended to protect against.
Flood-Related Erosion The collapse or subsidence of land along the shore of a lake or other body of water as a result of undermining caused by waves or currents of water exceeding anticipated cyclical levels or suddenly caused by an unusually high water level in a natural body of water, accompanied by a severe storm, or by an unanticipated force of nature, such as a flash flood or an abnormal tidal surge, or by some similarly unusual and unforeseeable event which results in flooding.
Flood-Related Erosion Area A land area adjoining the shore of a lake or other body of water, which due to the composition of the shoreline or bank and high water levels or wind-driven currents, is likely to suffer flood-related erosion damage.  Also referred to as the Flood-Related Erosion Prone Area.
Flood-Related Erosion Area Management The operation of an overall program of corrective and preventive measures for reducing flood-related erosion damage, including but not limited to emergency preparedness plans, flood-related erosion control works, and floodplain management regulations.
Flood Routing The process of determining progressively downstream the timing and stage of a flood at successive points along a river.  Also, the determination of the attenuating effect of storage on a flood passing through a valley, channel, or reservoir.
Flood Stage (1) An elevation for the water level at high flows.  (2) The elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured.
Flood Stage Profile A graph of flooding condition water surface elevation versus distance along a river or stream.  The profile may correspond to an historic flood event or an event or a specified recurrence interval.  The channel bottom, as well as bridges, culverts, and dams, are usually shown on the flood stage profile.
Flood, Standard Project (SPF) A hypothetical flood that might result from the most severe combination of meteorological and hydrological conditions that are reasonably characteristic of the geographical region involved.  The SPF is the usual basis for design of flood control structures.
Flood Tide, also Floodtide The incoming or rising tide; the period between low water and the succeeding high water.
Floodwall Flood barrier constructed of manmade materials, such as concrete or masonry.
Flood Warning The issuance and dissemination of information about an imminent or current flood.
Floodwater The water of a flood.  Often used in the plural (Floodwaters).
Floodwater Detention Capacity That part of the gross reservoir capacity which, at the time under consideration, is reserved for the temporary storage of floodwaters.  It can vary from zero to the entire capacity (exclusive of dead storage) according to a predetermined schedule based upon such parameters as antecedent precipitation, reservoir inflow, potential snowmelt, or downstream channel capacities.  Also referred to as Flood-Control Capacity.
Floodwater Retarding Structure A structure providing for temporary storage of floodwater and for its controlled releases.
Floodwater Retention The capacity of Wetland sediments and vegetation to hold excess pulses of water for subsequent discharge.
Flood Wave A distinct rise in stage, culminating in a crest and followed by recession to lower stages.
Floodway (1) The channel of a river or other watercourse and the adjacent land area that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height.  (2) A regulatory floodplain under the National Flood Insurance Program (NFIP) 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).  (3) Portion of the regulatory floodplain that must be kept free of development so that flood elevations will not increase beyond a set limit - a maximum of 1 foot under NFIP.  The floodway usually consists of the stream channel and land along its sides.  Also referred to as Regulatory Floodway.
Floodway Encroachment Lines The lines marking the limits of Floodways on federal, state, and local floodplain maps.
Floodway Fringe The area of the floodplain on either side of the Regulatory Floodway where encroachment may be permitted.
Floodway Hazard Boundary Map See Flood Hazard Boundary Map (FHBM).
Flood Zone (1) The land bordering a stream which is subject to floods of about equal frequency; for example, a strip of the floodplain subject to flooding more often than once, but not as frequently as twice in a century (100-Year Flood).  (2) (FEMA) Zones that begin with the letters V and A are in the Special Flood Hazard Area (SFHA).  Zones B, C, D, or X are within the floodplain but not in the SFHA and, therefore, are not considered to be areas requiring flood insurance for structures located in those areas.
Floor A generic term for the nearly level, lower-part of an inter-montane basin (a bolson or semi-bolson) or a major desert stream valley.
Flora (1) A term used to describe the entire plant species of a specified region or time.  (2) The sum total of the kinds of plants in an area at one time.  All plant life associated with a given habitat, country, area, or period.  Bacteria are considered flora.
Floriston Rates (California and Nevada) Currently represents the primary operational criteria of the Truckee River between its source (Lake Tahoe) and its terminus (Pyramid Lake).  The rates originated in a 1915 decree (Truckee River General Electric Decree) in which the U.S. Bureau of Reclamation (USBR) gained an easement to operate the Lake Tahoe outlet dam in return for providing year-round flow rates for run-of-the-river usershydropower and a pulp and paper mill.  Along with the Orr Ditch Decree (1944) and the Truckee River Agreement (1935), which has been incorporated into the Orr Ditch Decree, these requirements govern the Truckee River flows.  The Floriston rates essentially constitute a minimum instream flow in the river, as long as water is physically available in Lake Tahoe and Boca Reservoir to support the rates.  Water may only be stored in Lake Tahoe and Boca Reservoir when rates are being met.  The precise definition contained in the Truckee River Agreement is as follows:
(1)  Floriston Rates means the rate of flow in the Truckee River at the head of the diversion penstock at Floriston, California (to be measured at the Iceland gage, but currently measured at the Farad gage) consisting of an average flow of 500 cubic feet of water per