F [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 F = r/k, where F is called the
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 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
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. 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(F° -
FAIRFIELD-HARDY DIGESTER—(Water Quality) A machine
that decomposes garbage, sewage sludge, industrial and other
organic wastes by a controlled continuous Aerobic-Thermophilic
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:
 The cooling of surface waters;
 A density change in surface waters producing convection
currents from top to bottom;
 The circulation of the total water volume by wind action;
 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.
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
FALLS—A waterfall or other precipitous descent of
FANGLOMERATE—Heterogeneous materials that were
originally deposited in an Alluvial Fan but since
deposition have been cemented into solid rock.
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
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
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
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:
 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
 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;
 Historically Active Fault—A
fault active within the last 200 years;
 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
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 44.5C,
and which can grow with or without oxygen. 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.
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 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)—The
federal agency responsible for administering the National Flood
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 act contains
a clause modeled after a clause in the Reclamation Act of
1902 which disclaims any intent to affect state water
rights law. Subsequently, in a number of decisions dating back
to the 1940s, the U.S. Supreme Court held that the provisions of
both the Reclamation Act and the Federal Power Act preempted
inconsistent provisions of state law. Decisions under both acts
found that these clauses were merely "saving clauses"
which required the United States to follow minimal state
procedural laws or to pay just compensation where vested
non-federal water rights are taken. Later the Supreme Court
overturned a number of its earlier decisions and required that
the Bureau of Reclamation (USBR) comply with conditions
in state water rights permits unless those conditions conflict
with "clear Congressional directives." However, no
such reversal of the Federal Power Act's provisions followed and
more recent decisions (Sayles Hydro Association v. Maughan,
February 1993) reinforced this fact by holding that federal law
has "occupied the field," preventing any state
regulation of federally licensed power projects other than
determining proprietary water rights. This precedent has
far-reaching implications over states' rights to regulate water
projects and stream flows within their borders. There have been
instances where holders of Federal Power Act licenses have
claimed preemption from state safety of dams requirements,
minimum stream flow requirements, and state designation of wild
and scenic streams. Also see Equal Footing Doctrine (U.S.
Constitution) and Public Trust Doctrine.
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.
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
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
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
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
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.
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 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
FILTRATE—Liquid that has been passed through a
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
FIRM CAPACITY—For public drinking water supplies,
the system delivery capacity with the largest single water well
or production unit out of service.
FIRM YIELD—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.
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
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
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 the
Department of Agriculture. In 1939, these functions were
transferred to the Department of the Interior. Then in 1940,
these functions were formally consolidated and redesignated as
the Fish and Wildlife Service. Further reorganization came in
1956 when the Fish and Wildlife Act created the Bureau of Sport
Fisheries and Wildlife. An amendment to this act in 1974
designated the Bureau as the U.S. Fish and Wildlife Service.
Today the USFWS consists of a headquarters in Washington, D.C.,
eight regional offices, and over 700 field units and
installations. Included are more than 470 National Wildlife
Refuges, comprising more than 90 million acres, 57 fish and
wildlife research laboratories and field units, 43 cooperative
research units at universities across the country, nearly 135
national fish hatcheries and fishery assistance stations, and a
nationwide network of law enforcement agents and biologists. The
functions of the USFWS primarily includes the following:
 Acquires, protects and manages unique ecosystems
necessary to sustain fish and wildlife, such as migratory
birds and endangered species;
 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;
 Operates fish hatcheries to support research, develop new
techniques and fulfill the public demand for recreational
 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;
 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;
 Renders financial and professional assistance to states,
through federal aid programs, for the enhancement and
restoration of fish and wildlife resources;
 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
 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
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
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.
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.
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 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
FLAT-WATER—Of or on a level or slow-moving
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.
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.
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).
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
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
FLOOD, or Flood Waters—(1) An overflow of water
onto lands that are used or usable by man and not normally
covered by water. Floods have two essential characteristics: The
inundation of land is temporary; and the land is adjacent to and
inundated by overflow from a river, stream, lake, or ocean. (2)
As defined, in part, in the Standard Flood Insurance Policy
(SFIP): "A general and temporary condition of partial
or complete inundation of normally dry land areas from overflow
of inland or tidal waters or from the unusual and rapid
accumulation or runoff of surface waters from any source."
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 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
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 FLOODWAY MAP (FBFM)—Official map of
a community where 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 POOL—Reservoir volume reserved for
flood runoff and then evacuated as soon as possible to keep that
volume in readiness for the next flood.
FLOOD CREST—The maximum stage or elevation 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 DURATION CURVE—A cumulative frequency curve
that shows the percentage of time that specified discharges are
equaled or exceeded.
FLOOD FORECASTING—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
FLOOD FREQUENCY—A statistical expression or
measure of how often a hydrologic event of a given size or
magnitude should, on an average, be equaled or exceeded. For
example, a 50-year frequency flood (2 percent change of
occurrence) should be equaled or exceeded, on the average, once
in 50 years. Also see Hundred-Year 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
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 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
 Flood Zone A—Area of
special flood hazard without water surface elevation
 Flood Zones A1-30 & AE—Areas
of special flood hazard with water surface elevations
 Flood Zone AO—Area of special
flood hazard having shallow water depths and or unpredictable
flow paths between one and three feet;
 Flood Zone A-99—Area of
special flood hazard where enough progress has been made on a
protective system, such as dikes, dams, and levees, to
consider it complete for insurance rating purposes;
 Flood Zone AH—Area of special
flood hazard having shallow water depths and or unpredictable
flow paths between one and three feet and with water surface
 Flood Zones B & Shaded X—Areas
of moderate flood hazard;
 Flood Zones C & Unshaded X—Areas
of minimal hazard;
 Flood Zone D—Area of
undetermined but possible flood hazard;
 Flood Zone E—Area of special
flood-related erosion hazards;
 Flood Zone M—Area of special
mudslide or mudflow hazards.
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 interested persons to purchase
insurance against loss resulting from floods.
FLOOD INSURANCE RATE MAP (FIRM)—Official map on
which the Federal Emergency Management Agency (FEMA)
has delineated both the areas of special flood hazards and the
risk premium zones applicable to the community.
FLOOD INSURANCE STUDY (FIS)—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
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 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) A strip of
relatively smooth land bordering a stream, built of sediment
carried by the stream and dropped in the slack water beyond the
influence of the swiftest current. It is called a Living
Flood Plain if it is overflowed in times of high water but
a Fossil Flood Plain if it is beyond the reach of the
highest flood. (2) The lowland that borders a stream or river,
usually dry but subject to flooding. (3) That land outside of a
stream channel described by the perimeter of the Maximum
Probable Flood. Also referred to as a Flood-Prone Area.
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
FLOODPLAIN MANAGEMENT—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.
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 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 OF AGGRADATION—A flood plain formed by
the building up of the valley floor by sedimentation.
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.
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—A graph showing the relationship of
water surface elevation to 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—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
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
FLOOD-RELATED EROSION PRONE 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.
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—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
FLOODWATER—The water of a flood. Often used in the
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
FLOODWATER RETENTION—The capacity of Wetland
sediments and vegetation to hold excess pulses of water for
FLOOD WAVE—A distinct rise in stage, culminating
in a crest and followed by recession to lower stages.
FLOODWAY—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
FLOODWAY ENCROACHMENT LINES—The lines marking the
limits of Floodways on federal, state, and local
FLOODWAY FRINGE—The area of the floodplain on
either side of the Regulatory Floodway where
encroachment may be permitted.
FLOODWAY HAZARD BOUNDARY MAP (FHBM)—Official map
of a community where the boundaries of the flood, mudslide and
related erosion areas having special hazards have been
designated as Flood Zones A, M, and E. Supersedes the Flood
Boundary Floodway Map (FBFM).
FLOOD ZONE—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).
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 users—hydropower 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:
 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 second each day during the
period commencing March 1 and ending September 30 of any year,
and an average flow of 400 cubic feet per second each day
during the period commencing October 1 and ending the last day
of the next following February of any year.
 Reduced Floriston Rates means rates of
flow in the Truckee River, measured at the Iceland gage
(currently the Farad gage), effective and in force during the
period commencing November 1 and ending the next following
March 31 of each year, determined as follows:
(a) 350 cubic feet per second whenever the elevation of
the water surface of Lake Tahoe is below 6226.0 feet above
sea level and not below 6225.25 feet above sea level; and
(b) 300 cubic feet per second whenever the water surface
elevation of Lake Tahoe is below 6225.25 feet above sea
Also see Truckee River Agreement [Nevada and California].
FLOTATION, also Floatation—(1) The act, process,
or condition of floating, also called Flotage. (2) The
process of separating different materials, especially minerals,
by agitating a pulverized mixture of the materials with water,
oil, and chemicals. Differential wetting of the suspended
particles causes unwetted particles to be carried by air bubbles
to the surface for collection.
FLOW—The rate of water discharged from a source
given in volume with respect to time.
FLOWAGE—(1) The act of flowing or overflowing. (2)
The state of being flooded; a body of water, such as a lake or
reservoir, formed by usually deliberate flooding. (3) An outflow
FLOW AUGMENTATION—The addition of water to a
stream especially to meet instream flow needs.
FLOW BOUNDARIES—Anything which inhibits ground
water flow, such as a ground water divide or an impermeable
FLOW DURATION CURVE—A cumulative frequency curve
that shows the percentage of time that specified discharges are
equaled or exceeded.
FLOWLINE (STREAMLINE)—(1) The general path that a
particle of water follows under laminar flow conditions. (2) The
line indicating the direction followed by ground water toward
points of discharge. Flow lines are perpendicular to Equipotential
FLOW METER—A device which allows for measurement
of stream flow by measuring velocity in a given cross-sectional
(GROUND WATER) FLOW MODEL—(1) A digital computer
model that calculates a hydraulic head field for the modeling
domain using numerical methods to arrive at an approximate
solution to the differential equation of ground-water flow. (2)
Any representation, typically using plastic or glass
cross-sectional viewing boxes, with representative soil samples,
depicting ground-water flows and frequently used for educational
FLOW, LAMINAR—Flow of water in well-defined flow
lines in which the viscous force is predominant; in channels it
occurs at a Reynolds Number smaller than 500-2,000 and
through porous media at Reynolds Number smaller than 1-10.
FLOW, MODIFIED—That streamflow which would have
existed had the works of man in or on the stream channels and in
the drainage basin been consistent throughout the period of
record. Usually used with an adjective such as
"present" or specific year to mean that the flow
record was modified to represent the record that would have been
obtained had the "present" conditions prevailed
throughout the period of record. Modified flow is equal to Virgin
Flow minus the amount of Streamflow Depletion
occurring at the specified time.
FLOW, NATURAL—The rate of water movement past a
specified point on a natural stream from a drainage area which
has not been affected by stream diversion, storage, import,
export, return flow or change in consumptive use resulting from
man's modification of land use. Natural flow rarely occurs in a
FLOW, NET—A graphical representation of flow lines
and Equipotential Lines for two-dimensional,
steady-state ground-water flow.
FLOW, OVERLAND—The flow of rainwater or snowmelt
over the land surface toward stream channels. Upon entering a
stream, it becomes runoff.
FLOW PATH—The subsurface course a water molecule
or solute would follow in a given ground-water velocity field.
FLOW RATE—The rate, expressed in gallons or
liters-per-hour, at which a fluid escapes from a hole or fissure
in a tank. Such measurements are also made of liquid waste,
effluent, and surface water movement.
FLOW RESOURCES Versus STOCK RESOURCES—Flow
resources are resources that are not permanently expendable
under usual circumstances; they are resources which are
replaced. They are commonly expressed in annual rates at which
they are regenerated. Examples are fresh-water runoff and
timber. Stock resources can be permanently expended and whose
quantity is usually expressed in absolute amounts rather than in
rates. Examples are coal and petroleum deposits.
FLOW, STEADY—A flow in which the magnitude and
direction of the specific discharge are constant in time.
FLOWSTONE—A layered deposit of calcium carbonate,
CaCO3, on rock where water has flowed or dripped, as
on the walls of a cave. Also see Tufa.
FLOW, TURBULENT—A flow in which successive
flow particles follow independent path lines, and head loss
varies approximately with the square of the velocity. In stream
channels it occurs at a Reynolds Number greater than
FLOW, UNIFORM—A characteristic of a flow system
where specific discharge has the same magnitude and direction at
FLOW VELOCITY—The volume of water flowing through
a unit cross-sectional area of an aquifer. Also referred to as Specific
FLOW, VIRGIN—That streamflow which would exist had
man not modified conditions on or along the stream or in the
FLOWING WELL—An Artesian Well having
sufficient head to discharge water above the land surface; a
well where the Piezometric Surface lies above the
FLOWMETER—A gauge indicating the velocity of
wastewater moving through a treatment plant or of any liquid
moving through various industrial processes.
FLUE GAS SCRUBBER—A type of equipment that removes
fly ash and other objectionable materials from flue gas by the
use of sprays, wet baffles, or other means that require water as
the primary separation mechanism. Also referred to as Flue
FLUID—Having particles which easily move and
change their relative position without a separation of the mass,
and which easily yield to pressure; capable of flowing; liquid
FLUIDIZED—A mass of solid particles that is made
to flow like a liquid by injection of water or gas is said to
have been fluidized. In water treatment, a bed of filter media
is fluidized by backwashing water through the filter.
FLUID OUNCE—(Abbreviated fl oz, fl. oz.) (1) A
unit of volume or capacity in the U.S. Customary System, used in
liquid measure, equal to 29.57 milliliters (1.804 cubic inches).
(2) A unit of volume or capacity in the British Imperial System,
used in liquid and dry measure, equal to 28.41 milliliters
(1.734 cubic inches).
FLUID POTENTIAL—The mechanical energy per unit
mass of a fluid at any given point in space and time with
respect to an arbitrary state and datum. Loss of fluid potential
results as the fluid moves from a region of high potential to
one of low potential and represents the loss of mechanical
energy which is converted to heat by friction.
FLUME—(1) A narrow gorge, usually with a stream
flowing through it. (2) An open artificial channel or chute
carrying a stream of water, as for furnishing power, conveying
logs, or as a measuring device.
FLUORIDATE (FLUORIDATION)—To add a fluorine
compound to a drinking water supply, for example, for the
purpose of reducing tooth decay, particularly in children. Since
1962, the U.S. Public Health Service (PHS) has recommended an
"optimal" fluoride concentration of 0.7 to 1.2 mg/l
(milligrams per liter) to prevent dental caries and minimize
mottling (fluorosis). In 1986, the U.S. Environmental
Protection Agency (EPA) set the Maximum Contaminant
Level (MCL) for fluoride at 4 mg/l.
FLUORIDE—A binary compound of Fluorine
with another element; gaseous, solid, or dissolved compounds
containing fluorine that result from industrial processes.
Fluoride combines with tooth enamel to render it less soluble in
acid environments and fluoride compounds are added to public
water supplies to prevent tooth decay. Excessive amounts in food
can lead to Fluorosis. Fluorine is a halogen with the
chemical symbol F.
FLUORINE—A pale-yellow, highly corrosive,
poisonous, gaseous halogen element, the most electronegative and
most reactive of all the elements, used in a wide variety of
industrially important compounds. Fluorine is a halogen with the
chemical symbol F.
FLUOROSIS—An abnormal condition caused by
excessive intake of Fluorine, as from fluoridated
drinking water, characterized chiefly by mottling of the teeth.
FLUSH—(1) To flow suddenly and abundantly, as from
containment; flood. (2) To be emptied or cleaned by a rapid flow
of water, as a toilet. (3) To open a cold-water tap to clear out
all the water which may have been sitting for a long time in the
pipes. In new homes, to flush a system means to send large
volumes of water gushing through the unused pipes to remove
loose particles of solder and flux. (4) To force large amounts
of water through liquid to clean out piping or tubing, storage
or process tanks.
FLUSHLESS TOILET—A toilet that disposes of waste
without using water, especially one that utilizes bacteria to
break down waste matter.
FLUSHOMETER—A device for flushing toilets and
urinals that utilizes pressure from the water supply system
rather than the force of gravity to discharge water into the
bowl, designed to use less water than conventional flush
FLUVIAL—Of or pertaining to rivers and streams;
growing or living in streams ponds; produced the action of a
river or stream.
FLUVIOGLACIAL—Pertaining to streams flowing from
glaciers or to the deposits made by such streams.
FLUX—(1) A flowing or flow. (2) The flowing in of
the tide. (3) The measure of the hydraulic rate of flow of water
through a pressure osmosis membrane in gallons per square foot
of membrane per day (GFD).
FLUX DENSITY—The rate of flow of any quantity,
usually a form of energy, through a unit area of specified
FOAM—(1) A mass of bubbles of air or gas in a
matrix of liquid film, especially an accumulation of fine,
frothy bubbles formed in or on the surface of a liquid, as from
agitation or fermentation. (2) The sea.
FOG—Condensed water vapor in cloud-like masses
lying close to the ground.
FOLD—(Geology) A bend or flexure in a layer or
layers of rock.
FOOD CHAIN—A succession of organisms in an
ecological community that constitutes a continuation of food
energy from one organism to another as each consumes a lower
member and in turn is preyed upon by a higher member.
FORAGE FISH—Small fish which breed prolifically
and serve as food for predatory fish.
FORB—Any Herbaceous flowering plant,
other than a grass; especially one growing under range
FORCE MAINS—Pipes in which wastewater is
transported under pressure; the system is used in some areas
having small elevation changes with distance and therefore
needing to augment the gravity flow.
FORCE PUMP—A pump with a solid piston and valves
used to raise a liquid or expel it under pressure.
FORD—A shallow place in a body of water, such as a
river, where one can cross by walking or riding on an animal or
in a vehicle.
FOREBAY—The water behind a dam. A reservoir or
pond situated at the intake of a pumping plant or power plant to
stabilize water levels; also a storage basin for regulating
water for percolation into ground water basins. Compare with Afterbay.
FOREBAY RESERVOIR—A reservoir used to
regulate the flow of water to a hydroelectric plant; it may also
serve other purposes such as recreation. Also see Afterbay.
FORECAST (FORECASTING)—(Statistics) A
forecast is a quantitative estimate (or set of estimates) about
the likelihood of future events based on past and current
information. This "past and current information" is
specifically embodied in the structure of the econometric model
used to generate the forecasts. By extrapolating the model out
beyond the period over which it was estimated, we can use the
information contained in it to make forecasts about future
events. It is useful to distinguish between two types of
forecasting, ex post and ex ante. In an ex
post forecasts all values of dependent and independent
variables are known with certainty and therefore provides a
means of evaluating a forecasting model. Specifically, in an ex
post forecast, a model will be estimated using observations
excluding those in the ex post period, and then
comparisons of the forecasts will be made to these actual
values. An ex ante forecast predicts values of the
dependent variable beyond the estimation period using values for
the explanatory variables which may or may not be known with
FORECAST HORIZON—(Statistics) The number of time
periods to be forecasted; also, the time period in the future to
which forecasts are to be made.
FORESHORE—(1) The part of a shore that lies
between high and low watermarks. (2) The part of a shore between
the water and occupied or cultivated land.
FOREST HYDROLOGY—The study of hydrologic processes
as influenced by forest and associated vegetation.
FOREST INFLUENCES—The effects resulting from the
presence of forest or brush upon climate, soil water, runoff,
streamflow, floods, erosion, and soil productivity.
FOREST LAND—Land which is at least 10 percent
occupied by forest trees of any size or formerly having had such
tree cover and not currently developed for non-forest use. Lands
developed for non-forest use include areas for crops, improved
pasture, residential, or administrative areas, improved roads of
any width, and adjoining road clearing and power line clearing
of any width.
(UNITED STATES) FOREST SERVICE (USFS)—The largest
and most diverse agency of the U.S. Department of Agriculture,
the Forest Service provides leadership in the management,
protection, and use of the nation's forests and rangelands,
which comprise almost two-thirds of the nation's federally owned
lands. The creation of the Forest Service go back to 1891 when
the President was authorized to establish Forest Reserves from
forest and range lands in the Public Domain. In 1905
the responsibilities for the management and protection of these
Forest Reserves was transferred from the Department of the
Interior to the Department of Agriculture and the Forest Service
was formally established. The Forest Reserves were then renamed
National Forests. Today the Forest Services manages 156 National
Forests, 19 National Grasslands, and 16 Land Utilization
Projects that make up the National Forest System located in 44
states, Puerto Rico, and the Virgin Islands. Much of the
nation's fresh water supply flows from National Forest System
lands and insuring adequate yields of high quality water and
continuing soil productivity are primary aims of the Forest
Service's watershed management programs. The Forest Service
manages more than 14 percent of the nation's 1.2 billion acres
of forest range. This National Forest System (NFS) rangeland is
managed to conserve the land and its vegetation while providing
food for both domestic livestock and wildlife. The Forest
Service manages fish and wildlife habitat on the National
Forests and National Grasslands in cooperation with the
individual states' fish and game departments. Of the 191 million
acres of National Forests, 86.5 million acres are classified as
commercial forests, available for, and capable of, producing
crops of industrial wood. National Forest timber reserves are
managed on a sustained-yield basis to produce a continuous
supply of wood products to meet the nation's economic demands
while maintaining the productive capacity of these lands. In
1924 the Forest Service pioneered the establishment of
wilderness areas on National Forest lands. National Forest lands
are a major source of mineral and energy supplies with
regulatory and management responsibilities for mineral
activities shared with the Department of the Interior, Bureau of
Mines. The Forest Service, with one of the world's largest
wildland firefighting forces, provides direct fire protection
and control for National Forest System lands as well as
cooperative fire control on several million additional acres.
The Forest Service is responsible for the forest management
aspects of the Watershed Protection and Flood Prevention Program
administered by the Natural Resources Conservation Service (NRCS).
The Forest Service also participates in the forestry aspects of
the River Basin Program, which guides and coordinates water and
related land resource planning among several federal
departments. The Forest Service operates an extensive forestry
research program consisting of eight Forest and Range Experiment
Stations, a Forest Products Laboratory, and 75 research labs
located throughout the U.S., Puerto Rico, and the Pacific Trust
Territories. The Forest Service is organized into nine (9)
regions as listed below (regional headquarters are in
 Eastern Region (Milwaukee,
Wisconsin)—Maine, New Hampshire, Vermont, Massachusetts,
Rhode Island, Connecticut, New York, New Jersey, Pennsylvania,
Delaware, Maryland, West Virginia, Ohio, Indiana, Michigan,
Illinois, Missouri, Iowa, Wisconsin, Minnesota;
 Southern Region (Atlanta,
Georgia)—Virginia, North Carolina, South Carolina, Kentucky,
Tennessee, Georgia, Florida, Alabama, Mississippi, Arkansas,
Louisiana, Oklahoma, Texas;
 Rocky Mountain Region (Denver,
Colorado)—South Dakota, Nebraska, Kansas, Wyoming, Colorado;
 Northern Region (Missoula,
Montana)—North Dakota, Montana, Idaho (northern part only),
South Dakota (northwest corner only), Wyoming (northwest
 Intermountain Region (Ogden,
Utah)—Nevada, Utah, Idaho (except northern portion), Wyoming
(western portion only);
 Southwest Region (Albuquerque,
New Mexico)—Arizona, New Mexico;
 Pacific Northwest Region
(Portland, Oregon)—Washington, Oregon;
 Pacific Southwest Region (San
Francisco, California)—California, Hawaii;
 Alaska Region (Juneau,
FORFEITED WATER RIGHT—A water right that is no
longer valid because of five or more consecutive years of
nonuse. Also see Abandoned Water Right.
FORFEITURE—The invalidation of a water right
because of five or more consecutive years of nonuse. Also see Abandonment.
FORMATION—(Geology) A body of rock or soil of
considerable thickness that has characteristics making it
distinguishable from adjacent geologic structures.
FOSSIL WATER—Limited subterranean water deposits
laid down in past ages but drawn on by modern man.
FOUCAULT, Jean Bernard Léon (1819-1868)—A French
physicist who estimated the speed of light and determined that
it travels more slowly in water than in air (1850).
FOUNDATION (of a Dam)—The natural material on
which the dam structure is placed.
FOUNDER—To sink below the water.
FOUNTAIN—(1) An artificially created jet or stream
of water; a structure, often decorative, from which a jet or
stream of water issues. (2) A spring, especially the source of a
stream. (3) A reservoir or chamber containing a supply of liquid
that can be siphoned off as needed.
FOUNTAINHEAD—(1) A spring that is the source or
head of a stream. (2) The upper end of a confined-aquifer
conduit, where it intersects the land surface.
FRACTURE—A general term for any break in rock,
which includes cracks, joints, and faults.
FRACTURED BEDROCK AQUIFER—An aquifer composed of
solid rock, but where most water flows through cracks and
fractures in the rock instead of through pore spaces. Flow
through fractured rock is typically relatively fast.
FRAGILE AREA—Areas that, due to steepness, soil
type, exposure, and cover, are especially subject to soil
erosion and rapid deterioration. Also referred to as Critical
FRAGMENTATION (of Habitat)—Interruption of large
expanses of one type of habitat or vegetation by man-made
clearings. Generally used where roads or areas of cropland are
cleared within forested areas, thereby breaking a large
continuous area of forest into smaller parcels of forest.
FRAZIL (FRAZIL ICE)—A French-Canadian term for the
fine spicular ice, derived from the French words for cinders
which this variety of ice most resembles. When formed in slat
water it is known as Lolly Ice. When first formed,
frazil is colloidal and is not visible in the water.
FREEBOARD—The vertical distance between a design
maximum water level and the top of a structure such as a
channel, dike, floodwall, dam, or other control surface. The
freeboard is a safety factor intended to accommodate the
possible effect of unpredictable obstructions, such as ice
accumulation and debris blockage, that could increase stages
above the design water surface. (Nautical) The distance between
the water line and the uppermost full deck of a ship. For dams,
the terms "net freeboard", "dry freeboard",
"flood freeboard", or "residual freeboard"
refer to the vertical distance between the estimated maximum
water level and the top of a dam. "Gross freeboard" or
"total freeboard" is the vertical distance between the
maximum planned controlled retention water level and the top of
FREE FLOW—(Hydraulics) Flow through or over a
structure not affected by submergence or backwater.
FREE-FLOWING—Flowing without artificial
FREE-FLOWING STREAM—A stream or a portion of a
stream that is unmodified by the works of man or, if modified,
still retains its natural scenic qualities and recreational
FREE-FLOWING WEIR—A weir that in use has the
tailwater lower than the crest of the weir.
FREE-FLOWING WELL—An Artesian Well in
which the potentiometric surface is above the land surface. Also
see Potentiometric Surface.
FREE GROUND WATER—Water in interconnected
pore spaces in the Zone of Saturation down to the first
impervious barrier, moving under the control of the water table
FREE LIQUIDS—(Water Quality) Liquids capable of
migrating from waste and contaminating ground water. Hazardous
waste containing free liquids may not be disposed of in
FREE MOISTURE—Liquid that will drain freely from
solid waste by the action of gravity only.
FREE WATER SURFACE (FWS) CONSTRUCTED WETLAND—A
type of constructed wetland, a man-made marsh-like area used to
treat wastewater. In this type of wetland, the effluent flows
through various aquatic plants, with the water level exposed to
the air. While this type of wetland is relatively easy to
construct, it is not as effective as the Subsurface Flow
(SF) Constructed Wetland with respect to associated odors,
potential for insect breeding, and risk of public exposure and
contact with the water in the system. Also see Wetlands,
FREEZE—(1) To pass from the liquid to the solid
state by loss of heat. (2) To acquire a surface of coat of ice
FREEZING—The change of a liquid into a solid as
temperature decreases. For water, the freezing point is 32F
(Fahrenheit) or 0C (Celsius).
FREEZING POINT—(1) The temperature at which a
liquid of specified composition solidifies under a specified
pressure. (2) The temperature at which the liquid and solid
phases of a substance of specified composition are in
equilibrium at atmospheric pressure.
FRENCH DRAIN—An underground passageway for water
through the interstices among stones placed loosely in a trench.
FREQUENCY ANALYSIS—A statistical procedure
involved in interpreting the past record of a hydrological event
to occurrences of that event in the future (e.g., estimates of
frequencies of floods, droughts, storage, rainfall, water
FREQUENCY CURVE—A graphical representation of the
frequency of occurrence of specific events. Also referred to as Frequency
FREQUENCY DISTRIBUTION—An arrangement of
quantities pertaining to a single event, in order of magnitude
and frequency of occurrence.
FRESH—(1) Not saline or salty. (2) Free from
impurity or pollution.
FRESHET—(1) A sudden overflow of a stream
resulting from a heavy rain or a thaw. (2) A stream of fresh
water that empties into a body of salt water.
FRESH-SALT WATER INTERFACE—The region where fresh
water and salt water meet.
FRESHWATER—(1) Of, relating to, living in, or
consisting of water that is not salty. (2) Water with salinity
less than 0.5 (parts per thousand) dissolved salts. (3) Water
that contains less than 1,000 milligrams per liter (mg/l) of
dissolved solids; generally, more than 500 mg/l of dissolved
solids is undesirable for drinking and many industrial uses. (4)
(Nautical) Accustomed to sailing on inland waters only.
FRESHWATER MARSH—A Circumneutral Ecosystem
of more or less continuously water-logged soil dominated by
emersed herbaceous plants, but without a surface accumulation of
FRET—To gnaw or wear away; erode. To form (a
passage or channel) by erosion. To disturb the surface of (water
or a stream); agitate.
FRICTION HEAD—Energy required to overcome friction
due to fluid movement with respect to the walls of the conduit
or containing medium.
FRICTION LOSSES—Total energy losses in the flow of
water due to friction between the water and the walls of a
conduit, channel, or porous medium, usually expressed in units
FRICTION SLOPE—The energy loss per unit of length
of open or closed conduit due to friction.
FRIENDS OF THE EARTH (FOE)—A conservation and
environmental organization, founded in 1969, dedicated to
preservation, restoration, and wise use of natural resources.
The United States headquarters is located in Washington, D.C.,
with affiliates offices in 37 countries. Through the Friends
of the Earth Foundation, the organization promotes public
education and monitors enforcement of environmental policies.
FRINGE WATER—Water occurring in the Capillary
FRINGE MARSH—A saturated, poorly drained area,
intermittently or permanently water covered, close to and along
the edge of a land mass.
FRONT—(1) Land bordering a lake or river. (2)
(Meteorology) A line of separation or interface between air
masses of different temperatures or densities.
FRONTAGE—Land adjacent to something, such as a
body of water.
FROST—(1) Thin ice crystals in the shape of
scales, needles, feathers or fans which are deposited by Sublimation
at temperatures of 32°F (0°C) or lower. (2) A temperature low
enough to cause freezing. (3) The process of freezing.
FROST HEAVE—Ruptured soil, rock, or pavement
caused by the expansion of freezing water immediately beneath
FROST LINE—The depth to which frost penetrates the
FROST POCKETS—A low area or depression at the base
of a slope where frost collects.
FROTH—A mass of bubbles in or on a liquid; foam.
FROZEN—(1) Made into, covered with, or surrounded
by ice. (2) Very cold.
FULL COST (USBR)—A water rate defined by Congress
in the Reclamation Reform Act of 1982 intended to represent the
federal government's actual cost in providing project water to
irrigators. The full-cost rate for each project or district is
calculated by amortizing the expenditures for construction
properly allocable to irrigation facilities in service,
including all operation and maintenance deficits funded, less
payments, over such periods as may be required under federal
reclamation law or applicable contract provisions. Interest on
all charges accrues from October 12, 1982, on costs outstanding
at that date or from the date incurred of costs arising
subsequent to October 12, 1982. The term Full-Cost Rate
means the full-cost charge plus actual operation, maintenance,
and replacement costs.
FULL-COST RATE (USBR)—An annual rate as determined
by the Secretary of the Interior that shall amortize
construction expenditures that are properly allocable to
irrigation facilities in service, including all operation and
maintenance deficits funded, less payments, over such periods as
may be required by reclamation law or applicable contract
provisions, with interest on both accruing from October 12,
1982, on costs outstanding at that date, or from the date
incurred in the cast of costs arising subsequent to October 12,
FULLY PERMANENT SPRINKLER SYSTEM—An irrigation
system usually composed of buried enclosed conduits carrying
water under pressure to fixed orifices to distribute water over
a given area.
FUMAROLE—A hole or orifice in a volcanic region,
and usually in lava, from which issue gases and vapors at high
FUNCTIONAL EQUIVALENT—A term used to describe the U.S.
Environmental Protection Agency's (EPA) decision-making
process and its relationship to the environmental review
conducted under the National Environmental Policy Act (NEPA).
A review is considered functionally equivalent when it addresses
the substantive components of a NEPA review.
FUNCTIONAL RELATIONSHIP—(Statistics) A
hypothetical relationship that describes the effect of one or
more Independent Variables on a Dependent Variable,
of the general form:
Y = f(X1, X2, ... , Xn)
where Y represents the dependent variable whose behavior is a
function of, f( ), the values of the independent
variables, X1, X2, ..., Xn. A
fundamental assumption of a functional relationship is that
changes in the independent variables, also referred to as the Exogenous
Variables, prescribe or determine changes in the dependent,
or Endogenous Variable, consequently leading to a flow
of causation from the independent variables to the dependent
variable. As such, a functional relationship is not exactly
comparable to a mathematical equation in which variables may be
moved from one side of the equation to the other without
changing the validity of the equality. In a functional
relationship by contrast, once the flow of causation has been
prescribed (the Specification), the equation's
(model's) structure is fixed.
FUNGI (Singular: Fungus)—Molds, mildews, yeasts,
mushrooms, and puffballs, a group of organisms lacking in
chlorophyll (i.e., are not photosynthetic) and which are usually
non-mobile, filamentous, and multicellular. Some grow in soil,
others attach themselves to decaying trees and other plants
whence they obtain nutrients. Some are Pathogens,
others stabilize sewage and digest composted waste.
FURROW—A long, narrow, shallow trench made in the
ground by a plow for planting and irrigation.
FURROW DAMS—Small earth ridges or rows used to
impound water in furrows.
FURROW IRRIGATION—Spreading water by directing it
into small channels across the field. Also referred to as Corrugation
FURROW STREAM—The size of water flow released into
the furrow; the size of the stream is adjusted to prevent
erosion, limited in amount to the capacity of the furrow, and as
needed for the intake rates of the soil involved.