Basic Water Definitions

See USGS Water Basics Glossary for more water terms

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

Bank - The sloping ground that borders a stream and confines the water in the natural channel when the water level, or flow, is normal.

Bank Full Stage - An established river stage at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach.

Cubic foot per second (ft3/s, or cfs) - Rate of water discharge representing a volume of 1 cubic foot passing a given point during 1 second, equivalent to approximately 7.48 gallons per second or 448.8 gallons per minute.

Conservation Pool
A specified amount of water area that is dedicated water storage, which may be used to meet water needs such as municipal, domestic, industrial, agricultural and recreational purposes.

Constant Level - No body of water has a constant level. Some fluctuate more than others, but all change on a daily basis, some rapidly during floods.

Datum - The basis for relating river stage to mean sea level. (datum + stage = elevation in feet above mean sea level)

Elevation - The height above mean sea level that water in a reservoir has reached. Brazos River Authority and U.S. Army Corps of Engineers lakes are actually reservoirs, or parts of the Brazos River basin impounded by dams. (A free-flowing river is measured in stages, that is, by depth from the river bed to the surface.)

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

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

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

Flood Elevation Study - An examination, evaluation, and determination of flood hazards, and if appropriate corresponding water-surface elevations. The resulting reports are used to develop Flood Insurance Rate Maps. (Also known as Flood Insurance Study)

Floodplain - Any land area susceptible to inundation by water from any source.

Flood Pool - A specified area within a lake and surrounding land that may only be used during periods of flooding to store flood water. This allows the flow of flood waters to be regulated and released in a safe manner. Only U.S. Army Corps of Engineers lakes in the Brazos River basin have flood pools. Brazos River Authority lakes – Possum Kingdom, Granbury, Limestone, and Alan Henry – have Conservation Pools only.

Flood Stage - An established gage height within a creek or river above which a rise in water surface level is defined as a flood.

10-Year Flood -
A flood which has a ten percent chance of occurring in any given year.

50-Year Flood -
A flood which has a two percent chance of occurring in any given year.

100-Year Flood -
A flood so large it has a one percent chance of occurring in any given year. The term “100-year” is a measure of the size of the flood, not how often it occurs. Several 100-year floods can occur within the same year or within a few short years. Also called the base flood.

500-Year Flood -
A flood which has a two-tenths of one percent chance of occurring in any given year.

Gaging station - A particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.

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

National Flood Insurance Program - Federal insurance program under which flood-prone areas are identified and flood insurance is made available to residents of participating communities that agree to adopt and enforce floodplain management ordinances to reduce future flood damage.

Streamflow - The discharge of water in a natural channel.