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USGS ILLINOIS WATER SCIENCE CENTER GLOSSARY OF HYDROLOGIC TERMS 

For more information, please see Circular 1123: The Stream-Gaging Program of the U.S. Geological Survey.
Control
designates a feature downstream from the gage that determines the stage-discharge relation at the gage. This feature may be a natural constriction of the channel, an artificial structure, or a uniform cross section over a long reach of the channel.
Control structure
a structure on a stream or canal that is used to regulate the flow or stage of the stream or to prevent the intrusion of salt water.
Cubic foot per second
(ft3/s also CFS) is the rate of discharge representing a volume of 1 cubic foot passing a given point during 1 second and is equivalent to 7.48 gallons per second or 448.8 gallons per minute or 0.02832 cubic meter per second.
Discharge
is the volume of water (or more broadly, volume of fluid plus suspended sediment) that passes a given point within a given period of time.
Drainage area
of a stream at a specified location is that area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream above the specified point.
Drainage basin
is a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.
Gage height
(G.H.) is the water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term "stage," although gage height is more appropriate when used with a reading on a gage.
Gaging station
is a particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.
Hydrologic unit
is a geographic area representing part or all of a surface drainage basin or distinct hydrologic feature as delineated by the Office of Water Data Coordination on State Hydrologic Unit Maps; each hydrologic unit is identified by an eight-digit number.
Instantaneous discharge
is the discharge at a particular instant of time.
Mean discharge
(MEAN) is the arithmetic mean of individual daily mean discharges during a specific period.
National Geodetic Vertical Datum of 1929
(NGVD) is a geodetic datum derived from a general adjustment of the first order level nets of both the United States and Canada. It was formerly called "Sea Level Datum of 1929" or "mean sea level". Although the datum was derived from the average sea level over a period of many years at 26 tide stations along the Atlantic, Gulf of Mexico, and Pacific Coasts, it does not necessarily represent local mean sea level at any particular place.
Sea level
refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.
Stage
See Gage height
Stage-discharge relation
is the relation between gage height (stage) and the volume of water per unit of time (discharge) flowing in a channel.
Streamflow
is the discharge that occurs in a natural channel. Although the term "discharge" can be applied to the flow of a canal, the word "streamflow" uniquely describes the discharge in a surface stream course. The term "streamflow" is more general than "runoff" as streamflow may be applied to discharge whether or not it is affected be diversion or regulation.
Surface area
of a lake is that area, in acres, outlined on the latest USGS topographic map as the boundary of the lake and measured by a planimeter. In localities not covered by topographic maps, the areas are computed from the best maps available. All areas shown are those corresponding to the stage existing at the time when the planimetered map was made.
Water year
is the 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 1992, is called the "1992 water year."

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