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. 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. Flood stage is the stage when exceeded has the potential for inundating areas beyond the river channel in the area of the gage where the flood stage is determined. Gaging station is a particular site on a stream, canal, lake, or reservoir where systematic observations of hydrologic data are obtained.
Instantaneous discharge is the discharge at a particular instant of time.
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. Streamflow is the discharge that occurs in a natural channel. 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.