Stream flow information is essential for many important uses across a broad range of scales, including global water balances, engineering design, flood forecasting, reservoir operations, navigation, water supply, recreation, and environmental management. Growing populations and competing priorities for water, including preservation and restoration of aquatic habitat, are spurring demand for more accurate, timely, and accessible water data. To be most useful, stream flow information must be collected in a standardized manner, with a known accuracy and for a long and continuous time period.
The U.S. Geological Survey (USGS) operates over 7000 stream gauges nationwide, which constitute over 90% of the nation's stream gauges that provide daily stream flow records, and that are accessible to the public. Most stream flow records are not based on direct measurement of river discharge, but are derived from continuous measurements of river elevations or stage. These stage data, recorded to 3-mm accuracy are then converted into discharge by use of a stage/discharge relation (rating) that is unique for each stream gauging location. Because stream beds and banks are not static, neither is the stage discharge rating. Much of the effort and cost associated with stream gauging lies in establishing and updating this relation. Ten years ago, USGS personnel would visit stream gauging stations 8 to 10 times a year to make direct measurements of river depth, width, and velocity using mechanical instruments: a sounding rod or cable, a tagline, and a current meter. From these data, flow rates were computed.The range of measured flow and concurrent river stages were then used to build the rating curve for each site and to track changes to the rating curve.