An instrument or installation that measures the water level (stage) of a river or stream continuously. Stream gauges provide the real-time data essential for flood warning, water supply management, and hydrological research.
A stream gauge (also spelled gage in USGS terminology) is the instrument or set of instruments used to continuously measure and record the water surface elevation (stage) in a river, stream, or canal. Traditional stream gauges use a float and counterweight system inside a stilling well connected to the stream via intake pipes, with the float position recorded by a shaft encoder or data logger. Modern alternatives include non-contact sensors such as radar level sensors and ultrasonic distance sensors mounted above the water surface, as well as submersible pressure transducers that measure hydrostatic pressure. The USGS Streamgaging Program, authorized in 1889, is the largest and most comprehensive in the world, operating over 13,000 continuous-record stations that transmit data in near-real-time via GOES satellite and cellular telemetry to the National Water Information System (NWIS). Stream gauge data serve as the foundation for flood forecasting by the National Weather Service, water supply and allocation decisions, dam safety monitoring, infrastructure design criteria, environmental flow assessment, and long-term hydrological trend analysis. The data are freely available in real-time at waterdata.usgs.gov. Some USGS stream gauges have been in continuous operation for over a century, providing invaluable long-term records for understanding hydrological variability and climate change impacts on water resources. The Cooperative Water Program partners federal, state, and local agencies to share the costs of operating this critical national monitoring infrastructure.