A stream gauge is a station on a river, creek or canal that records the water level, called stage, and is usually used to work out how much water is flowing past, called streamflow or discharge. The U.S. Geological Survey (USGS) spells it streamgage, and National Weather Service (NWS) river pages commonly write gage.
What it measures
Stage is the height of the water surface above a fixed reference point at the site, usually reported in feet. One common USGS method is a gas-purge system, pushing gas through a tube into the stream and recording the pressure changes as height.
Discharge is the volume of water moving past the site. It is computed from the area of water in a channel cross section multiplied by the average velocity through that section.
From stage to flow
Measuring flow directly takes a field crew, so a gauge typically relies on a rating curve, the relationship between stage and streamflow built from field measurements. The USGS Utah Water Science Center says its technicians visit each gauge about once every six weeks and also measure high flows when they occur.
Channels erode, fill in and grow vegetation, so ratings are adjusted over time. A stage reading turns into a flow estimate only through that current rating.
Real-time data and river forecasts
The USGS says its gauges typically record river height and streamflow at 15-minute intervals, with data usually reaching USGS computers within an hour and more often during extreme events. As of October 2024, the agency measured continuous water levels at more than 12,000 gauges.
River Forecast Centers (RFCs) use these observations at forecast points along each river. The Arkansas-Red Basin RFC notes that most of its forecast points sit at Army Corps of Engineers or USGS gauging stations, and it turns forecast flow back into stage with the rating curve. The National Water Model also assimilates USGS and Corps streamflow observations in several of its configurations.
What a gauge cannot tell you
A gauge reports conditions at one point. Flash flooding on small streams, dry washes and urban streets can be severe with no gauge nearby, so radar rainfall estimates and spotter reports fill the gaps.
A flow value is only as good as the rating behind it, which is why crews make extra measurements during high water. Stage also has meaning only at that site, which is why each river forecast point carries its own flood stage.