Rivers & gauges
How Does a River Gauge Work? USGS Streamgages Explained
By the Water Level Atlas team · Last reviewed Oct 21, 2026
A USGS streamgage is a small station beside a river that measures the height of the water surface, usually every 15 minutes, and sends those readings to the U.S. Geological Survey. The water height (called stage or gauge height) is then converted into streamflow using measurements that USGS field crews make by hand at the same spot. Almost every river number on Water Level Atlas starts at one of these stations.
What is a streamgage?
USGS runs more than 13,500 streamgages across the United States. The first one was set up in 1889 on the Rio Grande in New Mexico. A typical station is a small shelter or box on the river bank or a bridge, holding the sensor, a data recorder and a transmitter.
The station does not measure flow directly. It measures stage: the level of the water surface above a fixed reference point at that site. Stage is usually given in feet. See gauge height vs streamflow for how the two are related.
How does it measure the water level?
USGS uses several kinds of sensors. Which one a site uses depends on the river, the bank and the cost.
| Method | How it works |
|---|---|
| Stilling well | A pipe or chamber in the bank or on a bridge pier, connected to the river by underwater pipes. The water inside matches the river level but stays calm. A float, or a pressure, optical or acoustic sensor, reads the level. |
| Bubbler | Gas is pushed through a tube that opens under water. The pressure needed to push out bubbles is directly related to the depth of water above the tube's outlet. Used where a stilling well is not practical. |
| Pressure transducer | A sensor under water that reads water pressure. Listed by USGS among the newer methods. |
| Radar | A sensor that reads the water level using radar signals. Also listed by USGS among the newer methods. |
| Staff gage | A painted ruler fixed in the water. Read by eye during storms and site visits to check the electronic sensor. |
USGS says stage at its gauges is typically accurate to within 0.01 ft (about 3 mm) or 0.2 percent of the stage, whichever is greater.
How often are readings taken and sent?
The recorder typically stores a reading every 15 minutes. During fast rises from intense rain, some stations record as often as every 5 minutes.
Most stations send their data by satellite. Other sites relay data by telephone or radio. In normal conditions, data usually reach USGS about once an hour, or every few hours at some sites. During floods and other critical events, recording and transmission can be more frequent, as often as every 15 minutes. Once the data arrive, they can be viewed online within minutes.
Who checks the gauge?
USGS hydrographers (field technicians and hydrologists) visit each station regularly. USGS describes visits about every 6 to 8 weeks at most gauges, with extra trips to catch very high and very low flows, which are rarer and harder to measure.
On each visit they:
- measure the actual streamflow by hand;
- check the sensor against the staff gage and fixed reference marks;
- service the equipment and fix damage.
To measure flow, the crew works out the river's cross-section area and the water's speed. Flow equals area times velocity. They use current meters, which are lowered at around 25 to 30 points across the channel, or an acoustic Doppler current profiler (ADCP), which uses sound to measure water speed across the channel.
The gauge structure itself is also surveyed against nearby benchmarks from time to time, because floods and settling can move it.
How does a height reading become a flow?
Each hand measurement gives a pair of numbers: the stage at that moment and the flow at that moment. Over many visits, from droughts to floods, these pairs build a "rating curve" for that site. USGS computers then apply the rating curve to every 15-minute stage reading to estimate a continuous flow record.
Rivers change. Banks erode, sand bars move, weeds grow and ice forms. When new field measurements show the river no longer fits the rating curve, USGS adjusts it. This is one reason recent numbers are marked provisional. See provisional vs approved data.
What can go wrong?
Equipment can fail, sensors can be blocked by debris or ice, and floods can damage or destroy a station. USGS notes that equipment or database problems can occasionally produce wrong values for short periods until corrected. Real-time numbers are screened automatically for impossible values, but they are not reviewed by a person until later.
Using this on Water Level Atlas
Our river pages show the latest flow in cfs and gauge height in feet from the USGS station, with the time of the reading, so you can see how fresh it is. Values are labelled provisional when USGS has not yet approved them. Each page also links to nearby upstream and downstream gauges and explains its data sources on our methodology page. To understand the flow number itself, see what is cfs.