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How Streamflow is Measured

If you're a teenager, I imagine your favorite activity is to sit with your parents on a quiet river bank, drink your glass of lemonade, and ponder the complexities of life. Probably the first question you ask is "How much water is flowing in this river?" You've come to the right place for an answer. The U.S. Geological Survey has been measuring streamflow on thousands of rivers and streams for many decades and by reading this set of Web pages you can find out how the whole streamflow-measurement process works.

Often during a large rainstorm you can hear an announcement on the radio like "Peachtree Creek is expected to crest later today at 14.5 feet." The 14.5 feet the announcer is referring to is the stream stage. Stream stage is important in that it can be used (after a complex process described below) to compute streamflow, or how much water is flowing in the stream at any instant.

Stream stage (also called stage or gage height) is the height of the water surface, in feet, above an established altitude where the stage is zero. The zero level is arbitrary, but is often close to the streambed. You can get an idea of what stream stage is by looking at this picture of a common staff gage, which is used to make a visual reading of stream stage. The gage is marked in 1/100th and 1/10th foot intervals.

Introduction to USGS Streamgaging

The U.S. Geological Survey (USGS) started its first streamgage in 1889 on the Rio Grande River in New Mexico to help determine if there was adequate water for irrigation purposes to encourage new development and western expansion. The USGS operates over 7,000 streamgages nationwide. These streamgages provide streamflow information for a wide variety of uses including flood prediction, water management and allocation, engineering design, research, operation of locks and dams, and recreational safety and enjoyment.

Streamgaging generally involves 4 steps. Click on the links below to explore each topic.

1. Measuring stream stage—obtaining a continuous record of stage—the height of the water surface at a location along a stream or river
2. The discharge measurement—obtaining periodic measurements of discharge (the quantity of water passing a location along a stream)
3. The stage-discharge relation—defining the natural but often changing relation between the stage and discharge, and
4. Converting stage information to streamflow information—using the stage-discharge relation developed in step 3 to convert the continuously measured stage into estimates of streamflow or discharge

Streamflow summary

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Sources and more information

• Discharge measurements at gaging stations, USGS Techniques manual
• A Day in the Life of a USGS Water Scientist

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How the stream height (stage) relates to the amount of water flowing in a stream
Real-time USGS streamflow data

The water cycle: Streamflow