The USGS Water Science School

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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 3 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; using the stage-discharge relation to convert the continuously measured stage into estimates of streamflow or discharge

## Streamflow summary

Streamgaging involves obtaining a continuous record of stage, making periodic discharge measurements, establishing and maintaining a relation between the stage and discharge, and applying the stage-discharge relation to the stage record to obtain a continuous record of discharge. The USGS has provided the Nation with consistent, reliable streamflow information for over 115 years. USGS streamflow information is critical for supporting water management, hazard management, environmental research, and infrastructure design. For more information on USGS streamgaging, go to the USGS Web site at http://water.usgs.gov. The National Streamflow Information Program offers more information on this topic., Go to the USGS Office of Surface Water Web site for more information on surface-water activities, and the USGS WaterWatch site gives you current streamflow conditions nationwide or in your area.

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