U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3001

January 2011

How Does a U.S. Geological Survey Streamgage Work? 

Printed on recycled paper

Information on the flow of rivers and streams is a vital 

national asset that safeguards lives, protects property, and 

ensures adequate water supplies for the future. The U.S. Geological Survey (USGS) operates a network of more than 9,000 

streamgages nationwide with more than 500 in Texas. 

If you have ever crossed a highway bridge and noticed a 

metal structure with an antenna by the side of it, you’ve probably seen a USGS streamgage (fig. 1). A streamgage contains 

instruments that measure and record the amount of water 

flowing in the river or stream, or its discharge. Generally, these 

measurements occur automatically every 15 minutes or more 

frequently in times of flooding.

Words that have the same meaning used throughout this text:

Stream stage = gage height = water level 

Streamflow = discharge = flow

Streamflow station = streamgage = gaging station = gage

Figure 2. Diagram of a typical streamgage installation with 

equipment used to measure stream stage (by L.S. Coplin, U.S. 

Geological Survey). 

Figure 1. Examples of gage structures located at U.S. Geological 

Survey streamgaging stations. 

USGS computers convert the water-level data into information about the flow of the river. The flow and gage-height data 

are then made available to users over the Internet at http://water.

usgs.gov/nwis/.

How Streamflow Is Measured 

Measuring streamflow in rivers or streams is similar to 

taking the pulse of blood flow in a human body. It is important 

to measure streamflow so that scientists and water managers can 

make informed decisions about a river’s or stream’s health.

Measuring streamflow generally involves several steps. 

These include recording continuous water levels, taking 

discharge measurements in the river or stream, developing a 

mathematical relation between stage and discharge, and then 

applying this relation to the continuous stage record to compute streamflow. Because it is not practical for a streamgage to 

continuously measure streamflow, the gage height of a stream is 

continuously measured. There is a strong mathematical relation 

between this height and streamflow and, as a result, a continuous record of streamflow can be calculated from the record of 

stream stage. 

Measuring the relative height of streams is done many 

times a day through an underwater tube contained in the 

streamgage (fig. 2). Gas is continually pushed through the tube 

into the stream. As the depth of water above the tube opening 

increases, more pressure is required to push the gas through the 

tube. As the water decreases, less pressure is needed to push 

the gas out. This level of pressure is used to calculate the height 

of the water above the tube. These changes in pressure are 

recorded as changes in height, or stage. Stage data are measured 

usually every 15 minutes. When intense rainfall and runoff 

cause the river or stream to rise quickly, the time intervals 

sometimes are shorter. The time interval can be as short as every 

5 minutes. These data are transmitted to a satellite on a preset 

schedule which is usually every 1 to 4 hours. During high water 

or other emergency situations, data will be sent to the satellite 

every 15 minutes to provide more timely data. 
Figure 3. Example of a typical stage-discharge relation or rating 

curve (Nielsen and Norris, 2007, fig. 2).

Figure 4. Diagram of a channel cross section with subsections 

(Olson and Norris, 2007, fig. 3). 

Figure 5. Diagram of how streamflow data are transferred from 

streamgage to the Internet (Sauer and Turnipseed, 2010, fig. 48). 

Publishing support provided by

Lafayette Publishing Service Center

For additional information, contact 

Director, USGS Texas Water Science Center

World Wide Web: http://tx.usgs.gov/ 

E-mail: gs-w-txpublic-info@usgs.gov

The Relation of Height to Streamflow 

The continuous record of stage is converted to streamflow 

by applying a mathematical rating curve. A rating curve (fig. 3) 

is a graphic representation of the relation between stage and 

streamflow for a given river or stream. USGS computers use 

these site-specific rating curves to convert the water-level data 

into information about the flow of the river. 

The rating curve is developed from a number of physical 

discharge measurements collected over a period of time and 

over a range of stages (from low flow to flood stage). Each 

point on the graph represents one discharge measurement. 

Streamflow is measured in cubic feet per second, which 

is equivalent to a box of water measuring 1 by 1 foot with a 

flow rate of about 7.5 gallons per second. The stage-discharge 

relation depends on the shape, size, slope, and roughness of 

the channel at each gage and is different for every streamgage. 

A rating curve often changes after a flood when the physical force of high water movement can change the dimensions 

of the streambed or stream channel. To keep the rating curve 

accurate and up-to-date, USGS hydrologic technicians visit 

each streamgage about once every 6 weeks to measure the flow 

directly and also measure high flows when they happen.

Making Discharge Measurements

In general, river discharge is computed by multiplying the 

area of water in a channel cross section (fig. 4) by the average 

speed, or velocity of the water in that cross section:

Discharge = Area x Velocity

The most common method used by the USGS for measuring velocity is with a current meter. However, a variety 

of advanced equipment can also be used to sense stage and 

measure streamflow. In the simplest method, a current meter 

turns with the flow of the river or stream. The current meter is 

used to measure water velocity at predetermined points (subsections) along a marked line, suspended cableway, or bridge 

across a river or stream. The depth of the water is also measured 

at each point. These velocity and depth measurements are used 

to compute the total volume of water flowing past the line during a specific interval of time. Usually a river or stream will be 

measured at 25 to 30 regularly spaced locations across the river 

or stream. 

Streamflow Data on the Internet

All USGS stage and streamflow information is available in near real time through the National Water Information System Web Interface (NWISWeb) for Texas (fig. 5) at: 

http://waterdata.usgs.gov/tx/nwis/rt). In addition to real-time 

streamgage data, NWISWeb also provides access to daily 

discharges and annual maximum discharges for the period of 

record for all active and discontinued streamgages operated by 

the USGS. 

This information is used by the National Weather Service to forecast flooding and by emergency managers to make 

informed decisions to protect public safety in times of flooding. 

References

Nielsen, J.P., and Norris, J.M., 2007, From the river to you—USGS 

real-time streamflow information: U.S. Geological Survey Fact 

Sheet 2007–3043, 4 p. (Also available at http://pubs.usgs.gov/

fs/2007/3043/.)

Olson, S.A., and Norris, J.M., 2007, U.S. Geological Survey 

streamgaging...from the National Streamflow Information Program: 

U.S. Geological Survey Fact Sheet 2005–3131, 4 p. (Also available 

at http://pubs.usgs.gov/fs/2005/3131/.)

Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement at gaging 

stations: U.S. Geological Survey Techniques and Methods, book 3, 

chap. A7, 45 p. (Also available at http://pubs.usgs.gov/tm/tm3-a7/.)

—Dee L. Lurry