What is a rating curve? Why does it change over time?

In order to convert water height (or “stage”, usually expressed as feet) into a volume of water (or “discharge”, usually expressed as cubic feet per second), USGS hydrographers must establish a relationship between them. This stage-discharge relationship is called a rating curve. It’s developed by making frequent direct discharge measurements at stream gaging stations.

The rating curve depends on the hydraulic characteristics of the stream channel and floodplain, and will vary over time at almost every station. There might be subtle changes to a stream channel, such as the growth of aquatic vegetation in the summer, frequent shifting of a sand-bed stream bottom, catastrophic changes due to floods, or man-made changes such as construction of a bridge. These changes might require only minor or temporary adjustments to streamflow records, or could require a complete reevaluation of the rating curve.

The USGS Waterwatch Toolkit includes a Customized Rating Curve Builder to generate rating curves for individual gaging sites. More information about site-specific rating curves is available from the USGS Water Science Center that manages the site.

Learn more:

Related Content

Filter Total Items: 8

Is there a way to get alerts about streamflow conditions?

Yes! The USGS offers two services: WaterAlert - automated emails or text messages are sent to you whenever certain parameters (that you define) are exceeded at one of our gaging stations. WaterNow - Send an email or text message to WaterNow@usgs.gov containing the USGS Site Number of the gage you want to query (optionally add parameter codes to...

Why does the USGS use the spelling "gage" instead of "gauge"?

The spelling of “gage” is part of our very rich USGS history. In 1888, USGS Director John Wesley Powell met a very forward-thinking graduate student named Frederick Haynes Newell. Powell was so impressed that he made Newell the first full-time appointee to the new Irrigation Survey, which was created to investigate the potential for dams and...

Does "stage" tell you how much water is flowing in a stream?

Not directly. You cannot say that because a stream rises (doubles) from a 10-foot stage to a 20-foot stage that the amount of flowing water also doubles. Think of a cereal bowl with a rounded bottom. Pour one inch of milk in it. It doesn't take much milk to make it up to the one inch level because the bowl is at its narrowest near the bottom. Now...

What does the term "stream stage" mean?

Stream stage is an important concept when analyzing how much water is moving in a stream at any given moment. "Stage" is the water level above some arbitrary point in the river and is commonly measured in feet. For example, on a normal day when no rain has fallen for a while, a river might have a stage of 2 feet. If a big storm hits, the river...

Why are there sometimes differences between USGS and National Weather Service river stages?

At some USGS stream-gage installations, NOAA’s National Weather Service (NWS) maintains a separate stage sensor that is serviced by NWS technicians. Calibration of any sensing device can occasionally drift from a "true" value, so there might be differences between USGS and NWS data reports. USGS personnel visit installations on an interval of 6...

Why might USGS streamflow data be revised?

Real-time USGS streamflow data are PROVISIONAL, meaning that the data have not been reviewed or edited. These data might be subject to significant change and are not official until reviewed and approved by the USGS. Real-time streamflow data can be affected by: backwater from ice or debris such as log jams algae and aquatic growth in the stream...

Sometimes the USGS real-time stage data seems too high (or too low). Are the USGS data inaccurate?

There can be occasional equipment or database problems where erroneous data are reported for short periods of time until corrections can be made. This is why it is important to look at a record of streamflow (like the 7-day hydrograph plots) rather than a single point in time. However, most of the time the USGS has a high level of confidence in...

Where can I get real-time and historical streamflow information?

The best starting point for USGS streamflow data is the interactive National Water Information System (NWIS): Mapper website. Zoom in to your area of interest or use the search options in the left navigation window. The map displays active surface-water sites by default, but you can change the type of water site (surface-water, groundwater,...
Filter Total Items: 3
Date published: February 1, 2017

The Vital Nature of Streamgaging

Gary Moore spent the last three days of 2015 stacking hefty bags of sand in front of a fellow church member’s brick home. With only 1,000 feet between the house and the swelling Mississippi and Meramec Rivers, Moore and other volunteers worked quickly, in frigid temperatures, to assemble a 10-foot high, 1,000-foot-long sandbag wall to ward off floodwaters.

Date published: August 22, 2016

Fighting the Floods

The USGS response to the Louisiana floods is part of the larger USGS flood science mission...

Date published: February 26, 2013

Stay Current on Your Rivers with USGS WaterNow

For the first time, anyone can find out the current conditions on thousands of rivers and streams across the country, right from their phone, using USGS' latest system WaterNow.WaterNow makes the water conditions monitored by more than 16,000 streamgages and other sites across the country available via text or email. 

Filter Total Items: 14
A U.S. Geological Survey hydrologic technician, uses an Acoustic Doppler Velocimeter to measure stream flow on a Florida creek.
September 2, 2016

Measuring streamflow

Neil Yobbi, U.S. Geological Survey hydrologic technician, uses an Acoustic Doppler Velocimeter to measure stream flow on Curlew Creek in Tampa, Florida. While Hurricane Hermine might have made landfall almost 200 miles away in St. Marks, Florida, a USGS rain gauge in Pinellas County, Florida, still measured more than 16 inches of total rain during the past three-day storm

Image: Ice Jam Remnants
April 28, 2015

Ice Jam Remnants

On April 29, hydrologic technicians Anthony Underwood and Jeremiah Pomerleau visited the USGS gaging station on the St. John River at Ninemile Bridge and found a sea of broken up, dirty ice left behind by a recent ice jam.

According to Anthony, photos don't do the size and scale of the ice chunks any justice.

February 24, 2015

Under The Ice

On February 26, Nick Stasulis and Charlie Culbertson visited USGS station 01054200 Wild River at Gilead, Maine to make a streamflow measurement through the ice. For these measurements, 20-30 holes are drilled through the ice and a current meter (the spinning yellow cups) is used to measure velocity. River depths and distance across the channel are also measured. After this

Image: Time-Lapse of Measuring Streamflow
December 8, 2014

Time-Lapse of Measuring Streamflow

This time-lapse photo shows the process that U.S. Geological Survey hydrographers use to measure streamflow across the cross-section of a wadeable river. USGS hydrographers follow standard, documented techniques and methods to ensure high-qualtiy, reliable data.

December 11, 2012

Streamflow Gaging Station and Measurement on San Pedro River, AZ

USGS employee, Hanna Coy, talks about stream gauging.

November 13, 2012

Stage Discharge Ratings Class

USGS employees talk about the importance of getting accurate data from stream gauging and the benefit of the stage discharge ratings class.

Attribution: Water Resources
Image: Measuring Streamflow with an Acoustic Doppler Current Profiler
May 10, 2011

Measuring Streamflow with an Acoustic Doppler Current Profiler

USGS boat crew measuring the streamflow with an acoustic doppler current profiler (ADCP) at the White River at Chlarendon, AR.  Water Stage was 37.5 ft at 3:00 pm on May 10, 2011. Driving the boat is Dwight Lasker of the Arkansas Water Science Center, to the back right is Brandon Cobb from the Tennessee Water Science Center, and in the foreground operating the computer is

Attribution: Water Resources
Image: Streamflow and Water Level Measurements
April 8, 2011

Streamflow and Water Level Measurements

USGS scientists John Miller and Brent Hall measure the streamflow and water levels at the flooded Ditch 14 near Mapleton, ND. What appears to be a lake is actually a flooded cornfield.

Image: Measuring Streamflow
October 5, 2010

Measuring Streamflow

USGS hydrologic technician measuring streamflow in the Merced River, near Happy Isles stream gage in Yosemite Valley.

Attribution: Water Resources
October 14, 2009

Streamgages: The Silent Superhero

Whether you drink water from your tap, use electricity or canoe down your local river, chances are you benefit from USGS streamgage information. So what is a streamgage and what does it do for you? This CoreCast episode gives you the inside scoop on your silent superhero.

Transcript and captions available soon.

Diagram showing the stream stage (height) and streamflow (discharge) relation

The stream stage (height) and streamflow (discharge) relation

An example of a stage-discharge relation is shown in the diagram. The stage-discharge relation depends upon the shape, size, slope, and roughness of the channel at the streamgage and is different for every streamgage.

The development of an accurate stage-discharge relation requires numerous discharge measurements at all ranges of stage and streamflow. In addition,

River Stage: Is the water level measured in feet above a point, usually somewhere below the river bed.

USGS Storm Words: River Stage

The USGS has many missions before, during and after a major storm. Here is an explanation of a word we commonly use with our science.

River Stage: Is the water level measured in feet above a point, usually somewhere below the river bed. The USGS’ real-time streamgage network is able to indicate when a monitored river reaches a stage where the water will start