Colorado River: Spring Disturbance Flow

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Detailed Description

For five days in March of 2021, the Glen Canyon Dam released a historically low flow of 4,000 cubic feet per second into the Colorado River. The USGS took this opportunity to collect data in an attempt to verify the measurements being reported by the Lee’s Ferry gaging station, which showed a discrepancy between the amount of water being released at the dam and the water flowing past the gage. This video showcases the data collected and methods used during this likely once-in-a-lifetime opportunity. For more information visit:


Date Taken:

Length: 00:03:59

Location Taken: Lees Ferry, AZ, US

Video Credits

Kurt Schonauer, USGS, Arizona Water Science Center, Hydrologist/Flagstaff Field Office Chief,

Dan Evans, USGS, Arizona Water Science Center, Data Chief,


- The Colorado River is a lifeline to the Southwestern United States. It provides water to nearly 40 million people, is used to irrigate over 5.5 million acres of land, generates billions of dollars in revenue from outdoor recreation, and can produce upwards of 4,200 megawatts of electrical power per year. The water in this system is precisely allocated to states through numerous laws, agreements, and treaties, and every drop is important. A 20-year drought, and especially the extraordinary lack of rainfall in the past year has made measuring discharge below Glen Canyon Dam vitally important to downstream communities all the way to and beyond the Mexican border. Normally the average amount of water flowing out of the Glen Canyon Dam is around 12,000 cubic feet per second, or CFS. Recently the Bureau of Reclamation conducted a historically low and steady release of 4,000 CFS to lower water levels enough to complete dam maintenance. This provided the USGS with a rare opportunity to confirm and/or improve the accuracy of our streamgage at Lees Ferry. An interesting bit of data emerged during the planned low-flow event. The steady discharge coming from the dam indicated a 185 CFS difference with the water being measured 15 miles downstream at the Lees Ferry gage. The 185 CFS might not seem like a large amount of water when compared to the 12,000 CFS average, but to put it in perspective, this 185 CFS difference works out to be 83,000 gallons per minute. So verifying the accuracy of the gauge and determining what section of the 15 miles of channel this additional water was coming from was a crucial part of our mission during this study. The water could be entering the river from springs between the dam and the gage. It could be coming directly from the dam area or the perceived extra water could simply be an error in equipment calibration at the gage. To find the answers we embarked on the most intensive and accurate discharge measuring effort ever performed on the Colorado River between the dam and our Lees Ferry gage. We collected discharge measurements from five cross sections beginning just downstream of the dam, 1.5 miles down from the dam, 4.5 miles from the dam, 9 miles from the dam, and at the Lees Ferry gage. The teams employed highly accurate, Acoustic Doppler Current Profilers, or ADCPs, to make multiple discharge measurements at each cross section over a two-day period. And to further reduce potential instrument error, the teams made measurements with two different types of ADCPs at a few of the cross sections to verify measure discharges. The 15 measurements taken at the gage itself were also used to define the accuracy of the lower portion of the stage versus discharge rating. An opportunity that has not presented itself in 30 years. Our data showed an average flow of 4,200 CFS at these cross sections. And we did not measure any significant gain or loss over the entire 15 mile reach. The intense measurements over the course of the five-day, low-flow event on the stretch of river from Glen Canyon Dam to the Lees Ferry gage, has not only confirm the accuracy of the gaging station, but has also given scientists valuable pieces of data to assist in accurately accounting for the treasured water of the Colorado River.