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Reclamation publishes overview of Colorado River evaporation history

Media Contact: Michelle Helms 702-726-1921
For Release: Feb 8, 2024

BOULDER CITY, Nev. - The Bureau of Reclamation today published an overview of historical natural losses along the lower Colorado River. The Mainstream Evaporation and Riparian Evapotranspiration report looks at water surface evaporation, soil moisture evaporation, and plant transpiration. It will be used by Reclamation as a source of data as it manages regional water operations and to improve the agency’s modeling efforts.   

 “Reclamation’s approach to water management in the Colorado River Basin and across all Reclamation states is based on best available science, transparency, and inclusivity.” said Reclamation Commissioner Camille Calimlim Touton. “The release of the Mainstream Evaporation and Riparian Evapotranspiration study today evidences this commitment by informing our partners and the public about river and reservoir evaporation and transpiration in the Colorado River Basin.”   

 The report provides an overview of average mainstream losses from both river and reservoir evaporation, as well as the evaporation and transpiration associated with vegetation and habitats along the river. The report states that approximately 1.3-million-acre feet of losses occur annually along the lower Colorado River mainstream. Based on data from 2017 to 2021, approximately 860,000 acre-feet of Colorado River water is lost to evaporation occurring annually from Lake Mead to the border with Mexico. A further 445,000 acre-feet is lost to evaporation and transpiration from natural vegetation and habitats.   

Reclamation is committed to addressing the challenges of climate change and drought in the Colorado River Basin, using science-based, innovative strategies. As Reclamation continues working cooperatively with the basin states, tribes, stakeholders, partners, and the public who rely on the Colorado River, we are also deploying historic funding and resources from President Biden’s Investing in America agenda that increase near-term water conservation, build long term system efficiency, and prevent the Colorado River System’s reservoirs from falling to critically low elevations that would threaten water deliveries and power production. As a result of the commitment to record volumes of conservation in the Basin, as well as recent hydrology, the Interior Department announced in October 2023 that the chance of falling below critical elevations has been reduced to eight percent at Lake Powell and four percent at Lake Mead through 2026. Lake Mead is currently about 40 feet higher than it was projected to be at this time last year. 

 The Mainstream Evaporation and Riparian Evapotranspiration report is available on the Reclamation website.  


The Bureau of Reclamation is a federal agency under the U.S. Department of the Interior and is the nation's largest wholesale water supplier and second largest producer of hydroelectric power. Our facilities also provide substantial flood control, recreation opportunities, and environmental benefits.

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