American River Basin: declining snowpack through century-end

rising temperatures expected over rest of 21st Century

The American River Basin in the Sacramento, California region faces declining annual snowpack through the end of this century according to a study released by the U.S. Bureau of Reclamation (USBR).

The American River Basin Study reports “an increased variability of fall and winter precipitation will amplify the severity of droughts and flooding in the basin.”

The 3,600 square mile area of interest in central California spans the Sacramento valley through foothills to the top of the Sierra Nevada. A noteworthy site affected in the basin is the multi-county 198.72 MW hydroelectric powerhouse at Folsom Dam and Lake.

Maximum temperatures are projected to increase. The most significant increase of 7.3 degrees Fahrenheit during the summer months by the end of the 21st century.

Projections of average annual precipitation are uncertain, but climate projections indicate a change in past and present precipitation timing and variability. More variability in rainfall is projected into the future. The timing is likely to shift away from fall and spring rains to winter and summer precipitation.

In addition, snow precipitation will decrease due to warming, shifting the timing of peak snowmelt runoff by more than a month by the mid- to late century.

Among other adaptation strategies are under way to increase agricultural and urban water use efficiency. New adaptation strategy portfolios were also developed for further evaluation by Reclamation and its collaborators to maintain a balance between supply and demand.

For example, adaptation portfolios included evaluating:

  • Use of existing diversion facilities on the Sacramento River and exchange water supply to reduce reliance on Folsom Reservoir and the American River
  • Raising Folsom Dam to increase flood control space
  • Releasing flood water earlier to recharge groundwater and create additional regional water supply
  • The effectiveness of the flow management standard for the Lower American River in the 2015 update of the Sacramento Water Forum Agreement to reduce the effects on the river’s ecosystem and fisheries from climate change

“Water management in the basin is expected to be more challenging in the future due to climate pressures that include warming temperatures, shrinking snowpack, shorter and more intense wet seasons and rising sea levels,” said California-Great Basin Regional Director Ernest Conant.

The basin study was selected in 2017 and built upon the Sacramento and San Joaquin Rivers Basin Study, a USBR technical report completed in 2016.

Reclamation developed the study in collaboration with the Placer County Water Agency, City of Roseville, City of Sacramento, El Dorado County Water Agency, city of Folsom and Regional Water Authority. Non-federal partners coordinated with the Sacramento Area Flood Control Agency to address flood risks.

Other contributors to the report included the California Department of Water Resources, University of California Davis, The Water Forum, Sacramento Municipal Utility District (SMUD), and El Dorado Irrigation District.

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