Over the past century, the amount of snowmelt runoff from the Sierra Nevada flowing into the Sacramento River and San Joaquin River during the spring has declined.

In the spring, with warmer air temperatures, more sunlight and solar radiation, the snowpack in the Sierra Nevada melts and releases water as runoff. This snowmelt runoff – which typically occurs from April through July – feeds the Sacramento and the San Joaquin Rivers, which serve as major sources of water for the state. For more information, download the Snow-melt Runoff chapter.

This graph presents the percentage of total runoff that flows into the Sacramento River between April and July. Percentages are shown for each water year (from October through September of the following year).

• The fraction of snowmelt runoff that flows into the Sacramento River between April and July (“spring”) has decreased by about eight percent per century and varies greatly from year to year.

• A similar decline has been observed for spring runoff into the San Joaquin River.

Runoff in a Sierra Nevada mountain stream in Placer County, taken April 17, 2012.

Credit: Dale Kolke/California Department of Water Resources

• Spring runoff has historically provided about one-third of the state’s annual supply for agriculture and urban needs.

• Increased tree deaths and wildfires have been linked to declining snowmelt.

• Reduced streamflow impairs cold water habitats, making conditions unfavorable for fish such as trout and salmon.

• Changes in the timing of peak runoff challenge California’s infrastructure, which was designed for historical conditions. 

• A changing climate affects the timing (and volume) of snowmelt runoff. During the winter, warmer temperatures mean less snow and more rain, resulting in less snowpack. The earlier arrival of warmer temperatures in the spring causes snow to melt earlier in the year.

• California Department of Water Resources, Hydroclimate Report (multiple years)