Photo: the damaged spillway at Oroville Dam last week (source: yahoo.com)
California’s State Water Project collects snowmelt from the Northern Sierra in Lake Oroville and moves the water in aqueducts to Central California farms and Southern California cities. The project’s design is based on two key tenets: that the Sierra snowpack stores up to 30 percent of the state’s water supply through the winter, and that the snow melts just in time to meet the summer water demands of crops and cities. But data tells a different story.
In 1906, the State of California’s Department of Water Resources began measuring how much water flows into the Sacramento River when the Sierra snowpack melts. The first 50 years of data show river flow peaking in April. From the mid-1950s to 2007, however, the Sierra snowpack melted earlier and river flow peaked in March (See Figure 3-20 from the California Water Plan). Warmer temperatures caused the change, and continued warming could melt the snowpack even earlier in the decades ahead (See Figure 3-23 from the California Water Plan).
This is no small change for California’s dam operators, who lower reservoir levels in January, February and March to make room for the runoff caused by winter storms. This is critical to prevent flooding in Northern California. Oroville Dam, for example, captures the precipitation that falls in the 6,000 square mile expanse of the Feather River watershed. Now, and in the decades ahead, dam operators need to capture the melting Sierra snowpack in January, February and March, and manage flood control simultaneously.