California reservoirs rebound after spring melt

NASA Earth Observatory published this original article on June 7, 2023. Edits by EarthSky.

California reservoirs rebound

Shasta Lake, California’s largest reservoir, filled to nearly 100% capacity in May 2023, reaching levels not seen for four years. Since 2019, a prolonged period of extreme drought resulted in dwindling reservoir levels. In the early months of 2023, heavy rains and meltwater from an above-average mountain snowpack caused a notable turnaround.

The rising water level is apparent in the images above. The first shows Shasta Lake on November 18, 2022. On this date, the lake stood at 31% capacity, according to the California Department of Water Resources (DWR). In the middle image, from January 29, 2023, the Operational Land Imager (OLI) on the Landsat 8 satellite showed Shasta at 56% capacity. And by May 29, an image from Landsat 9 showed the lake was 98% full. The tan fringe, or “bathtub ring,” around its perimeter had vanished.

The color of the water in the center image likely appears greener because of suspended sediment. In the right image, some portions of the lake surface appear lighter due to an optical phenomenon known as sunglint, and suspended sediment may also be present.

Other California reservoirs

Lake Oroville, the state’s second-largest reservoir, was also near capacity on May 29, at 97% full. Both Lake Oroville and Shasta Lake are critical not only for water storage, but also for flood control, recreation, irrigating cropland in the Central Valley, and preventing saltwater intrusion into the Sacramento-San Joaquin Delta.

Full reservoirs do not ensure plentiful water for years into the future. The past four years are a testament to how drastically reservoirs can change over the course of one or two years. In addition to the many demands for water, lake levels need to be drawn down to create capacity for flood control in wetter seasons. The California DWR is collaborating with other agencies to incorporate better forecasting and observation technologies in order to optimize water releases.

Surface water vs. groundwater

Plentiful surface water does not necessarily equate to replenished groundwater stores. In California’s Central Valley, groundwater may account for 2/3 of agricultural water use during drought years. A recent study using data from NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellite missions found that groundwater depletion in the Central Valley has been accelerating since 2003.

Officials in California are working to leverage the recent influx of water. While some groundwater recharge happens naturally, resource managers can employ other strategies to send water underground, such as diverting it into canals or ponds and injecting it into the subsurface through wells.

Bottom line: The heavy snowpack over the winter led to a spring thaw that is filling up California reservoirs again. Learn more about surface water such as reservoirs and groundwater storage.