Storms boost state’s already rising major reservoirs
Nearly all of California’s major reservoirs are holding more than 100% of their historic average water levels for this time of year after recent storms across the state gave several a boost.
Average total water storage across the state’s 48 biggest reservoirs has also been climbing since the beginning of the year, topping 70% of capacity as of Thursday, according to state data. California’s reservoirs overall are holding 118% of their average levels of water for this time of year.
California’s reservoirs, which are a major part of the state’s water supply, are not just affected by rainfall — runoff from the state’s snowpack, the elevation of and terrain around the reservoirs and flood control measures all impact how much water is in an individual reservoir.
Fluctuations in reservoir levels in the winter rainy season are largely driven by flood-control requirements, not by storms, said Jeanine Jones, interstate resources manager with the California Department of Water Resources. Officials are carefully managing reservoirs, planning releases to ensure they do not overflow and cause floods while also maintaining enough water in case of another drought, according to the state water department.
As reservoirs encroach into their flood-control space, which is governed by a rule developed by the U.S. Army Corps of Engineers, reservoir managers make releases, she explained, “metering it out carefully so it doesn’t cause problems downstream.”
She also noted that reservoirs overall have been carrying more water than average, thanks to last year’s wet winter. Statewide reservoir levels have been elevated since storms in late 2022 and early 2023 brought a deluge of water to the state — bumping overall water
storage in reservoirs over 100% of the historic average last year for the first time since 2019, after years of dwindling water levels due to drought.
After powerful storms last weekend and early this week, nearly all of the state’s major water supply reservoirs saw a slight increase in the total amount of water they were holding compared with the week before.
While most of the major reservoirs were already holding more than their historic average amount of water for this time of year a week ago, the recent rains bumped a couple — Trinity Lake in Northern California and Castaic Lake in Southern California — over the 100% mark.
Lake Casitas in Southern California and Millerton Lake in Fresno County are both just under 100% of their historic average water levels, at 97% and 95%, respectively. The San Luis Reservoir near Pacheco Pass is at 81% of its historic average. The off-stream reservoir is a holding facility for the State Water Project and the federal Central Valley Project and stores water pumped in from the Sacramento-San Joaquin River Delta for use in the San Joaquin Valley and Southern California, according to the state parks department.
Reservoirs that supply water to the Bay Area, such as Hetch Hetchy and Pardee, also saw a small bump in water levels over the past week. Hetch Hetchy went from 86% of its total capacity to 89%, or 141% of its historic average for this time of year, while Pardee went from 85% to 86%, or 99% of its historic average.
Jones also noted that groundwater and snowpack are also important parts of California’s water picture. The state’s snowpack this year has been lagging compared with typical winters, though the recent storms gave it a boost.
With all of the factors at play, Jones explained, one storm or series of storms doesn’t have much of an impact on California’s water supply, she said.
“It takes a season’s worth to make a difference,” she said.