When it rains, it truly makes a significant impact—and this is particularly true for California’s water resources. After a somewhat sluggish beginning to the winter season, a series of atmospheric river storms have unleashed an impressive amount of rain over the past three weeks, delivering billions of gallons into the state’s reservoirs. This influx has calmed the worries of water officials and greatly diminished the chances of facing water shortages during the upcoming summer months.
Since December 16, Shasta Reservoir, which is California's largest body of water stretching 35 miles near Redding, has experienced a remarkable increase in water levels, rising by 36 feet. As of Monday, it was filled to 77% capacity, surpassing its historical average for this time of year by reaching 129%.
Similarly, Oroville Reservoir, the second-largest in the state, has seen its water level rise by an impressive 69 feet during the same period. This reservoir, created by the Oroville Dam in Butte County, was reported to be 73% full and at 134% of its historical average as of Monday. The volume of water added in just the past 24 hours was nearly equal to the total capacity of the Crystal Springs Reservoir in San Mateo County when full.
Jeffrey Mount, a professor emeritus at UC Davis and a senior fellow at the Public Policy Institute of California’s water center, expressed confidence in the current situation: "We’re in great shape."
In total, Shasta and Oroville reservoirs have captured an astonishing 1.6 million acre-feet of water over the last three weeks. To put that into perspective, that’s equivalent to filling four reservoirs the size of Hetch Hetchy, which is the largest reservoir serving San Francisco, enough to supply water for approximately 9 million people for an entire year.
In addition, there has been a notable improvement in the Sierra Nevada snowpack, which accounts for one-third of California’s annual water supply. On December 16, the snowpack was at a mere 18% of its normal levels, causing ski resorts to struggle for business. However, following a weekend of heavy snowfall, it surged to 90% of normal levels by Monday, thanks to several feet of fresh powder across the Lake Tahoe area.
California typically receives the majority of its rainfall between December and March, and with the wet season still ongoing, Mount pointed out, "We’re not even halfway through the wet season. We can’t predict how it will all pan out. In previous years, we’ve had seasons with early rains followed by prolonged dry spells. Nevertheless, the substantial increase in water levels over the past few weeks significantly reduces the risk of summer shortages."
The past three winters have been either average or above average in Northern California, where most major reservoirs are located. This trend has led to higher starting water levels as they entered this winter season.
As a result, nearly all of California's significant reservoirs were reported to be above their historical average capacity for this time of the year as of Monday. For instance, the San Luis Reservoir, an expansive inland body of water located between Gilroy and Los Banos, was at 70% capacity, while Los Vaqueros Reservoir, the largest in Contra Costa County, was filled to 90%.
In Southern California, the Cachuma Reservoir, which holds the title of the largest in Santa Barbara County, reached full capacity at 100% and began spilling on Monday. Even further south, Diamond Valley Lake in Riverside County, critical for supplying water to 20 million residents in the Los Angeles area, was reported to be 94% full.
Smaller communities also reflected similar positive trends. All seven reservoirs managed by the Marin Municipal Water District were at 99% capacity. Additionally, Loch Lomond, which serves around 100,000 residents in the Santa Cruz area, reached full capacity of 100% on Sunday and started releasing water down the spillway.
Chris Coburn, deputy director of the Santa Cruz City Water Department, noted, "We’ve received about a foot of rain since Christmas Eve in the Santa Cruz Mountains. As someone responsible for managing water resources, this is certainly uplifting. It provides reassurance that we are well-equipped to meet customer demand for at least the next year. We’re always grateful for the rain."
Operators at various reservoirs, including Shasta, Oroville, and Folsom near Sacramento, have recently increased water releases to create additional capacity, allowing them to capture more water safely if more storms hit in the coming weeks. Mount emphasized, "If they fill the reservoir too quickly, the next storm could cause overflow, which is something they want to avoid. It’s essential to manage the flow carefully to minimize flooding risks downstream."
The East Bay Municipal Utility District, which serves 1.4 million residents in Alameda and Contra Costa counties, reported that its seven reservoirs were at 82% capacity on Monday. Andrea Pook, a spokesperson for East Bay MUD, remarked, "This is all encouraging news. We still have several months ahead of us, and we will have a clearer picture by April. However, these critical wet months are vital for replenishing our reservoirs. We hope to see continued storms throughout the remainder of winter and into early spring."
Pook also offered advice to homeowners, saying, "Be sure to turn off your irrigation systems."
Meanwhile, the nine reservoirs operated by the Santa Clara Valley Water District, catering to 2 million South Bay residents, were at 50% capacity on Monday. This lower percentage can be attributed to the South Bay receiving less rainfall than other regions in recent weeks and due to state regulations that prevent certain reservoirs from being filled to capacity without earthquake retrofitting. Notably, Anderson Reservoir, the largest in the district near Morgan Hill, remains nearly empty as it undergoes construction to meet seismic safety standards.
So far, Northern California has avoided significant flooding, and this fortunate trend is expected to continue, as forecasts indicate a stretch of ten days of dry weather commencing Tuesday.
Jan Null, a meteorologist from Golden Gate Weather Services, stated, "The positive aspect is that nearly every area in the state has recorded above-normal precipitation for this time of year. However, starting Tuesday, we’re entering a drier phase. This year showcases stark contrasts—periods of extended dryness followed by intense wetness."
