Record Lows to Near Brimming: North Bay Reservoirs Stage Remarkable Recovery

Two North Bay reservoirs are flush with water just a year after their levels fell to record-setting lows.

Heading into the second week of July, Lake Sonoma and Lake Mendocino — at 66% and 79% full, respectively — are both well above their average levels for this time of year, and roughly twice as full as they were last summer.

“Unless you count the first year it was filled, Lake Sonoma had dropped to the lowest level ever,” said David Rabbitt, a Sonoma County supervisor and a Sonoma Water director. “To rebound like that in one year was beyond anyone’s imagination.”

As the summer heats up, reservoir levels statewide are collectively at around 87%, the result of the massive amount of precipitation that storms dumped on the state this year. But while other reservoirs across California depend on runoff from the Sierra Nevada snowpack, Lake Mendocino and Lake Sonoma — serving over 600,000 customers in Sonoma and Marin counties — rely on seasonal rains in the hills above the Russian River.

And although this flash-in-the-pan system normally goes through wet-to-dry extremes, human-caused climate change has significantly widened that chasm.

“The weather patterns are so volatile,” said Rabbitt. “Three years of drought can be followed by one year of intense rainfall can be followed by three or more years of drought again. And we seem to be in this pattern.”

He notes that even with the grand reentrance of El Niño this summer — which could usher in another wet, stormy winter — his agency has to prepare for what may be a dry year ahead.

Regardless of what comes from the skies, Sonoma Water, which collectively manages both reservoirs with the Army Corps of Engineers, uses satellite forecasting to watch for coming storms to decide in real time when to release water. When a storm is on its way, the agency can decide to let water out or keep it in the reservoir if there is no storm on track. While it sounds like a no-brainer, the concept is new to how the reservoirs are managed because of rules set up decades ago. This practice allowed the agencies to keep 19,000 acre-feet of water in Lake Sonoma and 5,000 acre-feet of water in Lake Mendocino this winter, instead of releasing it into the ocean.

“While we think of Lake Sonoma as a large reservoir, compared to some of the others in the state, it’s not,” said Nicholas Malasavage, chief of the operations and readiness division at the Army Corps’ San Francisco office. “Imagine the amount of water that could be saved if [the state] utilized that tool to its fullest potential.”

Sonoma Water also uses a series of stationary radar units across the Bay Area to better understand where these storms will drop the majority of their rainfall.

But Malasavage said fuller reservoirs and the possibility of another wet year also raise a concern that was avoided last winter along the Russian River: disastrous flooding.

“Last year we were just hoping for rain,” he said. But this year, with reservoirs fuller, the agency has “to be more methodical and more mindful” of potential flooding when winter begins.

During the rainy season, managers in charge of flows from the reservoirs also allowed farmers to use excess water from the Russian River to submerge their land, a process meant to slowly recharge groundwater aquifers.

“They were just simulating what naturally would have been a rainy season,” said Elizabeth Salomone, general manager for the Russian River Flood Control and Water Conservation Improvement District. “Some of these farmers who did that saw better yields in 2022. Word of mouth got around and now folks are opening up a lot more to it.”

But even with all the rain this year, Salomone said she is worried about both too much and too little rainfall next winter.

“I need to dust off my crystal ball because as a water supplier, I always have to consider the worst-case scenario,” she said.

She says she gets calls from farmers all the time asking whether the drought is over, and tells them they’re asking the wrong question.

“We need to stop focusing on whether we are out of a drought, but instead, ask this question: What if it doesn’t rain when we expect it to rain?” Salomone said. “Because one thing we can expect is that it’s likely not going to occur the way we have expected it to in the past. The historical patterns of precipitation are no more.”