Thanks to three consecutive wet years, Marin’s reservoirs are above average levels headed into a brewing La Niña winter, officials said.
Last month, federal forecasters said La Niña has arrived and is expected to continue through the winter. The phenomenon, which happens when Pacific Ocean waters off South America are cooler than normal, has been thought to signal a drier season, but experts say the impact in Northern California is less severe — and maybe even inconsequential — compared to southern parts of the state.
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“We are observing La Niña conditions, but at this point it’s quite uncertain what that means for us,” Lucy Croy, water resources manager at the Marin Municipal Water District, said Tuesday while providing a water supply outlook to the district board. “It could mean wet, it could mean slightly dry, but we’ll hopefully get a few atmospheric rivers to keep things wet.”
Atmospheric rivers are storms that drag narrow, concentrated bands of rain from the tropics. These are the severe storms that have been blasting the county for the past three years, and are credited for the district’s plentiful supply.
Last year ended with the district collecting 54.7 inches of rain, which is 104% of normal. The average annual rainfall is about 52 inches, Croy said.
The prior year, the district recorded 62 inches, or about 120% of average. In the 2022-23 season, the rainfall topped off Marin reservoirs and extracted the county from a significant drought. The district recorded 77 inches of rain, or about 150% of the average of 52 inches.
After the three wet winters, reservoirs were left replenished enough to put the district in a strong position.
Then the rainy season got off to an early start this year with storms in October. The district recorded 2.8 inches of rain for the month. As of Nov. 1, storage was above 65,000 acre feet, or about 80% of the total capacity. That put the reservoirs at 123% of average for this time of year, the district said.
“Overall, we don’t expect to be in the place where we’d have to call for conservation come next April,” Croy said.
“It’s nice to be in a good spot and have some time to plan for the dry years, too,” board president Matt Samson said.
The district’s seven reservoirs comprise about 75% of its water supply, holding up to 80,000 acre-feet of water, equal to about a two-year supply. An acre-foot is about 326,000 gallons. The district serves about 191,000 residents.
Croy said the rain on Wednesday brought the district an estimated 1.5 to 2.5 inches.
“It’s been a welcome start to our wet season and encouraging to see some creek levels rise and run off into the reservoirs already,” Croy said. “The early storms will help saturate the soils and set us up for better runoff conditions as the season progresses.”
The district also receives about 25% of its supply from the Sonoma County Water Agency. Last month, the agency and partner organizations agreed on a new water control manual for the Coyote Valley Dam and Lake Mendocino.
Lake Mendocino is a key drinking water source for approximately 650,000 people in Mendocino, Sonoma and Marin counties. The revised manual redefines operating rules using “forecast-informed reservoir operations,” or FIRO, to enhance flood risk management and water supply security.
Croy said the FIRO approach allows the district to look at when storms are coming, or when dry periods are expected, to decide when to store or release water. It enables water managers to plan for up to an additional 11,650 acre-feet of water storage when needed.
Board member Larry Russell expressed concern about the FIRO model, saying there is a “hidden risk” if water is stored and then an unexpected storm hits.
“It could get a little tricky,” he said.
This past year, the North Marin Water District recorded about 25 inches of rain at Stafford Lake, its reservoir in Novato. Its average is about 27 inches.
“We are looking pretty good heading into the winter with a Stafford Lake level slightly above average at this time of the year (182.9 feet) and a planned shutdown of the treatment plant this month as demands reduce,” Tony Williams, general manager of the district, said in an email.
The district serves about 61,000 residents in the greater Novato area and parts of western Marin. It receives about 80% of its water supply from the Sonoma County Water Agency.
“Fortunately, Sonoma Water’s Lake Sonoma is currently at a very good level heading into the winter, similar to last year, at about 90% of storage,” Williams said. “We will be closely following the long-range climate predictions as we do every year but early indication are that the first half of the winter will be less impacted by La Niña conditions and this is usually how Stafford fills up.”
