Over the next four to six weeks, the district plans to remove between 60 and 160 acre-feet of water — roughly the volume of 29 to 70 Olympic-size swimming pools. The drawdown is intended to leave more room in the reservoir to capture runoff when early-season storms arrive.

“This effort is being done in advance of the winter,” said Jessie Underhill, a Marin Municipal Water District spokesperson. The added capacity, Underhill said, could help absorb runoff from the first storms of the season.

Phoenix Lake, west of Kentfield and Ross, supplies drinking water to more than 191,000 Marin County residents. Water released from the reservoir can flow into Ross Creek and eventually Corte Madera Creek. County officials said a lower lake level after the 2021 drought helped capture runoff that otherwise would have entered those waterways.

The potential flood reduction is not guaranteed. Officials said results will depend on when storms arrive, how intense and prolonged they are, and how much water is in the lake at the time.

The preparations come after heavy rain and king tides flooded parts of Marin in January, causing road closures, property damage and interruptions to emergency services. Forecasters and climate scientists have warned that a possible strong El Niño could bring a wet winter, though the effects of individual storms remain uncertain. The Marin County Board of Supervisors approved an additional $3 million for winter-storm preparedness last month.

Residents can sign up for emergency notifications through AlertMarin. County officials say preparations can reduce risks but cannot eliminate the possibility of flooding.