Marin County plans to use Phoenix Lake for flood-mitigation effort

By John Ramos

As Marin County residents prepare for what could be increased flooding this winter, officials have developed a plan to use an existing reservoir as a buffer for the heavy rains to come. And on Monday, they got a small reminder of why that may be necessary.

By noon, the water was already pouring into the street in Larkspur. It wasn’t a king tide, but a graph on the NOAA website showed it was about 2 feet higher than expected. Fueled by El Niño, or Kelvin waves, or some other unseen force of nature, Wendie Odwyer was in a good spot to see it happen.

“Not a normal high tide,” she said, looking at the water seeping under the sandbags guarding the back parking lot. “No, this is special.”

Odwyer is the office manager for Pipe Spy, a plumbing company that is located just where all the water from the Tamalpais watershed flushes into the Bay. And if it wasn’t for a 3-foot-high berm out front, it looked like the water would have been into the building.

“I think they’ve anticipated it, so it wasn’t quite that quickly,” she said as the water began to recede. “Because they knew with El Niño and the high tides — That’s why we have all the sandbags and the berms.”

No damage was done on Monday, but with winter storms on their way, Marin County and the Marin Municipal Water District have been developing plans to control how much water will be coming down off of Mt. Tamalpais.  

“One thing that was identified was the opportunity to draw down Phoenix Lake to provide more room to absorb the early season runoff from storm events into a lower lake capacity,” said Jessie Underhill, a communications specialist for Marin Water.

Phoenix Lake is the smallest of the district’s seven reservoirs, and it sits right above the town of Ross. The plan is to drain off a large portion of the water, leaving room as a catch basin for any atmospheric rivers that may come along, preventing increased flooding downstream. And the idea was born, a bit, by accident.  

Marin County Supervisor Brian Colbert said, in 2021, the area was suffering from a prolonged drought, and Phoenix was at a dangerously low level. But then, in late October of that year, a series of heavy rainstorms moved in.

“And then people were paying attention and noticed, ‘Oh, my goodness, Phoenix Lake is capturing a lot of the water.’ Which would then be running downstream, or could potentially lead to increased flooding or ground saturation later on in the winter,” Colbert said. “And so, we kept that in mind, and here we are. This seems like a particularly good time to revisit that, thinking, ‘What can we do to get ahead and prepare?”

The reservoir was never built to be a flood control facility, but that’s what it will be used for now. It currently stands at 120 percent of normal for this time of year, but by using it as the district’s primary source of normal water delivery, the level should draw down fairly quickly.

“We’re not releasing the water,” said Underhill. “We’re using the water in our system, and using this as an opportunity just to lower that reservoir level.”

As for Odwyer, she just appreciated that people are planning ahead, so days like Monday don’t have to come as a surprise to anyone.

“They know this is happening,” she said. “In fact, my boss called me a little while ago and said, ‘How high IS it?”

Phoenix Lake won’t be drained completely; that would degrade water quality. But the district said it will be brought down to about the level it was in 2021.  And they said the plan marks the beginning of a new collaboration in the county’s flood control efforts.