Phoenix Lake might be prized more for its scenic trails than its drinking water, but the drought has water managers exploring a multi-million-dollar project to extract every drop it can from the small reservoir.
Built in 1905-06, Phoenix Lake is the Marin Municipal Water District’s second-oldest and second-smallest reservoir, making up only about a half-percent of the total water supply for 191,000 residents.
The lake is not connected to other reservoirs in the district, making it time-consuming to extract water when needed. Water quality issues such as large amounts of sediment and low mineral content also make the water difficult to treat, leading the district to tap only a portion of the water.
“Right now it’s not really connected to the water system,” said Paul Sellier, district operations director. “We have to do some fairly labor-intensive pipework to set it up so we can pump it to the treatment plant.”
As a result, the district only taps Phoenix Lake during dry water years such as last year. Prior to 2021, the district had not used the lake since the 2013-2014 drought, Sellier said.
One solution being explored would be to connect Phoenix Lake to the nearby Bon Tempe Lake, about one mile to the west, using a pipeline.
District staff said mixing the Phoenix Lake water into Bon Tempe Lake would work to address water quality issues, making it easier to treat and adding more drinking water to the system. In the rainy season, the district would also be able to extract water from Phoenix Lake more frequently, allowing it to draw on the reservoir multiple times as it refills after rains.
While the district can extract about 200 to 250 acre-feet of water from the 411-acre-foot reservoir, the upgrades could allow it to obtain an additional 300 to 600 acre-feet, depending on the options, according to district staff.
The proposal is also reviving an idea by the county government to potentially use the lake to capture runoff that would otherwise flood nearby communities such as Ross and Kentfield during storms. The idea would be to have the district draw down Phoenix Lake in anticipation of upcoming storms, allowing the lake to capture runoff that could swell Ross and Corte Madera creeks.
County officials told the district board that this concept was demonstrated during the heavy storm in October, when Phoenix Lake was low.
“We really did notice the value of Phoenix Lake, at least in the first half of that storm,” Liz Lewis, a manager at the Marin County Flood Control and Water Conservation District, told the MMWD board this month. “In the community, we heard from a lot of folks that really felt like Phoenix Lake being empty prior to that event probably resulted in less flooding in Ross and downstream.”
The project, which would include pump upgrades and about 2,200 feet of new pipe, could cost $3 million to $5 million depending on the options, according to staff estimates. Part of this cost could be shared by the county.
The concept received mostly positive reviews from the district board during a presentation this month.
“I really like this project,” said board member Larry Bragman. “I think it could have a lot of other future uses.”
Board member Monty Schmitt said that if the flood reductions were large enough, they “could have implications for people’s flood insurance and other things that reduce costs for our customers.”
Larry Russell, president of the board, raised concerns about the cost. He said paying up to $5 million for a half-percent of the district’s water supply seemed “very high.”
While the flood control proposal interested him, Russell said the county and the district would have separate priorities on how the lake should be managed.
“From our end, we need to keep it full,” Russell said. “From their end, we need to keep it empty.”
The idea of using Phoenix Lake as a flooding buffer is not a new one. In the early 2010s, the county explored raising the Phoenix Lake dam. But the high cost — estimated at about $20 million — did not make sense financially, according to Marin County Public Works Director Rosemarie Gaglione.
One benefit of Phoenix Lake is that it already exists, so it doesn’t require the construction of new stormwater detention ponds, which can be controversial in some communities, Gaglione said. Additionally, the water transferred from Phoenix to Bon Tempe Lake would be used as part of the district’s water supply, as opposed to spilling during storms into Ross Creek and out into the bay.
“This is just the start of a lot of information gathering,” Gaglione said. “We’re just grateful to start the conversation.”