MMWD explores projects to add storage capacity

The Marin Municipal Water District is taking a closer look at storage expansion projects that could increase capacity for billions of gallons of additional water to defend against drought.

After several months of study, district officials and consultants are considering projects that could include raising dam heights and some possibilities for creating new dams. Each option would increase the storage capacity by about 20,000 acre-feet.

The proposals include expansions of Alpine Lake, Kent Lake and the Soulajule and Nicasio reservoirs. The district is also looking at constructing new reservoirs in the areas of Devil’s Gulch, Halleck Creek and upper Nicasio. The proposals were presented to the water board at its meeting on March 19.

“We’ve got options,” Paul Sellier, the district water resources director, told the board.

Sellier said these are “generational projects” and the staff is working to narrow the alternatives, expected to cost millions of dollars, for a more focused discussion in May.

The discussion last was an update on the district’s water supply roadmap, approved last year, that set the stage for the district to significantly increase water supply for the first time since the 1980s.

The plan seeks seeks to add another 12,000 to 20,000 acre-feet of annual supply by 2035.

The effort follows the 2020-2021 drought that threatened to deplete the utility’s seven reservoirs. Rains in late 2021 nearly refilled the basins and ended the crisis.

The district serves 191,000 residents in central and southern Marin. Its seven reservoirs make up about 75% of the district’s water supply. The reservoirs can hold up to about 80,000 acre-feet of water, about a two-year supply. An acre-foot is about 326,000 gallons of water.

The water supply roadmap estimated the agency would need at least 8,500 acre-feet of additional water per year to weather a severe four-year drought.

Last year, the district adopted historic rate increases to help pay for the estimated $35 million it plans to invest in the projects through mid-2027.

By comparison, each proposed expansion plan had its advantages and disadvantages.

For example, Alpine Dam would need to be raised 75 feet to accommodate the new 20,000 acre feet of capacity, while Nicasio would only call for an 18-foot-tall extension. An expansion at the Peters Dam at Kent Lake would heighten the dam 37 feet, and at Soulajule by 39 feet.

Peters Dam at Kent Lake would require nearly 3 billion cubic yards of fill to create the expansion, about three times the amount needed for Soulajule.

Expansion at the Nicasio reservoir would come with the most widespread new inundation, including roads and a big portion of the town.

“Inundating the town of Nicasio, I would say right out of the gates, is not feasible,” said board member Monty Schmitt.

However, Schmitt said in order to understand the cost of each project alternative, he would like to know the cost of the privately owned land that would be within an expanded reservoir’s footprint. He said the cost to acquire the land and the impact on the communities would be factored in to the project total.

When it comes to creating a new dam, consultants said the areas of Devil’s Gulch and Halleck Creek might be undesirable because they lie within narrow valleys. That would force a V-shaped reservoir with dam walls exceeding 270 feet tall.

A new reservoir at upper Nicasio could be built at 103 feet and achieve the desired storage capacity, staff said.

Additionally, staff said adding spillway gates at Nicasio Reservoir could add an additional 3,000 acre feet of storage.

Board member Matt Samson said the information was helpful, and he would also like to see what structures may be impacted by expanded or new reservoirs.

“It’s really interesting to see how this process is starting to evolve, and we’re starting to really see where the rubber meets the road,” Samson said.

“In addition to cost, we obviously have to look at the environmental impact on our critical habitat and our native species and our watershed,” said board member Jed Smith. “I’d like to make sure that that is at the top of our additional criteria.”

Board chair Ranjiv Khush said that when thinking about enhanced storage, the district is considering its ability to capture more runoff and potentially store water imported from Sonoma County.

Khush suggested that staff also consider looking at options for a phased approach with projects building off one another to incrementally reach the desired 20,000 acre feet of storage.

Sellier said staff will look at approaches for different capacity levels.

One of the biggest questions about the proposals is how it would affect the district’s water rights. A 1995 order from state water regulators says that any additional reservoir storage capacity the district creates must be used for environmental water releases for protected fish, such as coho salmon.

Ben Horenstein, the general manager of the water district, said the staff is simultaneously consulting with the state regulators on the proposed projects.