The Nicasio Land Owners Association has a simple message for Marin Water as the agency prepares to raise the level of the Nicasio Reservoir: slow down.

Elevating the reservoir’s spillway is one of several projects the district is planning to increase its storage capacity to cope with droughts. But association members fear raising the level could exacerbate flooding along the banks of Halleck and Nicasio Creeks, which they say occurs routinely during the rainy season.

Marin Water owns much of the property around the reservoir where the waters would rise, but water is also expected to encroach on parts of about a dozen privately owned parcels, including those on which Nicasio School is located, according to inundation maps it prepared.

“Our community understands what Marin Water wants to do in terms of improving the resilience of its water supply, but that goal has to be balanced with consideration of the serious impact that raising Nicasio Reservoir could have on our properties and our community,” said Martha Davis, a board member of the landowners’ association.

The water district is preparing to conduct an environmental impact report assessing the $15 million project and will conduct two public meetings to discuss the scope of that study later this month. 

The E.I.R., required by the California Environmental Quality Act, must consider any potential adverse impacts and ways to mitigate them. The district is accepting public comment until 5 p.m. on June 4.

The project involves installing a 280-foot-long, 4.4-foot-high inflatable rubber gate spanning the top of the spillway. The gate would be inflated during the wet season to increase storage in the reservoir, which is fed by Halleck and Nicasio Creeks. It would be lowered during the dry season.

The heightened spillway would increase the reservoir’s capacity by 3,700 acre-feet, an increase of roughly 15 percent. (An acre-foot is about 326,000 gallons, or enough water to cover an acre of land about 1 foot deep.) Construction would begin in 2027.

“We want to hear from any members of the public on any concerns or issues they may have that will help inform what we investigate,” said Adriane Mertens, a district spokeswoman. “This is going to be an involved process, and public input at this juncture is really important.”

Among the areas where the reservoir would expand is a stretch of land that follows the path of Nicasio Valley Road and includes the mouths of both creeks. It runs behind the school and several privately owned parcels near the center of town.

District staff have met several times with residents to discuss their plans, including a session last week with the school board and members of the landowners’ association. The sessions did not assuage the residents’ concerns, according to Matt Pickett, who serves on the boards of both groups.

Nicasio Creek runs behind his house, about a mile south of the school, outside the area covered by the inundation maps. Even there, he said, the creek rises by as much as 8 feet multiple times a year, and the situation is far worse closer to the school.

“We’re getting massive amounts of water backup and massive amounts of flooding on some people’s property already,” Mr. Pickett said. “Every single year, we have flooding issues at the schoolyard. Raising the spillway will compound the problem.”

At last week’s school board meeting, water district staff promised the project would not inundate the school or other nearby properties and that it would take steps to leave them less vulnerable. 

“If there’s a fantastical world where they can improve the existing flooding problem, raise the dam, get more water and not negatively impact the school, then it’s a win-win,” Mr. Pickett said. “But there’s a lot of real work to be done to understand if that’s even feasible, much less how to do it.”

Marin Water’s seven reservoirs hold nearly 80,000 acre-feet of water, with Nicasio Reservoir holding about 22,000 acre-feet. About 75 percent of the district’s water originates from rainfall in the Mount Tamalpais watershed and the hills of West Marin. With supplemental recycled water and water from the Russian River watershed, along with customer conservation measures, the district has always been able to meet demand.

But after the 2021 drought severely tested supply, the district adopted a water supply roadmap that included various short- and long-term projects to expand its storage capacity.

Raising the spillway is the second of those to spark controversy in West Marin. The district also considered expanding the Soulajule Reservoir, a project that would have flooded several multigenerational ranches in Hicks Valley. After residents protested, the district put the idea on the back burner.

Marin Water will hold scoping meetings on May 22 from 6 to 8 p.m. and May 27 from 1 p.m. to 3 p.m. on Zoom. The first can also be attended in person at Marin Water’s offices at 220 Nellen Ave., in Corte Madera. Send comments to [email protected]. Find more information at https://www.marinwater.org/node/1261