The Marin Municipal Water District has launched an environmental study of a plan to increase water supply at the Nicasio Reservoir.

The proposal calls for modifying the spillway gates at Seeger Dam, which was constructed on Nicasio Creek in 1960 to create the reservoir. The reservoir has a storage capacity of 22,340 acre-feet of water.

The district’s seven reservoirs comprise about 75% of its water supply, holding up to 80,000 acre-feet of water, equal to about a two-year supply. An acre-foot is about 326,000 gallons.

The project would install a 280-foot-long, 4.4-foot-high inflatable rubber gate spanning the spillway crest to increase capacity by about 3,700 acre-feet.

An environmental impact report is required under the California Environmental Quality Act. As one of the initial steps in the process, the district is inviting the public to comment on the scope of the review.

“The purpose of the scoping is for any party to bring forth issues that may be of concern so the district can be sure to investigate those further during the EIR,” said Adriane Mertens, spokesperson for the agency.

The district is planning two “scoping” meetings where residents can learn more about the project and submit their comments. One is from 6 to 8 p.m. May 22 in the district boardroom at 220 Nellen Ave. in Corte Madera. Teleconference access will be offered.

A second online-only session is set for 1 to 3 p.m. May 27. Instructions for how to participate are at marinwater.org/SpillwayModifications.

Written comments will also be accepted through 5 p.m. June 4. Comments can be mailed to Elysha Irish at the district headquarters or emailed to eirish@marinwater.org.

The spillway modification project is one of the near-term water supply projects being pursued by the district. After the 2020-2021 drought threatened to deplete the reservoirs, district officials huddled up to devise a defense plan.

The resulting water supply “roadmap” approved in 2023 set the stage for the district to significantly increase water supply for the first time since the 1980s.

The district says the project is a relatively cost-effective way to achieve additional water storage capacity in a shorter timeframe compared to other, longer-term water supply projects that were identified in the water supply roadmap.

In November, the district board voted unanimously to hire Environmental Science Associates to perform the environmental impact report at a cost not to exceed $1.9 million, which includes a 15% contingency. So far, the district has spent about $1 million on design and review, Mertens said.

Construction is expected to cost about $5 million. The total cost estimate, including design, planning and construction, is about $15 million, Mertens said.

If the district decides to seek federal grants to support the project, it will need clearance from the National Environmental Policy Act, which could involve another set of specific studies.

District officials aim to continue study and design through spring 2026 and have a project ready for construction in 2027.

Larry Minikes, a Marin Conservation League board member who has been following the district’s water supply planning progress, said the group supports this project as “a cost-effective and timely solution to bolster water storage capacity.”

“This project represents a proactive step toward enhancing the district’s water resilience in the face of increasing climate variability,” Minikes said.

“From an environmental perspective, we acknowledge the importance of comprehensive assessments to ensure that the project does not adversely affect local ecosystems,” Minikes said. “We anticipate that environmental considerations will be thoroughly addressed through the ongoing, established review processes.”