A proposal to enlarge the Nicasio Reservoir has collapsed after months of study, planning and heated community debate—and after residents unearthed forgotten records showing the district had proposed, and abandoned, the same idea more than 40 years ago.
Marin Water, which spent nearly a year pursuing a plan to raise the spillway, announced last week that it would abandon the project, citing runaway costs and limited benefits.
What began as a $10 million to $15 million effort had ballooned to an estimated $37 million to $54 million undertaking after the scoping process revealed the extent of the work required.
“I say we just cancel it right now and put the community at rest,” Matt Samson, the district’s board chair, said at a meeting last Tuesday. “It’s no longer economically viable and it’s no longer a responsible way to spend district ratepayer money.”
Though the board took no formal vote, it directed staff to halt the environmental review process, which had been gathering scoping comments since May.
The idea emerged in the wake of the 2020–2021 drought, which left the municipal district with only a few months of water supply for the 191,000 residents it serves in central and southern Marin. It was the only Bay Area utility to declare a water crisis. Searching for ways to expand capacity, officials turned to Nicasio Reservoir, the second largest of the district’s seven water sources.
The plan called for installing a 280-foot-long, 4.4-foot-high inflatable rubber gate across the crest of Seeger Dam. Inflated during the rainy season, the gate would have increased the 22,340-acre-foot reservoir’s capacity by about 17 percent, or 3,700 acre-feet. (The industry metric, an acre-foot is enough to cover an acre of land with a foot of water, or roughly what two typical households use in a year.)
At first, staff pitched the gate as a relatively easy and inexpensive way to add significant storage.
“You have to remember how excited they were when they thought they had discovered a fairly modest project that could be both inexpensive and provide a great deal of water,” said Martha Davis, a veteran water policy strategist who lives in Nicasio. “Like most things that look too good to be true, this was the case.”
From the outset, Nicasio residents opposed the idea. They rely on wells, not district water, and they knew that a higher reservoir level would worsen flooding in town. Residents pointed to sediment buildup in Halleck and Nicasio Creeks, where clogged channels already force stormwater into roads and yards at homes and the school. Signs soon dotted the tiny town: “Don’t Flood Nicasio.”
Dams disrupt more than just water. They also trap the sand and silt that rivers naturally carry, cutting off the downstream flow and causing sediment to settle on the reservoir floor. Over time, that buildup shrinks storage capacity. Ms. Davis noted that much of the new storage would have merely offset what was already lost to silt.
“This was a project intended to provide more water storage,” she said, “but in reality, half of the storage that would have been created was just replacing what had already been lost.”
At last week’s meeting, Ms. Davis gave the board an 848-page sediment management handbook, urging them to create a sediment maintenance plan for Nicasio.
The debate took a remarkable turn this summer, when residents of Hicks Valley, fighting their own reservoir expansion proposal, filed a public records request and uncovered district minutes from 1980.
The documents revealed that officials had studied a nearly identical spillway gate in Nicasio after the 1976–1977 drought, only to abandon it once staff concluded that it would inundate homes, roads and the school. The district instead built Soulajule Reservoir and doubled the capacity of Kent Lake.
Those buried records, passed from Hicks Valley to Nicasio and then to the water district, came as news to today’s board and staff.
“The board now had independent proof from an earlier evaluation that the impacts on Nicasio were very real,” Ms. Davis said. Residents also showed officials where sediment had piled up behind the school and in neighborhood creeks—areas the district once regularly cleared.
“For whatever reason, the water district sort of stopped maintaining the creeks about 30 years ago,” said Thomas Wood, a longtime Nicasio resident.
He added, “I think one of the reasons that we prevailed on this one is that we showed how the original reservoir was so devastating to this community.”
Seeger Dam was built in 1960 at the height of America’s big western dam-building era. Six ranches were condemned in the process.
“I, on behalf of the district, apologize to Nicasio for the difficulties that have been faced out there from the time the reservoir was conceived,” district board member Larry Russell said last week. “Reservoirs are an important part of our water supply, and they’re always in somebody’s backyard. It’s unfortunate that the reservoir lies outside our district, but that’s just how things are.”
The collapse of the Nicasio plan is the second major setback in the district’s quest to strengthen its drought resilience. A plan to expand Soulajule Reservoir—also located beyond district boundaries in West Marin—was deprioritized last winter amid opposition from ranchers in Hicks Valley whose land would have been flooded.
The district still has a goal of adding about 8,500 acre-feet of water a year, according to Paul Sellier, its water resources director. One leading option is the “atmospheric river capture” project: a 13-mile, 36-inch-diameter pipeline that would divert excess winter flows from the Russian River into Nicasio Reservoir.
The Russian River watershed in Sonoma County is many times larger than the Lagunitas Creek watershed that feeds Nicasio. Even in drought years, a few atmospheric rivers can send torrents of water surging down the Russian, much of which empties into the Pacific at Jenner.
The pipeline, which could deliver 3,800 to 4,750 acre-feet annually, is designed to intercept a fraction of that surplus, supplementing Nicasio’s supply during dry years without increasing the reservoir’s capacity.
The district said the project’s environmental review and design are expected to take two years, followed by at least four years of construction.
Meanwhile, staff plan to archive the Nicasio project’s engineering studies and sediment research as an institutional record for future boards. That work will include a yearlong review of Seeger Dam’s stability.
“I’d like to see whatever kind of comprehensive documentation comes out of this—the reasons why the board passed on it, the reasons why staff recommended against it—preserved, so that the next board, 30 years from now, doesn’t have to go through the same exercise we did just to get to the same answer,” Mr. Samson said.