Marin Water wants to raise the height of Nicasio Reservoir, potentially flooding our school and parts of historic Nicasio Valley.

We recognize the need to improve drought resilience in Marin, but the citizens of Nicasio have serious concerns about the Marin Municipal Water District (MMWD) proposal to raise the height of the reservoir. Our small town is already subject to severe seasonal flooding, largely due to MMWD’s lack of proper maintenance of the reservoir’s two main tributary streams. Raising the reservoir level further will make that seasonal flooding much worse, inundating our school, roads and backyards. And for what? Due to the broad, shallow nature of the reservoir, the proposed additional storage will almost certainly evaporate during any extended drought, providing little to no benefit to MMWD customers when they need it most.

Marin needs a better, more effective solution for drought resilience.

Want to help? See our suggestions below

MMWD’s preliminary inundation maps

Note: on these maps, the light blue color is the existing water level of the current reservoir, the darker blue colors represent the proposed new water level. Severe seasonal flooding is not yet represented on their maps, but is expected to have additional impacts beyond the increased static water levels.

What is Marin Water Proposing?

Here’s the project description from the Marin Municipal Water District:

“Nicasio Reservoir was created by the construction of Seeger Dam, an earthen filled dam on Nicasio Creek in 1960. The reservoir's current storage capacity is 22,340 acre-feet. This project would install a 280-foot-long, 4.4-foot-high inflatable rubber gate spanning the width of the existing spillway crest of Seeger Dam to increase the capacity at Nicasio Reservoir by approximately 3,700 acre-feet. Modifying Nicasio Reservoir’s spillway is one of the near-term storage enlargement projects identified in the District's Water Supply Roadmap.”

According to Marin Water, the purpose of the proposed project is to increase Marin’s dry-year water yield.

MMWD links

> The Notice of Preparation (NOP)

All projects like this include an Environmental Impact Report (EIR). This is MMWD’s official Notice that they are preparing to conduct an EIR.

> Link to MMWD project page

Details about their specifics for this proposed project.

> Link to Marin Water Supply Roadmap

An overview of the larger set of projects and goals that MMWD is considering for the county.

Why Nicasio is concerned

Raising the reservoir higher will exacerbate the current flooding of our town.

The current reservoir (which was built on ranch land seized from the residents of Nicasio by MMWD in 1961) is fed by two tributary streams, Nicasio Creek and Halleck Creek. These streams currently flood during the heavy seasonal rains, though they didn’t used to. Since the creation of the reservoir, the creeks have begun to silt in, backing up water and causing them to overflow their banks. MMWD initially maintained these creeks, but stopped in the mid-1980s. Since then, flooding of our public school and several private properties along the creeks has gotten increasingly worse. Raising the reservoir even higher will dramatically increase our current flooding, particularly endangering our historic public school.

45 years ago, MMWD rejected a nearly identical proposal due to flooding impacts.

In 1980, Marin Water considered a similar proposal (install a rubber dam across the Nicasio spillway, creating roughly 4,000 acre-feet of storage) to raise Nicasio dam. The District did not implement the 1980 project due to the projected severe flooding impacts on Nicasio including the need to acquire at least four residences through eminent domain.  

If the Nicasio Spillway Modification Project was not considered implementable in 1980, recent flooding conditions in Nicasio underscore that the Project is certainly not implementable now.

Little actual impact on Marin’s water supply during drought years

MMWD claims that raising Nicasio dam will increase the storage capacity of the reservoir by 13%. Sounds great but remember that’s during a wet year when there is already plenty of water to go around. How much water will be available through an extended multi-year drought, which is purportedly the purpose of the project? Nicasio reservoir is broad and shallow and is thus much warmer than other Marin reservoirs (like Kent Lake). Water evaporation from Nicasio is a whopping 2% a month. How much of it will be realistically left after 3 or 4 dry years?

Bad flooding in wet years, no water in dry years. The costs to Nicasio greatly outweigh the benefits to the county.

Use the arrows on either side of the slides to navigate.

Use the arrows on either side of the slides to navigate.

This is the proposed new water level of the reservoir when the spillway is raised and the lake is full, even with no rain. This is taken directly from Marin Water’s NOP. The pink diamond represents the Nicasio School. The very light blue color highli

This is the proposed new water level of the reservoir when the spillway is raised and the lake is full, even with no rain. This is taken directly from Marin Water’s NOP. The pink diamond represents the Nicasio School. The very light blue color highlights the existing lake level. and the darker blue colors indicate predicted inundation from the spillway expansion. The upstream side of the reservoir will expand beyond Marin Water's existing property line which is highlighted in green.

Let's zoom in a bit. All of this is new inundation, not the existing reservoir. The school owns three parcels, and one (triangle in the upper left) will be almost completely inundated by the new lake level. The inundation will also cover a significan

Let's zoom in a bit. All of this is new inundation, not the existing reservoir. The school owns three parcels, and one (triangle in the upper left) will be almost completely inundated by the new lake level. The inundation will also cover a significant area of Rockwood Ranch on the other side of the creek.

Zooming in further shows the school yard, which would be among the most impacted. Water will come up and inundate the existing drainage culvert right next to this building, and it will inundate the septic tank, the well, and come also dangerously clo

Zooming in further shows the school yard, which would be among the most impacted. Water will come up and inundate the existing drainage culvert right next to this building, and it will inundate the septic tank, the well, and come also dangerously close to the water tank that stores water for the school. Permanent static water encroachment adjacent to the school could destabilize the foundation. The playground would be converted into a boggy marsh.

The inundation also affects private properties along Nicasio Valley Road, including Nicasio Valley Cheese and at least four private homes that sit close to the existing creek. The new lake level works its way upstream almost to the Town Square where

The inundation also affects private properties along Nicasio Valley Road, including Nicasio Valley Cheese and at least four private homes that sit close to the existing creek. The new lake level works its way upstream almost to the Town Square where the "Road to Ranches" bridge crosses the creek, almost a mile from the existing lake level.

What these inundation maps don’t show is how the raised water level will worsen the severe flooding during storms that already occurs along this creek and even further upstream of what these maps show. This image shows what happens at the Road to Ran

What these inundation maps don’t show is how the raised water level will worsen the severe flooding during storms that already occurs along this creek and even further upstream of what these maps show. This image shows what happens at the Road to Ranches bridge during a typical atmospheric river storm today even during relatively dry years, with the current dam height.

For a bit of context, here this is the same creek and the same bridge from below. Based on our measurements of the high water marks on the bridge pylons, the water level rises about eight feet above normal creek water level during a storm surge. For

For a bit of context, here this is the same creek and the same bridge from below. Based on our measurements of the high water marks on the bridge pylons, the water level rises about eight feet above normal creek water level during a storm surge. For a creek that is 50 feet wide here, that is a tremendous volume of fast-moving water that this creek needs to drain.

This level of flooding is not an isolated incident. It happens at all the properties up- and down-stream of the bridge. This is the Seybold property where it routinely floods their whole yard and nearly their house.

This level of flooding is not an isolated incident. It happens at all the properties up- and down-stream of the bridge. This is the Seybold property where it routinely floods their whole yard and nearly their house.

And it happens at the school too, where erosion and flooding are perennial. The culvert right outside this building fills with fast moving water and rushes through the middle of the school yard. This is the same culvert that is projected to be inunda

And it happens at the school too, where erosion and flooding are perennial. The culvert right outside this building fills with fast moving water and rushes through the middle of the school yard. This is the same culvert that is projected to be inundated when the spillway is raised. If the lake is filled to the proposed new height at the same time that we get a typical yearly storm, where will all this water go? It will flood our school.

The fact that the existing reservoir has caused significant sedimentation upstream, which worsens flooding, has been known for decades by locals that have to deal with this flooding on an ongoing basis. MMWD survey data from 1960-2006 shows that Nica

The fact that the existing reservoir has caused significant sedimentation upstream, which worsens flooding, has been known for decades by locals that have to deal with this flooding on an ongoing basis. MMWD survey data from 1960-2006 shows that Nicasio creek was rapidly filled with 8 feet of sediment between 1960 and 1970 after the original dam was constructed. After some excavation in the 70s that restored the creek level close to the original profile, maintenance stopped in 1986. By 2006, the creek bed had filled back in again and then some.

In 2006, Marin Water acknowledged that this sediment build-up would "increase water levels in the creek in the rainy season" but declined to do anything about it. They have not addressed the existing sedimentation problem in Nicasio Creek for 40 year

In 2006, Marin Water acknowledged that this sediment build-up would "increase water levels in the creek in the rainy season" but declined to do anything about it. They have not addressed the existing sedimentation problem in Nicasio Creek for 40 years, and now they propose raising the base water level by another 4.5 feet.

There is a clear direct trade-off between Marin Water’s desire to capture as much water as possible with this barrier and our community’s flood risk. Without a binding, public use plan for this gate, we cannot even begin to estimate the true risk to

There is a clear direct trade-off between Marin Water’s desire to capture as much water as possible with this barrier and our community’s flood risk. Without a binding, public use plan for this gate, we cannot even begin to estimate the true risk to our community. When we initially raised our flooding concerns a few months ago, Marin Water’s response was that they were "unaware" of the existing problem and had "no stream data." If they cannot model the existing eight-foot storm surge at the bridge, their models are wrong. How can they create a safe operational plan without accurate data and predictive models? There is a massive credibility gap here currently.

In 1980, Marin Water proposed a very similar project to raise this spillway. At that time, they concluded the project would require them to condemn and remove four homes and that 1,600 feet of county road would be inundated. They passed on the projec

In 1980, Marin Water proposed a very similar project to raise this spillway. At that time, they concluded the project would require them to condemn and remove four homes and that 1,600 feet of county road would be inundated. They passed on the project. Today’s proposal aims for a similar volume increase but Marin Water claims it is not necessary to condemn those same four homes—plus our school, which was built after 1980. How is this possible?

Let's talk about the actual benefit of this project for Marin Water ratepayers.. In the 1980 proposal, Marin Water estimated a net yield of only 500 acre-feet from a 4,000 acre-feet expansion. The significant difference between the yield expectation

Let's talk about the actual benefit of this project for Marin Water ratepayers.. In the 1980 proposal, Marin Water estimated a net yield of only 500 acre-feet from a 4,000 acre-feet expansion. The significant difference between the yield expectation and the reservoir volume is due to the fact that the extra volume of water captured during a wet year has to be distributed to customers over multiple dry years while that extra volume never gets replenished. During that dry year storage time, much of the captured water will evaporate.

MMWD’s yield simulation models are calibrated to data acquired during a particularly wet 17 year span during which the Nicasio reservoir only failed to spill once. However, this same spillway has failed to spill 4 years out of the past 12 years. In

MMWD’s yield simulation models are calibrated to data acquired during a particularly wet 17 year span during which the Nicasio reservoir only failed to spill once. However, this same spillway has failed to spill 4 years out of the past 12 years. In years when the existing spillway is dry, the raised spillway would capture nothing. Relying only on the ‘92-’09 dataset for model calibration greatly overestimates future yield.

The inflatable gate is designed such that it can either be fully inflated or fully deflated, and it must be deflated during any possibility of a storm surge to prevent catastrophic flooding. It takes 24 hours to safely deflate to avoid downstream fl

The inflatable gate is designed such that it can either be fully inflated or fully deflated, and it must be deflated during any possibility of a storm surge to prevent catastrophic flooding. It takes >24 hours to safely deflate to avoid downstream flooding, so the deflation decision must be made at least a day in advance and based off of rather uncertain rainfall predictions. It is likely that the 3,700 AF will be have to predictively drained even in cases where a storm doesn’t actually end up hitting the Nicasio watershed. This means that even in wet years when the existing reservoir spills, the inflatable spillway can really only be safely operated in the late season and may not even fill reliably in wet years. At what point in the previous 3 wet years would the spillway have been inflated? How much water would it have captured?

Nicasio Reservoir is large in surface area, but is shallow and warm. This drives the absolute evaporation rate of water off Nicasio Lake to be more than twice that of Kent Lake, even though Kent Lake stores 50% more water. Because of this, Nicasio is

Nicasio Reservoir is large in surface area, but is shallow and warm. This drives the absolute evaporation rate of water off Nicasio Lake to be more than twice that of Kent Lake, even though Kent Lake stores 50% more water. Because of this, Nicasio is probably the worst place in MMWD’s system of reservoirs to attempt to store water over multiple years. If this barrier does capture an extra 3,700 AF in a wet year, how much of it will be realistically left after 3 or 4 dry years? 500/22500=2% 200/32500=0.6%

Ultimately, we have major reservations about this proposal not only because it will have a devastating effect on our community, but also because this project proposal is incomplete. The use plan is unspecified, the mitigation plan is unbudgeted, and

Ultimately, we have major reservations about this proposal not only because it will have a devastating effect on our community, but also because this project proposal is incomplete. The use plan is unspecified, the mitigation plan is unbudgeted, and there is a long history of indifference from Marin Water regarding the existing impacts of their infrastructure on our community.

The Nicasio Land Owner’s Position

We understand that Marin Water’s intent is to increase the resilience of its water supplies, but this goal must be balanced with consideration of serious impacts that raising Nicasio Reservoir will have on our properties and community, especially in light of the flooding that currently occurs along Nicasio and Halleck Creeks.

We oppose a project to raise the Nicasio Reservoir that will inundate, flood or otherwise harm our historic school, private properties, roads, or upstream/downstream environmental resources.  

Further, Marin Water must address current flooding of our Nicasio Community related to poor maintenance and sediment build-up in the creeks where water flows into Nicasio Reservoir.

Marin Water has other options to increase the resilience of its water supplies without harming our Nicasio Community, including the expansion of its Kent Lake Reservoir.


NLOA Principles for Addressing the Proposed Nicasio Reservoir Spillway Modification Project

  • Our NLOA commitment is, to the best of our ability, to protect our Nicasio Community from ongoing and additional harm.

  • We will work with Marin Water to determine if there are shared solutions for Nicasio Reservoir that satisfy our core principle that Nicasio’s historic school, private properties, roads and upstream/downstream environmental resources are not inundated, flooded, or otherwise harmed by the Nicasio Reservoir Spillway Modification Project.  

  •  We will not accept proposed solutions that trade off harm to our historic school, private properties, roads and upstream/downstream environmental resources for other benefits.

IN THE NEWS

IMPORTANT DATES

Tuesday July 15: As part of the environmental review process, Marin Water is holding three public scoping meetings. These meetings are an opportunity for interested parties to learn more about the project and provide public comments. The information presented will be the same for all three meetings.

August 4th: Final day to submit comments. Click here to send an email directly to Marin Water.

What you can do to help

1) Write a letter
to MMWD

2) Attend a meeting

3) Volunteer & donate
to help NLOA

Public Comment Period runs from April 30 until August 4, 2025 for citizen’s to voice their concerns regarding the proposed plan to raise the Nicasio Dam Spillway and level of the Nicasio Reservoir. Comments must be submitted before August 4, 5:00pm in one of the following ways:

  1. By email to: Click here to send an email directly to Marin Water

  2. Or by U.S. postal mail to:
    Marin Water
    Attention: Elysha Irish
    220 Nellen Ave.
    Corte Madera, CA 94925

As part of the environmental review process, Marin Water is holding two public meetings. These meetings are an opportunity for interested parties to learn more about the project and provide public comments. The information presented will be the same for both meetings.

If you’d like to get more involved, the NLOA has several initiatives that we’d love help with, including raising funds to help pay for professional & legal consultants that we are working with to help support our position.

> Email NLOA to get involved!