Mill Valley Marsh and Sea Level Rise

The Bothin Marsh Open Space Preserve in Mill Valley, California is a 106-acre wetland on the northern arm of Richardson Bay. The marsh is fed by the Arroyo Corte Madera del Presidio stream, originating in downtown Mill Valley, and Coyote Creek, originating in Tamalpais Valley. Today, the preserve is home to hundreds of birds and wildlife species, and provides a popular multi-use trail which goes from Mill Valley to Sausalito.

Great place for a bike ride or walk

The Bothin Marsh is home to the Mill Valley-Sausalito multi-use pathway, a popular bike and walking path along Richardson Bay. This pathway is part of the longer San Francisco Bay Trail that circles the bay. A good place to start on this path is at the Mill Valley Recreation Center.

The ride into Sausalito is 3.7 miles and beginner friendly (no elevation). To learn about the unique attractions that Sausalito offers, see the Sausalito page at LivinginMarin.  You can continue through Sausalito along Bridgeway and Alexander Avenue to the Golden Gate Bridge and into San Francisco.

Effects of sea level rise

Rising sea levels in the bay will impact the shoreline, plants, wildlife, and the multi-use pathway. On recent high-tide days (about 30 a year), large portions of the pathway through Bothin Marsh are completely under water and the entire marsh is submerged. With one foot of additional sea level rise, predicted by 2030, this condition would occur on more than 220 days a year. With a two-foot rise, predicted for 2050, the marsh area would be flooded year-round.

Why are marshes important?

Marshes are areas of shallow water that support the growth of grasses that can grow in mud. These areas become the protected home and breeding grounds for water birds, amphibians, and a wide variety of other animals. Coastal marshes are crucial to the environmental health of the region, filtering nutrients and pollution from the water and protecting communities from rising sea level and harsh storms. 

The One Tam Evolving Shorelines website offers excellent videos and graphics to explain the history of this area, the current status, and possible solutions to counteract the effects of sea level rise.

Help is coming

The Evolving Shorelines Project plans to protect, restore, and enhance the preserve, elevating and realigning a one mile segment above its existing flood prone location. The Golden Gate National Parks Conservancy and Marin County Parks are co-managing this effort to adapt the tidal marsh complex to sea level rise and ensure continued public access to the shoreline through nature-based strategies.  

Community input from many public meetings sets top priorities as restoring the native plant habitat of the marsh and improving the multi-purpose trail to eliminate flooding.

The project includes various potential improvements, including: 

• Elevating or realigning a segment of the BayTrail along south Bothin Marsh.
• Constructing ecotone slopes along trail embankments with the purpose of creating a protective flood levee.
• Establishing a better connection between Coyote Creek and south Bothin Marsh by either re-aligning Coyote Creek through the marsh or by creating new channels.
• Creating an elevated marsh through constructed marsh mounds to combat sea level rise.
• Exploring opportunities to beneficially re-use dredged material through collaboration and partnerships with Marin County Flood Control. This would be a way to use recycled, dredged material, lower carbon emissions, and save county money.

If you are interested in the details, you can read the latest Adaptation Concepts, 2021 for the Bothin Marsh project.

Informative videos

These easy-to-digest videos present alternative solutions to develop a Bay Trail that is protected from flooding from flooding:

Concept 1: Raise theTrail in Place

Concept 2: Span the South Marsh

Concept 3: Ring the South Marsh

Funding Bothin Marsh improvements

The Bothin Marsh Preserve is in unincorporated Mill Valley, so Marin County has jurisdiction over planning and maintenance. Marin County Parks has supported planning for the marsh restoration through Measure A funds, obtained from a voter-approved quarter percent sales tax to support parks, open space, and sustainable agriculture. This fund supported the recent Evolving Shorelines, Adaptation Concepts, 2021 report.