Evolving Shorelines Project at Bothin Marsh
Bothin Marsh, Mill Valley, Marin County; Measure AA Region: North Bay
Safe, Clean Water and Pollution Prevention Program; Vital Fish, Bird and Wildlife Habitat Program; Integrated Flood Protection Program; Shoreline Public Access Program.
Marin County Parks
This project will develop a conceptual design into schematic design (35% level of design) to protect, restore, and enhance the Bothin Marsh Open Space Preserve and elevate and realign a one-mile segment of the Bay Trail out of 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.
The Bothin Marsh Open Space Preserve (the Preserve) is Marin County Parks’ most visited park unit and the most vulnerable to sea level rise. The Bay Trail segment that runs through the Preserve will be increasingly compromised by tidal flooding. The Preserve’s 67 acres of tidal wetlands are particularly vulnerable to sea level rise due to the relatively low ambient suspended sediment concentrations in this part of Richardson Bay. This limits the wetlands’ ability to grow vertically to keep up with sea level rise by capturing sediment. Although the Bothin Marsh complex is relatively small compared to other marsh complexes in the San Francisco Bay, it is the largest remaining tidal marsh habitat in Richardson Bay, supports higher than average biodiversity, and provides critical habitat for wildlife in the region.
The current Bay Trail alignment was constructed on a former railroad levee that substantially alters the hydrologic connectivity between Richardson Bay and South Bothin Marsh, which affects marsh ecological function and habitat quality.
The project includes various potential improvements, including the following plans:
• Elevating and realigning a segment of the Bay Trail along South Bothin Marsh and elevating a segment of the Bay Trail along North Bothin Marsh, to accommodate over
three feet of sea level rise and a 100-year storm surge.
• Constructing ecotone slopes along trail embankments through thin-lift sediment placement to create high marsh habitat, with the potential to be engineered as a flood
• Establishing fluvial connectivity between Coyote Creek and South Bothin Marsh by either realigning Coyote Creek through the marsh or with the creation of new
• Enhancing tidal action and connectivity between Richardson Bay, South Bothin Marsh, and North Bothin Marsh with restored tidal prism and drainage function and
process with the expansion of tidal channels.
• Creating high marsh habitat through constructed marsh mounds from tidal channel excavation spoils.
• Protecting against marsh erosion with construction of up to four acres of beach habitat.
• Enhancing subtidal mudflat habitat with sediment augmentation and tidal creek channel creation.
• Exploring opportunities to beneficially re-use dredged material through collaboration and partnerships with Marin County Flood Control and nearby marina managers.
The project also strives to include scientific research on tidal marsh adaptation approaches.