by Eva Buxton and Ann Elliott

Sea level rise threatens to inundate Bothin Marsh Open Space Preserve. Following studies of its geomorphology, ecology, and options for conserving the natural environment, public comments were invited on proposed plans for conserving the marsh and locating its multi-use trails. Marin Chapter is evaluating these plans’ effects on the marsh’s native plant communities and associated wildlife habitat. Adaptation of Bothin Marsh may become an example for other planning agencies along the San Francisco Bay’s shoreline.

Bothin Marsh is a 106-acre preserve along Richardson Bay near Tam Junction on Hwy 1. Although both the very popular and well used Mill Valley/Sausalito Multi-Use Path (Bay Trail) and the Charles McGlashen Multi-Use Path traverse it, the marsh hosts unique plant communities and wildlife habitat. The largest population in San Francisco Bay of Point Reyes bird’s-beak (Chloropyron maritimum ssp. palustre), Chloropyron maritimum ssp. palustre Point Reyes Salt-marsh Bird's-beakChloropyron maritimum ssp. palustre (Pt. Reyes salt-marsh bird’s-beak)a CNPS Rank 1B species (rare and endangered throughout its range) occurs in the marsh.  Ridgway’s rail and California black rail, both federal and state listed bird species, are also present.

Much of the marsh and portions of the trails are inundated during king tides now. Further sea level rise of 10 inches is predicted in the next ten years, with 3-4 feet inundation of the marsh by 2050, and up to 8 feet by 2100. The rising water, strong waves, longer periods of drought, and intense storms caused by climate change would likely erode the marsh shoreline, converting the vibrant marsh to mudflat.

Marin County Parks collaborated with scientists to study the effects of sea level rise on Bothin Marsh, publishing Bothin Marsh Geomorphology, Ecology, And Conservation Options. They asked other local agencies and organizations to consider the studies therein and resulting nature-based solutions. Then they partnered with Golden Gate Parks Conservancy (via One Tam) to launch this Evolving Shorelines planning project.

The Initial Planning Memo, dated June 2020, outlined three alternative conceptual plans to conserve the trails and save the marsh:

  • 1 – Raise the trail 4 feet in place and widen it to 18 feet;
  • 2 – Span the South Marsh with a causeway to raise it above the marsh and renew tidal action;
  • 3 – Ring the South Marsh’s perimeter and move portions of the trail parallel to Shoreline Hwy.

All three alternatives would create marsh ecotones on the slopes of levees and upland habitat on their tops. “Mounds” would be constructed inside the levees as refugia for plants and animals.  Adding silt to the marsh in “thin lifts” to raise the level of the marsh above a mud flat is also being considered.

The project will require collaboration among the community, public agencies, and partners. Outreach to the community has included biking and hiking tours, guided kayaking trips, historical ecology talks, etc. An Evolving Shorelines questionnaire queried the public’s preferences for use of the trail: walking, biking, bird watching, commuting, as well as input on the proposed alternatives. According to Veronica Pearson, the County Co-Manager for the Bothin Marsh project, the result of the public’s responses will be published later.

Lack of funding has delayed project planning beyond the conceptual phase. The CEQA process will not begin in 2021 as planned. Hopefully, the California Coastal Conservancy will award a grant for definitive project planning, environmental analysis, and permitting.

Links for more information

Photo by Vernon Smith