Geotechnical and Archaeological Surveys and Informatin Collection

Geotechnical and Archaeological Surveys and Informatin Collection

Marin County Parks (MCP) is proposing to collect geotechnical and archaeological data to support design and environmental review, and regulatory permitting associated with the proposed Bothin Marsh Evolving Shorelines Project. The proposed Evolving Shorelines Project would include tidal marsh restoration and re-alignment of a segment of the San Francisco Bay Trail. Bothin Marsh consists of a mosaic of tidal wetlands including tidal marsh (62.77 acres), tidal channels (12.25 acres), tidal ponds (6.05 acres), mudflats (39.74 acres), and a fringing isolated freshwater forested wetland (0.35 acres). The Bothin Marsh Open Space Preserve is bordered by the open waters of Richardson Bay, Arroyo Corte Madera del Presidio, Coyote Creek, and urban development along its entire west side. The marsh is also bisected by the San Francisco Bay Trail. The marsh has a rich assemblage of tidal wetland vegetation communities and is ecologically important for the region. The marsh elevation is low relative to other marshes in the region and is converting to low marsh and mudflat from a combination of tidal and fluvial hydrologic disconnection, muted tidal prism, erosion, and sea level rise. Bothin Marsh contains habitat for California Ridgway's rail (Rallus obsoletus) and salt marsh harvest mouse (Reithrodontomys raviventris), species, which are special status species. Point Blue Conservation Science has completed annual surveys Bothin Marsh for Ridgway's rails since 2006. Until recent years, Ridgway's rails had been reliably detected in the northeast portion of Bothin Marsh, and in some years between 2008 and 2015 they had also been detected in the southwest portion of the marsh. Ridgway's rails were not detected at Bothin in 2023, 2024, or 2025. The California Department of Fish and Wildlife, Bay-Delta Region conducted a small mammal trapping at Bothin Marsh, August 28-30, 2017 and found that all the harvest mice sampled were determined to be common western harvest mice (Reithrodontomys megalotis). Chinook salmon (Oncorhynchus tshawytscha) and Coho salmon (Oncorhynchus kisutch) may be present in Richardson Bay but not within the areas for the proposed data collection. Bothin Marsh also provides habitat for Point Reyes birds beak (Chloropyron maritimum spp. palustre), which has a California Rare Plant Rank of 1B.2, meaning that it is rare, threatened, or endangered in CA and elsewhere. Surveys for Point Reyes birds beak have been conducted annually since 2022 with approximately 30,000 individuals estimated in 2024. The geographic distribution of birds beak within the project area has been documented and patches would be avoided during exploration activities. The proposed data collection activities would consist of hand auger borings within the tidal wetlands of Bothin Marsh. In total, 42 borings would be conducted, all within mapped tidal wetlands Hand auger borings are a minimally invasive subsurface testing method intended minimize impacts to Waters of the United States and Waters of the State, and a portion of archeological Test Area 3 would be located below the Mean High Water Mark (5.45 ft NAVD88). All borings would be completed with hand tools and utilize an eight-centimeter diameter soil auger bit. The cores would be advanced to a maximum of 12 feet below existing grade or refusal, whichever occurs first. In total, the data collection would result in a maximum of 2.65 square feet of ground disturbance. To reach each of the boring locations, field staff would need to traverse the vegetated tidal marsh by foot, estimated at 1,331 square feet. The proposed geotechnical data collection is a continuation of geotechnical data gathering initiated in 2022 when 4 geotechnical borings were completed. The proposed additional borings would support preliminary engineering for a retaining wall structure that would parallel Almonte Boulevard and support the proposed realigned Bay Trail. The specific boring locations have been selected to identify depths of bay mud and bedrock conditions to inform the suitability of retaining wall structure types, such as sheetpile or mechanically stabilized earth, and the overall footprint of the trail structure. The geotechnical engineer has proposed a minimum of six borings along three linear transects and a maximum of twenty borings along eight linear transects, which would result in a maximum of approximately 1.15 square feet of ground disturbance. Temporary pedestrian access to the boring sites would require traversing tidal wetlands characterized by the Distichlis spicata alliance vegetation mapping unit. The proposed archaeological data collection is based on the recommendations of a previously completed Archaeological Resources Survey Report, which was prepared to identify historic properties within the Area of Potential Effects (APE) in compliance with Section 106 of the National Historic Preservation Act (NHPA) and the California Environmental Quality Act (CEQA) To further support NHPA and CEQA compliance, the proposed archaeological borings would investigate the potential presence of precontact archaeological deposits at five separate high sensitivity areas within the Area of Potential Effects (APE), which would result in approximately 1.5 square feet of ground disturbance within the following vegetation mapping units: Sarcocornia pacifica alliance, Limonium Californicum, Frankenia salina alliance, Californian Annual & Perennial Grassland, Distichlis spicata alliance, Spartina foliosa alliance, and/or Shrub Fragment. The consultant would coordinate the data collection with the Federated Indians of Graton Rancheria, the federally recognized tribe with ancestral ties to the APE, and share the results. The following Avoidance and Minimization Measures would be implemented to ensure the proposed activities would not result in an adverse effect to Waters of the United States, Waters of the State, special aquatic sites, threatened and endangered species, wetlands, or other wildlife: 1. Time borings to avoid periods of unusual high-water flows, wind, wave, and tidal actions. Work would not occur during high tides or storm events. 2. Time borings to avoid biologically critical time periods to protect plants and animals. Ideally, work would not occur between January 15 - August 31 to avoid breeding season for Ridgway's rail (Rallus obsoletus) unless the annual permitted protocol level surveys completed between mid-January and mid-April establish that no detections have occurred. 3. Locate boring and temporary soil holding areas to minimize smothering of organisms. 4. At the boring locations, a piece of plywood or plastic mat will be temporarily laid down to establish a stable working surface. 5. Return excavated soil to the same boring hole. After the work is complete, spoils from the hand auger borings would be used to backfill the boring holes and would be the same material composition as within the disposal site. 6. A biological monitor would survey work areas immediately prior to contractors entering work areas and allow for any wildlife species present to leave work areas on their own accord. The biological monitor would accompany the contractors and identify sensitive vegetation and wildlife species and recommend measures to minimize disturbance to resources. 7. Do not operate vehicles in the marsh. 8. Coordinate data collection with the Federated Indians of Graton Rancheria and share results. Regulatory permit authorizations from the United States Army Corps of Engineers and the San Francsico Bay Regional Water Quality Control Board would be required for portions of the proposed data collection that would occur within jurisdictional waters. All special conditions associated with these permits would be implemented and the portions of the proposed data collection subject to regulatory permits would not proceed until the required permits are secured. Portions of the proposed data collection, namely 4 of the archaeological boring locations, that are located outside of jurisdictional waters would proceed in Fall 2026 prior to securing regulatory permits. The estimated duration of the activities is approximately 4 days for archeological investigations and 5 days for geotechnical investigations, these study days may or may not overlap.

Mill Valley

Marin

San Francisco Bay Area

Marin