Two aging bridges on one of Marin County's busiest bicycle and pedestrian corridors are getting a $526,000 overhaul, as years of saltwater exposure in a protected tidal marsh have left corroded steel and rotting timber in need of replacement.
The bridges sit within Bothin Marsh, where the Mill Valley-Sausalito Multi-Use Path crosses branches of Coyote Creek between Almonte Boulevard and Shoreline Highway in Mill Valley. The path is more than a recreational amenity. It functions as a daily commuter route feeding into the Golden Gate Bridge bike path and a designated Safe Routes to School corridor for children traveling to Tamalpais High School, Tam Valley Elementary, and other nearby schools. A significant closure or failure of these bridges would push cyclists and pedestrians onto Highway 101 frontage roads.
The repair work involves replacing corroded steel hardware and fasteners, swapping out decayed wood structural members, and replacing the existing pressure-treated lumber decking with untreated wood. That last change is environmentally significant: pressure-treated lumber contains copper-based chemicals that can leach into waterways, a serious concern in a tidal marsh that provides habitat for endangered species including the Ridgway's rail and the salt marsh harvest mouse. The shift to untreated decking reflects either regulatory requirements or proactive environmental stewardship, or both.
Source: U.S. Census Bureau, American Community Survey.
All work stays within the existing bridge footprints, a constraint that limits ecological disturbance but also limits contractor flexibility. Construction in Bothin Marsh comes with significant restrictions: tidal and biological work windows, oversight from agencies including the U.S. Army Corps of Engineers and the California Coastal Commission, and a requirement to keep the path open to the public throughout the project. Those constraints help explain why repairs that might cost a fraction of this estimate elsewhere are running to half a million dollars.
The project is being bid through Marin County Parks, with Jacob Millard serving as the project contact. The work comes as Bothin Marsh faces mounting pressure from sea level rise. The marsh, at the northern edge of Richardson Bay, has experienced increased flooding and erosion, and king tide events have repeatedly made the path impassable in recent years. Marin County has been engaged in long-term adaptation planning for the area, but the immediate priority is keeping the bridges structurally sound for the thousands of people who use the path each week.
Contractor selection is underway, and once work begins, residents and commuters who rely on the path can expect controlled access rather than a full closure.
