How High’s the Water, Ma?

by Sandy Guldman

        How high’s the water, Ma? Two feet high and risin’
                                                   Johnny Cash (1959)
This song resonates with residents of Corte Madera Creek watershed each winter when we witness king tides. The flooding is obvious, but over the long term, rising sea level has other damaging impacts. If marshes are converted to open water and mudflats, we lose their many environmental benefits, including habitat for wildlife and nurseries for young fish, carbon sequestration, flood water capacity, and attenuation of storm surges.

The big question is what to do about it. The ultimate solution is to reduce the quantity of greenhouse gas we add to the atmosphere, but a substantial amount of sea-level rise is now inevitable and we need to respond to that inexorable rise.

The least environmentally friendly responses are hard surfaces, like concrete walls. The best solution is to retreat and provide space for wetlands and floodplains to expand as sea level rises and storms become more intense. But we live in a developed area, and there is a limit to how much developed land we can abandon. The tradeoffs are difficult and we face a balancing act.

Marin County’s coordinated planning for sea level rise along the bay shoreline, BayWAVE, is a planning effort led by multiple agencies, partners, and municipalities, including the County. It is studying a number of approaches to solve the crisis. One project, designed to respond to climate change and sea-level rise, is the Lower COM Corte Madera Creek Habitat Restoration Project. Friends received a grant from the Coastal Conservancy and hired a team of experts to prepare 65% complete designs for partial removal of the concrete channel in Kentfield downstream of Stadium Way. The strategy in this area is to remove or lower the concrete walls and create a gently sloping bank.

Initially, most of the newly established vegetation on the bank will be transitional between tidal marsh and upland. Transition zone plants are salt tolerant and provide refugia for marsh wildlife during high tides. As sea-level slowly rises, the transition zone plants will be replaced by high marsh and then low marsh plants, preserving valuable tidal marsh and its many benefits.

An artist’s impression of proposed alterations to the downstream end of the concrete channel. Image by Scott Walls, walls land+water

Predictions of the amount of sea level rise by 2100 vary widely. This design uses estimates of sea level rise based on the San Francisco Bay Tidal Datums and Extreme Tides Study (2016) prepared for the Federal Emergency Management Agency and the San Francisco Bay Conservation and Development Commission.

It is estimated that by midcentury, the mean higher high water level in Corte Madera Creek will be 18 inches higher than at present. In 2100 this level may rise further by the same amount.

We expect that unless the rate of sea level rise can be slowed by significant reductions in greenhouse gas emissions, by 2100 other measures like walls and berms will be required to protect schools, homes, and businesses in the area.