Along most of California’s coast, accelerated sea level rise is not expected to cause major problems, as it will along the sloping Eastern seaboard and on the Gulf of Mexico, where flooding is a chronic issue. Significant impacts are expected, however, in low-lying coastal areas, including estuaries and bays. So what will happen to coastal wetlands, including those now being protected and restored? Will they be inundated and lost?

In some habitat restoration projects, sea level projections for the next 50 to 100 years are being factored in. Each site has to be considered on its own because no two present the same issues. Normally, as sea level rises, wetlands retreat inland. This is still physically possible in some places, such as Ormond Beach, in Ventura County, where marshland and dunes are backed by farmed fields. The Coastal Conservancy’s plans for habitat restoration at Ormond Beach include conservation easements on adjoining farmlands.

In most other places, however, wetlands are caught in a squeeze between the ocean and roads, buildings, and other hard structures. Most marshes on San Francisco Bay, for example, cannot migrate up, pointed out David Schoellhamer, research hydrologist at the U.S. Geological Survey (USGS). “The question becomes,” he said, “Can marshes restore themselves by accretion? Is the sediment accretion rate high enough?” If it is, sediment will build up the marsh even as sea level rises.

This and other questions related to sea level rise will be considered by a multi-agency project team led by the Coastal Conservancy in planning for one of the most ambitious marsh restoration projects in the state: to reconvert 15,100 acres of salt ponds, recently acquired from Cargill Salt, to a mix of tidal wetlands and managed ponds. Schoellhamer is a scientific advisor for the project.

According to hydrologist Philip Williams, the key to success will be quick revegetation. “In the south bay we had a [de facto] experiment in the ‘40s, ‘50s, and ‘60s, when accelerated groundwater pumping led to subsidence that exceeded the highest levels of sea level rise projected for the next 100 years,” he said. “The marshes persisted and were fine. Once vegetation is established, San Francisco Bay marshes are amazingly resilient.”

If revegetation is part of the south bay salt pond restoration project, sea level rise will have to be considered in decisions on where to plant, noted R. Michael Erwin of USGS Patuxent Research Center, a member of the National Science Panel for the project. “If cordgrass is planted at currently optimal elevations, it will no doubt be inundated within the next 50 years,” he said.

To Williams, a pioneer in marsh restoration, the challenge posed by sea level rise presents “an opportunity to address broader restoration problems.” The Big Lagoon project in Marin County, for which his firm is the principal consultant to the National Park Service, is a case in point. The Park Service intends to restore wetlands near Muir Beach in Marin County while also reducing flood hazards.

About 150 years ago there was a big lagoon at the mouth of Redwood Creek, behind a sandy beach with rocky areas and tidepools at both ends. Sea level rise at that time exceeded the rate at which sediment was deposited by stream flow, and the lagoon was probably expanding. But then grazing, logging, and grading in the watershed led to erosion upstream, sediment delivery to the lagoon increased tenfold, and the lagoon filled in.

Although the Redwood Creek watershed is now protected within the Golden Gate National Recreation Area and major restoration work has been done on lower parts of the creek, the sedimentation rate is still about five times what it would be under natural conditions. In the last few years, as sediment built up in the stream bed, Redwood Creek overflowed during storms, impeding public access to Muir Beach and to homes. It became clear that a major storm could do serious damage to nearby public and private property.

Looking at historic sediment delivery trends and taking accelerating sea level rise into account, Philip Williams Associates projected how the system would evolve for the next 50 years under different restoration alternatives, including one that would re-create the “big lagoon.” The Park Service’s next step is preparation of an environmental impact report, then funding must be acquired for the restoration. “It’s easier and more effective to address many of these problems now,” Williams said. “Fifty years down the road they’ll be harder to deal with.”

—RG

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