USGS - Science for a changing world

San Francisco Bay Could Lose Marshes to Sea-Level Rise by 2100.

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San Francisco Bay — which has already lost the majority of its marsh habitat since the 19th Century — could lose even more marshes by the year 2100, due to sea level rise.

These are the findings of a U.S. Geological Survey report published today. Researchers from the USGS Western Ecological Research Center and the USGS California Water Science Center surveyed the elevation, water levels, sediment and vegetation at 12 marshes near Petaluma River, San Pablo Bay, Napa River and South San Francisco Bay.

Using a new computer model, they found that 95 percent (4,798 acres) of these 12 marshes will be inundated by 2100 under a four-foot sea-level rise scenario — losing their vegetation and being converted into tidal mudflat habitats.

“Future inhabitants of the Bay Area shoreline will see a very different set of wetlands and wildlife, according to our model,” says USGS ecologist and study author Karen Thorne. “Losing marshes means losing the wild birds and animal species that depend on them, and we also lose the natural infrastructure that marshes serve as — as buffers against extreme tides and floods.”

The study sites represent about 12 percent of the remaining marshes in San Francisco Bay overall. Results of the study are published in USGS Open-File Report 2013-1081, and videos of the sea level rise scenarios for the study sites can be viewed at the project website:

Researchers painstakingly surveyed the marshes using sophisticated “RTK GPS” instruments (see Tech Tidbit, below) — which are capable of measuring elevation within a two-centimeter (0.78 inch) resolution.  With this ability, researchers not only mapped the marsh topography, but they mapped the layout of the marsh plants as well — a previously unavailable, yet crucial dataset.

Marsh plants offer nesting grounds and daily high-tide refuges for native wildlife, including endangered species like the salt marsh harvest mouse (Reithrodontomys raviventris) and California clapper rail (Rallus longirostrus obsoletus).

Animations, graphs and data from the San Francisco Bay sea level rise study.

Alternate Futures?

The report has implications for habitat restoration efforts for San Francisco Bay’s national wildlife refuges. And the implications of the study are not necessarily bleak.

“San Francisco Bay is home to one of the world’s most vibrant centers of society and commerce, but it also means that our remaining marshes are often fenced in by urban barriers, and there’s little high ground left for marsh plants to naturally spread and adapt to rising seas,” says Don Brubaker, manager of the San Pablo Bay National Wildlife Refuge. “But if there are other things we can do to help our existing marshes become more resilient to future changes and give migratory birds and endangered wildlife more habitat and time to adapt to sea level rise effects, then we might be able to keep pace with sea-level rise.”

“This new report actually underscores the need to restore more marshes to San Francisco Bay,” says Eric Mruz, refuge manager at the Don Edwards San Francisco Bay National Wildlife Refuge, also one of the sites of the South Bay Salt Pond Restoration Project. “The study is based on current conditions. But if we change that equation, and have more acres of marshes and healthier marshes on hand to begin with, then maybe we’ll have a different outcome 100 years from now.”

The salt marsh harvest mouse (Reithrodontomys raviventris) is an endangered species dependent on San Francisco Bay marshes. Image Credit: USGS


Thorne and colleagues are currently applying the same research methods to forecast the impacts of sea level rise on marshes in San Diego, Orange, Ventura, San Luis Obispo and Humboldt counties, as well as marshes in Oregon and Washington states.

This research was made possible by the support of the USGS National Climate Change and Wildlife Science Center, with additional support from the U.S. Fish and Wildlife Service Inventory and Monitoring and Science Applications Program and the Department of Interior’s California Landscape Conservation Cooperative.

Local cooperators in San Francisco Bay include USFWS, NOAA, National Estuarine Research Reserve System, California Department of Fish and Wildlife, California State Parks, East Bay Regional Parks, City of Oakland and San Francisco International Airport.

Tech Tidbit: What is an RTK GPS?

RTK GPS stands for “real-time kinematic global positioning system”. However, unlike GPS units commonly used by consumers, RTK GPS systems require a “base unit” in addition to the measuring unit (the tall pole device held by the USGS technician in the photo). In urban areas like San Francisco Bay, local cell phone tower networks can sometimes function as the base unit.

A USGS researcher surveying marsh topography using a RTK GPS measuring unit. Image Credit: Kevin Buffington/USGS Western Ecological Research Center

The system then measures the relative difference in positions between the base unit and the measuring unit.

RTK GPS offers very high resolution within 2 cm for elevation measurement — the span of a U.S. nickel coin and easily the difference in height between low-growing and taller-growing plant species. And the horizontal x- and y-axis positions can be closer to 1 cm or less in resolution.

There are other technical differences, but in essence RTK GPS systems have excellent applications for land surveys, especially where a high-precision instrument is needed to discern elevation changes in areas with very little slope — much like the marsh vegetation and elevation surveys required of this USGS project.