PRBO Conservation Science
Quarterly Journal of PRBO Conservation Science, Number 165, Summer 2011: San Francisco Bay Sea Level Rise


Protecting Tidal Marsh as Sea Levels Rise in San Francisco Bay

Where Will All the Marshes Go?

Melissa Pitkin

Sea Level Rise in San Francisco Bay
Welcoming Wendell Gilgert

Tidal marshes are a mosaic of low, medium, and high marsh habitat. Photo by Beth Huning, San Francisco Bay Joint Venture.
San Francisco Bay’s tidal marshes are a great source of pride and beauty for the people who live and work in the region. They also provide essential ecosystem services such as filtering pollutants, reducing the impacts to human infrastructure of flood and sea level rise, and sequestering carbon. And for those of us who love birds, tidal marshes offer opportunities to observe a number of unique bird species found nowhere else on the planet!

Given these remarkable qualities, it’s no surprise that many of us are concerned about the future of our marshes. Considering the effects of climate change, we have begun to wonder what will happen to San Francisco Bay’s marshes in the next 100 years: will they persist? How can we plan conservation into an increasingly uncertain future? How can we make choices about the activities needed today to prepare for the future? Will there be enough marsh habitats to sustain bird populations and also provide people with the ecosystem benefits of healthy marshes? Should we continue to place new developments at sea level?

These are all questions that the conservation community in the San Francisco Bay area faces on a day-to-day basis. At PRBO, they are questions that we are helping to answer—through an interactive website that maps future sea levels in the bay’s marshlands.

PRBO’s New Conservation Tool
Clapper Rail. Photo by Tom Grey,

Newly released by PRBO is the San Francisco Bay Sea Level Rise Web Tool (, incorporating data from PRBO and our partners. The website was created for conservation practitioners, and it presents a set of interactive maps that allow the user to see where and what types of marsh will persist into the future as sea levels rise.

PRBO’s modeling tool is unique: it is the first to include more than just estimated increases in sea level rise. Our online tool goes one significant step further, combining projections on climate, salinity, sedimentation, and elevation with sea level rise. All of these factors are key pieces to the puzzle of how, where, and whether marshes can survive into the future.

In addition to projecting where marshes of varying types might occur under different scenarios, the PRBO sea level rise website overlays our 14 years of marsh bird data on the projections, enabling users to see maps of the potential impacts to birds (key indicators of the biological community). One can view where the Common Yellowthroat, Song Sparrow, Marsh Wren, Clapper Rail, and Black Rail are expected to occur from 10 to 100 years into the future, given what we know today. Users can also view how 14 different plant species may be affected by rising sea levels.

To understand these findings, a primer on marsh ecology may be helpful. Tidal marshes are ever-changing ecosystems, and their survival depends upon a balance between physical processes (e.g., sedimentation, tidal action, and erosion) and biological factors (e.g., plant growth adapted to inundation by salt water). Tidal marshes are highly sensitive to changes in sea level; they are formed and sustained only along relatively protected shorelines where sediment accumulation can keep pace with rates of sea level rise. As water levels rise, marshes are flooded, bringing sediments that get trapped by plants and other organic matter at different rates, depending upon the elevation of the marsh. The interaction between water level, elevation, and sediment delivery is important to a marsh’s persistence over time. If sea level rises too quickly, the chance that a marsh can keep pace diminishes.

Beyond the presence or absence of a marsh, another aspect of how sea level rise will alter marshes in the future involves the change in marsh type. There are three general types of marsh: high, mid, and low marsh (see box at right). Each of these has particular attributes and a distinct value to birds, wildlife, and people. Our sea level rise modeling tool shows the changes we expect to see in each type of marsh over the next 100 years.

What We Are Learning

The interactive sea level rise website projects where marsh and marsh birds may occur in the future. Unlike a prediction, a projection is what we expect to see, given our current understanding of factors influencing climate change (such as resource use and greenhouse gas emissions) and also our knowledge of birds’ occurrence (their distributions and habitat associations) today.

Some of the take-home points for San Francisco Bay marshes and marsh birds in a changing future are:

• Up to 92% of current mid and high tidal marsh habitat in the estuary could be lost by 2110, under a combination of higher sea level rise (1.65 meters, or 5 feet 5 inches) and lower sediment availability—given the constraints imposed by our current infrastructure (dikes, levees, and urbanization).

• Total marsh area in the San Francisco Bay area is projected to increase over the next 50 years—but to decrease significantly in the second 50 years.

• Tidal marsh restored in the next decade will have a much better chance at keeping pace with sea level rise than will marsh restored in the second half of the century.
Common Yellowthroat. Photo by Tom Grey,

• Black Rails, Common Yellowthroats, Marsh Wrens, and Song Sparrows are likely to decline in abundance over the next 100 years due to flooding of their high-marsh breeding habitat.

• These bird population declines may be reduced under high sediment scenarios, meaning that restoration and watershed management activities (such as creek restoration and strategic placement of clean dredge material) that increase sediment concentrations should benefit bird populations.

• There is no ‘standard approach’ to conservation. San Francisco Bay is diverse, with large regional differences in the responses of plant and bird populations to sea level rise impacts. Effective management must be tailored to specific regions.

• Areas identified as high-quality future habitat in our modeling should be prioritized for conservation actions today. Examples include the sediment-rich Petaluma and Napa river systems and South San Francisco Bay.

• Conservation and urban planning adaptation efforts should include projections from PRBO’s Sea Level Rise Web Tool to identify and protect uplands adjacent to present-day marshes as future marsh sites.

Applying These Results

Our hope is that land managers, conservation groups, restoration practitioners, and scientists will use this information to guide today’s decisions, enabling our current marshlands to adapt to rising seas. A main lesson learned is the importance of time scale: in the first 50 years, we expect to see an increase in the amount of marsh habitat present bay-wide, but the second 50 years projects the opposite—a dramatic decrease. This illustrates the challenges faced by conservation practitioners; planning efforts need to look ahead 100 years and include actions that help ensure that today’s investments have lasting outcomes for birds, other wildlife, and people.

Another key finding is the importance of sediment to the persistence of marshes. Sediments are physically added to the San Francisco Bay through upstream actions, land-use decisions, bay dredging activities, and deliberate habitat restoration projects, and are complicated to control due to the variety of public and private entities working in this realm. Sediments might be managed across sectors to benefit the marshes most likely to persist into the future.

Finally, these analyses make clear the need for preserving upland habitat in key locations today. Now is the time to identify the undeveloped uplands that can be made available to allow mid-marsh habitat to persist into the future. This is challenging, because the bay is flanked with roads, buildings, levees, and other barriers prohibiting marsh migration upland as the sea level rises.

Despite these challenges, multi-partner tidal marsh restoration efforts are already creating new marshlands across the estuary. In 2003, 15,000 acres of former salt ponds were transferred to public ownership, beginning the South Bay Salt Pond Restoration Project, the largest wetland restoration on the West Coast. Similar efforts are underway in the North Bay as former salt evaporation ponds are being restored. PRBO scientists are working with restoration, land acquisition, and science partners to share our new sea level rise tool. With their feedback, we are refining it and making it as useful as possible to guide and enhance marsh restoration around the San Francisco Bay.

The next steps for PRBO’s sea level rise models include: ranking and prioritizing places for future tidal marsh restoration; and developing marsh bird population viability models, showing the likelihood of a given population’s persistence into a climate-changed future. Finally, to ensure our web tool is delivering the best science available, we are continually updating our models as new data on vegetation, elevation, birds, other biota, and climate become available.

We are very excited about the potential to unlock our data, make it relevant to conservation, and help ensure the persistence of tidal marsh and the benefits it provides to birds, other wildlife, and our communities well into the future.

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