Natural Shorelines

The natural shorelines in the ART Subregional Project area range from fully tidal marshes that are either exposed to the open Bay or are protected from wave and tidal energy by offshore mudflats, to muted tidal marshes and ponds that are protected from the Bay by berms and levees and have water levels controlled by tide gates and other structures.

Photo credit: BCDC

These natural shoreline systems provide ecosystem service benefits such as flood risk reduction, water quality improvement, habitat for threatened and endangered species, carbon sequestration, and opportunities for education and interpretation.

ART evaluated the vulnerability and risk of twelve tidal and five managed marshes in collaboration with PRBO, Point Blue Conservation Science. The assessment used an online decision support tool developed by Point Blue. This tool allowed for analysis of several critical factors, including the rate of sea level rise, the current elevation of the natural shoreline area relative to the tidal frame (the elevation range between the lowest and highest tides), mineral sediment availability either from the Bay or nearby tributaries, and the rate of organic matter accumulation.

Key Findings

Historically, marshes have kept pace with sea level rise by accumulating mineral sediment and by moving upward and landward in the tidal frame, but current projections suggest that the rate of sea level rise is accelerating and the concentration of Bay suspended sediment is declining. Furthermore, much of the Bay shoreline, including the project area shoreline, is fairly well developed and there are few opportunities for inland migration.


Photo credit: Mercurywoodrose

The PRBO online decision support tool findings are consistent with these regional projections: tidal marshes in the project area will “downshift” from higher to lower elevation marsh habitat (e.g., from high to mid-marsh or mid- to low marsh), and eventually to mudflat. Under the more pessimistic sediment supply scenario (low availability), all marshes in the  project area will transition to mudflat by 2050. Under the high sediment scenario only one marsh will be lost by 2050 and only one additional marsh will be lost by 2090.

Tidal marshes and managed ponds help reduce incoming wave heights, protecting shoreline structures from wind waves and tidal energy. The loss of these natural shorelines can place shoreline communities at greater risk of flooding by increasing the likelihood that structural shoreline protection is overtopped or fails, and by increasing the cost of maintaining, repairing and upgrading these already expensive structural protection assets.

Photo credit: NOAA

Many of the marshes and ponds in the project area have been restored, representing a significant financial investment. These natural shoreline areas provide habitat to a number of state-listed or federally threatened and endangered species as well as migrating and wintering birds that rely on them for breeding, foraging, and high tide refugia. Additionally, they offer opportunities to view wildlife, provide access to the shoreline, and offer scenic and aesthetic benefits. The loss of these functions will have consequences for the people that use these areas for outdoor enjoyment or recreation, and will ultimately diminish the value of the Bay Area as a desirable place to live.

Project Findings and Materials

For more information:

  • Lindy Lowe
  • 415-352-3642