Abstract
We assess whether or not restored marshes in the San Francisco Estuary are expected to be sustainable in light of future landscape scale geomorphic processes given typical restored marsh conditions. Our assessment is based on a review of the literature, appraisal of monitoring data for restored marshes, and application of vertical accretion modeling of organic and inorganic sedimentation. Vertical accretion modeling suggests that salt marshes in San Pablo Bay will be sustainable for moderate relative sea level rise (3 to 5 mm yr-1) and average sediment supply (c. 100 mg L-1). Accelerated relative sea level rise (above 6 mm yr-1) and/or reduced sediment supply (50 mg L-1) will cause lowering of the marsh surface relative to the tide range and may cause shifts from high to low marsh vegetation by the year 2100. Widespread conversion of marsh to mudflat-"ecological drowning"-is not expected within this time frame. Marshes restored at lower elevations necessary to aid the natural development of channel systems (c. 0.5 m below mean higher high water) are predicted to accrete to high marsh elevations by the year 2100 for moderate relative sea level rise and sediment supply conditions. Existing rates of sediment accretion in restored fresh water tidal marshes of the Delta of greater than 9 mm yr-1 and slightly lower drowning elevations suggest that these marshes will be resilient against relatively high rates of sea level rise. Because of higher rates of organic production, fresh water marshes are expected to be less sensitive to reduced sediment availability than salt marshes. The ultimate long-term threat to the sustainability of tidal marshes is the interruption of coastal rollover-the process by which landward marsh expansion in response to sea level rise compensates for shoreline erosion. Bay front development now prevents most landward marsh expansion, while shoreline erosion is expected to accelerate as sea level rises.
Highlights
The large scale historic loss of tidal marsh habitat throughout the San Francisco Estuary has prompted major initiatives aimed at restoring extensive areas of diked former tidal marsh (Goals Project 1999; CALFED 2000; Steere and Schaefer 2001)
Assuming that marshes are restored to the required elevation for halophyte colonization, what evidence is there to indicate that accretion would continue to keep pace with sea level rise and viable future salt marsh would result? we evaluate the vertical response of restored marshes to predicted changes in sea level and sediment supply using empirical data and accretion modeling
Under conditions of greater sediment availability (150 mg L-1), the model predicts that the marsh plain is capable of sustaining relative sea-level rise of up 6 mm yr-1 (Figure 6)
Summary
The large scale historic loss of tidal marsh habitat throughout the San Francisco Estuary has prompted major initiatives aimed at restoring extensive areas of diked former tidal marsh (Goals Project 1999; CALFED 2000; Steere and Schaefer 2001). This indicates that once marshes have reached a mature state, they can maintain their relative tidal elevations under existing low rates of sea-level rise, even under conditions of low sediment availability. With continued low sediment availability, marshes continue to lower relative to the tidal frame after the year 2100 and drowning is predicted for the highest sea level rise conditions (Figure 6).
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