Physical Evolution of Restored Breached Levee Salt Marshes in the San Francisco Bay Estuary

Abstract

Since 1972 over 940 ha (2,300 ac) of leveed former salt marsh sites around San Francisco Bay have been restored to tidal action, purposely or through natural processes. The evolution of these sites can inform predictions of rates of marshplain evolution and establishment of tidal channel systems. A review of the history of 15 re-flooded sites ranging in size from 18 to 220 ha (45 to 550 ac) and in age from 2 to 29 years indicates that marshplain vegetation with more than 50% cover was established at nine of the sites within 4 to 20 years. The remaining six sites aged 2 to approximately 20 years continue to be less than 50% vegetated. The evolution of these sites is consistent with the following simple conceptual model of the physical evolution of restored tidal marshes in subsided breached sites. Initially, deposition of estuarine sediment builds up mudflats that allow vegetation establishment once elevations are high enough for vegetation to survive. Sites that are initially lower in the tidal frame take longer to vegetate than those that are initially higher. Three factors appear to retard the time frame for vegetation establishment: limited estuarine suspended sediment supply, erosion of deposited estuarine muds by internally generated wind waves, and restricted tidal exchange. These factors affect evolution more significantly in larger sites. The comparatively short time frame for vegetation coloni-zation and marshplain evolution experienced in earlier, smaller, and/or less subsided breached levee restorations may not necessarily be replicable by simple levee breaching on larger subsided restoration sites now being planned. Our review of the 15 sites also indicates that the formation of tidal channels within the marshes is greatly dependent on whether and how high the site was filled before breaching. Filled sites at high intertidal elevations (above approximately 0.3 m below mean higher high water) can vegetate quickly but after several decades may show little development of tidal channels.

Diet and growth of non-native Mississippi silversides and yellowfin gobies in restored and natural wetlands in the San Francisco Estuary

“…Coon Island, the natural reference wetland, also had the most vegetation diversity, the best defined assemblages, and the highest elevation of all the sites in this study. Bull Island and Pond 2A are both lower in elevation, and the youngest (most recent) restoring marsh (Pond 2A) has the least species diversity and elevation indicating that it is still actively maturing as a marsh (Williams & Orr 2002. These small differences in developing wetland vegetation may account for the diversity of prey available to these generalist fish.…”

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…The overall management implication of a coastal wetland, if validated to comply with multiple stable state theory, would be that the wetland is valuable and long-lived, but non-stationary, and once degraded very difficult to restore. In fact, high rates of failure in coastal wetland restorations already suggests such a case [46][47][48][49].…”

Identifying factors that influence expression of eutrophication in a central California estuary

“…Most water control structures at Elkhorn Slough and other estuaries around the world were constructed to allow farming in floodplains and prevent inundation of farmed fields with salt water (Caffrey & Broenkow 2002, Williams & Orr 2002. For Elkhorn Slough, there are 2 aspects of agricultural development that have negative consequences for the estuarine ecosystem: nutrient run-off and artificial tidal dampening; these work in concert to influence eutrophication expression.…”