Sediment dynamics at the newly restored Island Ponds, South San Francisco Bay

Sediment accumulation is a critical factor driving the development of restored tidal marshes as they build elevation to a point suitable for vegetation establishment. This issue is particularly important for highly subsided tidal wetland restoration sites (including South San Francisco Bay salt ponds) that may be up to 200 cm below target elevations for vegetation establishment. We are evaluating sediment dynamics at the Island Ponds, the first salt ponds to be restored through the South San Francisco Bay Salt Pond Restoration Project. We are measuring sediment accretion within Pond A21, using the sediment pin method (PVC pipes set 3 meters into the sediment). The dense gypsum layer (up to 25 cm) and the lack of vegetation preclude the use of other methods. For short-term, mass-based accumulation rates, we are using a modification of the “filter paper method”, with sample discs that are deployed over a two-week tidal period. There has been substantial sediment accumulation within Pond A21 since breaching in March 2006, with approximately 8-10 cm of sediment accumulating over the first six months in many areas and even greater accumulation in some locations. Rates at higher locations are variable but lower; however, even at these locations sediment accumulation is orders of magnitude higher than in most natural tidal wetlands. Short-term, mass-based measurements of accumulation reflect similar spatial variability across the pond and show that substantial sediment accumulation has occurred throughout most of the year. Suspended sediment concentrations in flooding Bay waters have also been measured over two-week tidal cycles to correlate inputs of suspended sediments with rates of marsh sediment deposition. These results give an indication of the potential for sediment accumulation during the critical initial restoration period for subsided tidal marshes.