Linking tidal-creek sediment fluxes to vertical sediment accretion in a restored salt marsh

Abstract

Despite growing interest and investment in salt-marsh restoration, relatively few marshes subjected to restoration efforts have been systematically monitored to assess physical restoration trajectory or success. In south San Francisco Bay, California, USA, where 83% of wetlands were lost via human manipulation, the largest wetland restoration effort on the US west coast is currently underway, restoring approximately 6000 ha of former salt-production ponds to mixed habitats. The Whale Tail–Cargill Mitigation salt-marsh complex in south San Francisco Bay has a century-long history of drainage, industrial use as salt-production ponds and subsequent restoration and recovery. Restoration of the 20 ha Cargill Mitigation Marsh was initiated in the late 1990s when the levee surrounding the subsided, former salt-production pond was breached in two locations, enabling conversion back to salt-marsh habitat in the subsequent decades. Here, we present time-series measurements of sediment fluxes in the primary tidal creek entering the salt-marsh complex, which are compared to decadal-scale sedimentation patterns determined from repeat elevation surveys and cores collected at the study site. All three methods show net sediment import to the restored marsh. The greatest equivalent sedimentation rates occurred early in the restoration, with generally decreasing rates through time. The long-term average, as determined from cores and expressed as a vertical sedimentation rate, is approximately 1.8 cm year⁻¹. Rates from the elevation data are between 1.4 and 2.6 cm year⁻¹, with higher rates earlier in the restoration. The most recent estimates, computed from time-series instrument deployments, indicate seasonal variability in sediment import. Annualized rates are lower in winter, approximately 0.1 cm year⁻¹, and higher in summer, approximately 1.7 cm year⁻¹. Although our measured long-term equivalent sedimentation rates are considerably greater than the current local relative sea-level rise (SLR) of 0.3 cm year⁻¹, an increase in SLR or decrease in available suspended sediment would threaten the ability of the marsh to keep pace with SLR and avoid drowning in the future.