Friedrichs and Perry (2001) have hypothesized that exogenous sedimentation is a vital part of a feedback system (with tidal height, marsh elevation and local vegetation) that keeps the marsh surface in equilibrium with sea level. This study investigated the relationship between local sedimentation rates, presence of overlying vegetation, distance from tidal channel, and local elevation in two marshes located in the San Francisco Estuary. Exogenous sedimentation rates measured using sediment traps on marsh plains at approximately MHW were found to be much lower than expected. Sedimentation rates were highest closest to tidal channels, regardless of overlying vegetation, but declined rapidly on the inland portions of the marsh and were never high enough to build the marsh plain at rates greater than 0.91–1.37 mm m⁻² yr⁻¹. Maintenance of tidal marsh elevations solely by exogenous sedimentation within the San Francisco Estuary seems unlikely. Observations that local marshes keep pace with sea level rise implicate local productivity as the source of increased sediments.