Spatial variability of suspended sediment concentration within a tidal marsh in San Francisco Estuary

The sustainability of existing marshes and feasibility of future marsh restoration projects in San Francisco Estuary and elsewhere are threatened by a potential imbalance between accelerating sea-level rise and tidal marsh accretion rates. Marsh accretion is, in large part, dependent upon the availability of suspended sediment supplied from adjacent waterways. As water and sediment move across a marsh plain, suspended sediment settles and is trapped by vegetation near the source, resulting in less suspended-sediment concentration (SSC) and deposition in the interior of the marsh. Measurements of deposition and limited observations of SSC within marshes have confirmed a decrease in sediment supply and accumulation from the marsh edge to the marsh interiors, but the spatial variability of SSC has not been quantified in a manner that allows for comparison to a theoretical sediment transport model. For this study, transects of SSC were collected within a marsh at China Camp State Park in the San Francisco Estuary which demonstrate that a dominant pattern of settling can be quantified and generally matches the exponentially decreasing pattern of SSC predicted by a simple advection-settling model. The observed pattern suggests that sediment settling and marsh flow characteristics are consistent both spatially (between transects) and temporally (between monthly sampling events). However, deviations from the predicted pattern occurred systematically at some locations and are likely related to resuspension of sediment from the marsh surface or small, unmapped creek channels that supply sediment to the marsh. Despite these deviations, our data show this simple 1-D model of advection and settling can be used to generalize within-marsh sediment transport as a function of distance from the nearest sediment source.