This study investigated how changes in salt marsh soil properties and topography on sediment fans related to shifts in salt marsh plant community composition in the Elkhorn Slough Watershed, California, USA. Several sediment fans have formed in this watershed as soil eroding from farms moved downslope, filling marshes, mudflats, and channels. Sandy sediment deposition increased marsh plain elevation and altered edaphic properties by increasing bulk density and decreasing soil moisture, salinity, and soil nitrogen compared to reference sites. These changes created a strong wetland-upland gradient and influenced the development of well-defined vegetation zones from wetland to upland: pickleweed (Salicornia virginica), cattail (Typha spp.) and bulrush (Scirpus spp.), and arroyo willow (Salix lasiolepis). Based on statistical analysis, arroyo willow grew in a distinct edaphic environment, and its expansion into the salt marsh was restricted by elevation in tidal areas greater than 1.80 m NAVD 88, spring soil moisture levels lower than 20%, and year-round salinity levels lower than 2.67 dS/m. Cattail and bulrush were present in transitional environmental conditions with fluctuating salinity and at an elevation similar to that of the pickleweed community. The hydrogeologic setting played a part in this change, as the contribution of upland sandy soils likely facilitated the emergence of new edaphic properties including lower salinity, lower soil moisture, and reduced soil nutrients. The findings in this study underline the importance of on-going erosion-control efforts to estuarine conservation in Central California.