University of California Water Resources Center

While Elkhorn Slough wetlands are protected by a State ecological reserve and NOAA research reserve, intensified farming in the watershed has led to high soil erosion rates and sedimentation into the slough, where several sediment fans have formed at the base of slopes. The goal of this study was to determine how watershed land use change (i.e., increasing agriculture) and associated sedimentation over 30 years influenced changes in salt marsh soil physical properties, and in turn, plant composition through the use of remote sensing, GIS, and field methods. Objectives were to use current and historic aerial photographs to map historic wetland plant communities and quantify the extent and type of salt marsh vegetation change on sediment fans from 1971 to the present, and to explore the driving factors of this change through field sampling of marsh soil properties and topographic features. Analysis of historic aerial photos demonstrated that the formation of sediment fans in pickleweed salt marsh led to the expansion of arroyo willow into the marsh plain. A cattail and bulrush plant community also formed on the sediment fans, forming a band between the willow and pickleweed. This vegetation change occurred as sedimentation led to lower salinity levels, lower soil moisture, higher bulk density, higher elevation, and an increase in soil sand content. Resource managers are actively working with farmers to reduce soil erosion and off-farm sediment. Evidence of habitat change from sedimentation documented in this study supports the need for these continued erosion control efforts. As these efforts continue to reduce sedimentation into the slough, it would be valuable to continue to monitor the sediment fans to determine if any recovery in salt marsh vegetation occurs.