Over the past several decades, the average annual sediment supply from the Petaluma River watershed to the channel network is estimated to be 59,100 t/yr, equivalent to an average annual sediment yield of 184 t/km2/yr. This sediment source assessment quantifies average annual sediment from the watershed from four sources: 1) channel bed and bank erosion, 2) soil creep and sheetwash, 3) landslides, and 4) road-related erosion. Field surveys were conducted for channel bed and bank erosion features–the largest source of sediment in the study area. A delivery ratio-based soil loss equation was applied to evaluate soil creep and sheetwash contributions. Ten-year lidar differencing allowed for volumetric soil loss rates to be calculated from recent landslide activity, and rainfall analysis showed that those rates are likely similar to a longer period of record. Road-related erosion rates were applied to stream crossings and the mapped road network.

This is the only accounting of sediment sources in the Petaluma Operational Landscape Unit (OLU) to-date, filling an important data need. Compared to similar studies in the region, the area-weighted sediment supply (i.e., yield) for the Petaluma OLU is most similar to those of the Laguna de Santa Rosa and Sonoma Creek watersheds and within the same order of magnitude as other watersheds in this region. The results of this study will be used as a baseline to evaluate future temperature and precipitation impacts to the sediment supply in the watershed, and guide management recommendations that take advantage of natural processes to provide more creek sediment to bayland areas, increase resilience to future environmental conditions, and protect channel health while maintaining flood protection levels.

GIS data accompanying this report can be freely accessed at the following link: https://www.sfei.org/data/petaluma-river-watershed-sediment-source-assessment-data