SEDIMENT SOURCE ASSESSMENT WITHIN THE PINOLE CREEK WATERSHED, CONTRA COSTA COUNTY, CALIFORNIA: A SEDIMENT BUDGET APPROACH

In many watersheds of the San Francisco Bay Area, sediment production and delivery to creeks is poorly understood. Because the Bay Area has a heterogeneous lithology, active tectonics, and variable climatic conditions across relatively small spatial scales, sediment production and transport in small watersheds that drain to the Bay is highly variable. In addition, sediment processes are influenced by over 150 years of land use and land management.

In response to landowner and stakeholder concerns regarding the control of sediment erosion, the maintenance of aquatic habitat (especially for steelhead trout), and maintaining flood conveyance, we conducted a field-intensive sediment source assessment in the Pinole Creek watershed, western Contra Costa County, California. Data collection and analysis was guided by a conceptual model of sediment processes, and included quantifying hillslope sediment sources such as landslides, gullies, road-related sources, and land use-related sources, quantifying in-channel sources including bank erosion and bed incision, and quantifying the creek's sediment load over Water Year 2004. This data was utilized to create a watershed-scale sediment budget to understand the processes and volumes of sediment production and transport. Sediment was primarily sourced from active landslides, active gullies, and erosion from paved road-related drainage. Sediment derived from bank and bed erosion was only a minor contributor to the overall sediment budget. Suspended sediment concentrations ranged between 5.6 mg/L and 13,238 mg/L, suspended sediment load for the wet season was 9,964 metric tonnes, and suspended sediment export was 252 t km-2. Despite the fairly large sediment load, we found that most sediment delivered from the watershed to the downstream reaches is currently being transported out into the Bay and not impacting flood conveyance in the flood control channel.

Local landowners and natural resource managers requested this data as a necessary precursor to decisions on how to best prioritize restoration opportunities and apply limited resources to sediment erosion and related impacts. In addition, this study contributes to the understanding of watershed scale sediment processes in the Bay Area – a subject on which there is surprisingly little published literature.