Tracking Sediment Movement with Pittag Surveys during Storm Events in the North Fork of Caspar Creek, California

Abstract:

Caspar Creek is a gravel-bedded stream near Fort Bragg in Jackson Demonstration State Forest in Northern California’s Coast Range. Since 1962, detailed measurements of streamflow, turbidity, and rainfall have been made at a weir just below the study area by the Pacific Southwest Research Station and California Department of Forestry and Fire Protection (CAL FIRE). However, little research has been done to identify how gravel-sized sediment moves through the system during storm flows. The North Fork of Caspar Creek drains a relatively small and steep watershed providing fast residence times that allow direct correlation between rainfall events and movement of the gravel-dominated (<20% sand) bed material. The study reach has an average discharge of 3.8x10^-2 m³/s, yet over the past two years has had 4 storm events with flows greater than 1.0 m³/s, a flow that has shown to be a minimum needed for bedload movement. Gravel sized rocks inserted with HDX pit tags were placed throughout the study reach and surveyed after each storm to track their movement through the system. The results show that bed movement responds nonlinearly to increased peak storm discharge with larger storms disproportionately moving sediment further downstream. The movement distances appear independent of local slope, and the distribution of transported sediment sizes does not show significant variance between different storm events. This info will contribute to a better understanding of gravel deposition and bar formation in rainfall-driven watersheds and may help predict sediment fluxes during storm events.