ROAD-RELATED SEDIMENT DELIVERY VOLUMES DERIVED FROM A “FORWARD-LOOKING” EROSION ASSESSMENT, HOLLOW TREE CREEK WATERSHED, MENDOCINO COUNTY, CALIFORNIA

Erosion and mass wasting originating from rural road systems is a common and significant sediment source to streams in wildland and managed watersheds. Road-related sediment production includes storm-triggered episodic fluvial erosion and landsliding, and chronic erosion of fine sediment from the road alignment, both of which impact salmonid habitat.

Traditional sediment source inventories, including sediment budget studies, employ void identification and measurement techniques to quantify historic sediment production (e.g., landslides and gullies). Over the last 10 years we have developed and tested an alternative methodology that provides a quantitative estimate of future road-related sediment delivery that is useful for proactive watershed protection or restoration planning. The assessment employs a systematic "forward-looking" inventory to identify, quantify and prioritize sources of future sediment delivery. It relies on repeatable data collection protocols employed during a single visit to each sediment delivery site and can be used to develop scientifically defensible, prioritized watershed-wide sediment reduction plans.

We recently employed this sediment assessment protocol in the 30 mi2 Hollow Tree Creek watershed in Mendocino County, California. The assessment was part of Trout Unlimited's North Coast Coho Project, funded by the California Dept of Fish and Game and US Fish and Wildlife Service. First, stereo air photo analysis identified 140 miles of road in the watershed (4.7 mi/mi2). Secondly, a field-based forward-looking erosion inventory was conducted along 114 miles of forest road. We identified 652 sites with the potential to deliver 136,530 yd3 of sediment to Hollow Tree Creek, if left untreated. Approximately 82,900 yd3 of sediment delivery was estimated from stream crossings, 7,640 yd3 from potential landslides, and 4,540 yd3 from other sites, mostly gullies. Approximately 30% of this volume would be episodically delivered in the next 50 years. In addition, we identified 42.4 miles of “connected” road surfaces and ditches that drain directly to stream channels. These would delivery up to 41,400 yd3 of chronic fine sediment to stream channels in the Hollow Tree Creek Watershed over the next decade, depending on road use, if no changes are made to road drainage.