METHODOLOGY TO ESTIMATE SEDIMENT SOURCES FOR TOTAL MAXIMUM DAILY LOAD STUDIES, VAN DUZEN RIVER, HUMBOLDT COUNTY, CALIFORNIA

Developing Total Maximum Daily Load (TMDL) sediment studies are problematic, due to the large size of wildland watersheds, terrain characteristics, accessibility (cooperating landowners), available funding, and high rates of “natural” erosion on the landscape. It is necessary to develop a strategy that can identify the key sources and causes of both natural and management-related sediment production in a particular watershed, as well as quantify the magnitude of past erosion and sediment delivery, to confidently develop TMDL allocations.

In 1999, Pacific Watershed Associates (PWA) conducted a sediment TMDL study for the 429 mi2 Van Duzen River (VDR) watershed. PWA utilized a statistically based approach to identify and characterize sediment sources such as debris slides, debris flow tracks, deep-seated slides and gullies, and identified the land use association, if any. The study approach involved 1) stratification of the VDR basin into similar geologic terrain types, 2) air photo analysis of the basin for large sediment source features, 3) field inventory of 80 randomly sampled 40 acre plots for smaller sediment source features, 4) extrapolation of small sediment sources to the entire VDR basin, 5) evaluation of total estimated sediment delivery that is controllable and uncontrollable by type of erosion and management time frame, and 6) statistically estimate the confidence limits for the estimates of sediment production. Basin wide results show that 61% of the sediment delivery was identified as non management-related or uncontrollable, and 39% was identified as management-related or controllable. Sub-basin sediment source estimates varied based on types and intensity of management-related activities.

Since the 1999 Van Duzen Sediment Source Investigation, the stratified random sampling method for estimating sediment sources for TMDL studies has been used in the North Fork Eel River, Upper Eel River and currently in the Middle Main Eel River project areas.