Assessment of soil creep sediment generation for total maximum daily load development in a northern coastal California watershed

Sediment budgets are often used to help determine the anthropogenic impact on water quality and channel morphology and are useful for prioritizing restoration activities to help recover endangered salmonid species. Soil creep, a process in which colluvium is slowly supplied to the stream banks and delivers sediment to streams, is often estimated in sediment budgets developed for steep watersheds. Previous sediment budgets have shown that soil creep is an important sediment source in pristine watersheds and may be a significant sediment source in logged watersheds. Many sediment budgets use an empirical soil creep formula to estimate the amount of sediment delivery from this process. However, relying on empirical formulae to estimate soil creep sediment delivery with little to no field evaluation could lead to large errors in its estimation.This study investigated soil creep sediment delivery and the methods used to estimate its magnitude with a focus on three small forested watersheds within the Elk River watershed in northern California. Elk River is listed as a sediment impaired water body under Section 303(d) of the Clean Water Act and a Total Maximum Daily Load (TMDL) is being developed for the watershed.Field surveys to determine stream density were conducted in logged and nearly pristine watersheds and showed that logging has increased the stream density. Two methods, measuring voids along a stream channel and large wood in channels, were used to estimate bank erosion and provide a check on soil creep estimates. Comparing the results to suspended sediment loads, measuring voids appears to be the superior method for estimating bank erosion. Furthermore, the bank erosion surveys showed logged watersheds had higher bank erosion rates.Finally, methods for estimating soil creep sediment delivery are reviewed. Soil creep sediment delivery was estimated for the three watersheds and compared to the bank erosion rates, suspended sediment loads and other sediment sources. The resulting sediment budget reveals that soil creep is likely a minor source, < 1%, of sediment in the logged portions of the Elk River watershed. Future efforts need to focus on estimating other sediment sources, particularly logging-related increases in bank erosion and on ways to minimize this source of sediment.