Hydraulic mining sedimentation in the Willow Creek Watershed, Yuba County, CA.

Hydraulic mining is the practice of using high-pressure water to mobilize sediment. In California, historical hydraulic mining mobilized over a billion cubic yards of sediment to work Eocene placer gold deposits in hillsides above modern creek beds of the Western Sierra Nevada Metamorphic Belt. This research utilized sediment mixing ratios to investigate the storage and movement of hydraulic mining sediment (HMS) in the Willow Creek Watershed. The percentage of white quartz was used in the Bear River watershed to determine the percent of hydraulic mining sediment (HMS) in a deposit (James, 1991). Applying sediment mixing ratios to the Willow Creek Watershed, sediment deposits that contained ≥ 50% rounded white quartz pebbles in the 6-64 mm range were considered to be comprised of 100% HMS, Sediment mixing ratios of the hydraulic mining deposits were divided into populations based on their location within the watershed to determine the longitudinal and lateral movement of HMS in the Willow Creek Watershed. Hydraulic mining was shown to increase the white quartz percentage of deposits an average of 7.6% over in-situ auriferous gravels in the Willow Creek Watershed. The sediment mixing ratio varied geomorphically and decreased from mine tailing deposits in high terrace samples (n=12, μ=100%) to gravel bar samples (n=12, μ=49.3%) found in the active channel. Results from this study indicate that sediment mixing ratios can be extended throughout the Western Sierra Nevada metamorphic belt wherever historical hydraulic mining occurred to determine the current location and movement of HMS.