USING SIO2 CONCENTRATIONS TO IDENTIFY HYDRAULIC MINING SEDIMENT IN THE WILLOW CREEK WATERSHED, YUBA COUNTY, CA

Hydraulic mining is the practice of using high-pressure jets of water to mobilize sediment. In California, hydraulic mining mobilized millions of cubic yards of sediment to work Eocene placer gold deposits in hillsides above modern creek beds. In 1893, the U.S. Congress mandated that for hydraulic mining to continue miners needed to prevent sediment mobilized during hydraulic mining from reaching “navigable waters.” In response debris control dams (DCDs) were constructed to retain the sediment.

Currently two high arched DCDs >10m high are located in the Willow Creek subwatershed of the Upper Yuba River watershed, storing sediment related to hydraulic mining. James (1991) demonstrated the amount of hydraulic mining sediment (HMS) in the Lower Yuba and Bear River watersheds is correlated to the percentage of white quartz found in HMS deposits. To understand the role DCDs play in modern HMS storage, an assessment of the white quartz content from deposits in the Willow Creek watershed was conducted. After sampling to determine background white quartz percentages, 49 samples were analyzed from sediment deposits behind 2 DCDs and 10 terraces in the Willow Creek watershed. Gravels >6mm were separated and counted to determine the white quartz percentage and, using James’ (1991) method, the percent HMS in each sample.

Preliminary results of this study suggest that despite hydraulic mining ceasing over 60 years ago in the Willow Creek watershed many of the features observed today in the watershed (e.g. terraces) are a direct result of historical hydraulic mining. Results from this study may be used to determine the percentage of HMS stored in the Upper Yuba River watershed. This knowledge may assist stakeholders in characterizing current conditions and remediating watersheds impacted by historical hydraulic mining in the Sierra Nevada foothills.