Long-term hydraulic mining sediment budgets: Connectivity as a management tool

The Sierra Fund’s (TSF) Science Director Dr. Carrie Monohan co-author a peer-reviewed article accepted for publication in the journal Science of the Total Environment titled “Long-term hydraulic mining sediment budgets: Connectivity as a management tool.” Lead author Dr. Allan James is a professor at the University of South Carolina and has partnered with TSF on our work to address the impacts of hydraulic mining on Sierra Nevada headwaters. Co-author Brandon Ertis is a recent graduate of CSU, Chico and was co-advised by Dr. Monohan and Dr. James.

Hydraulic mining altered the topography of the Sierra Nevada headwaters. Powerful water canons washed away mountaintops and cut into hillsides, and much of this material remains in the headwaters today. This study used LiDAR, a remote sensing technique that generates 3-D representations of the landscape, to measure and analyze the volume and location of hydraulic mining sediment in areas of the Yuba and Bear River watersheds. The amount of material that washed off of mine sites was calculated by reconstructing the topographic contours of the landscape, such as hydraulic mining pits. The amount of material that is still stored in areas of the watershed, such as in terraces, tailings fans and channel beds, was calculated by reconstructing the slopes of canyon walls.

The techniques developed in this study and can be used across mining-impacted regions to create local sediment budgets and to learn where hydraulic mine sediment is being stored. This in turn informs the implementation of strategies to reduce erosion and transport of sediment and mercury downstream.

Mapping and modeling tools were used to create a local sediment budget.

What’s the issue?
Hydraulic mining sediment is contaminated with mercury, which was applied to the sluices and tunnels at mining operations to improve gold recovery. In aquatic ecosystems, mercury can methylate, or convert into a more toxic form that can be incorporated into the food web and pose a risk to humans and wildlife via the consumption of mercury-contaminated fish.

Management Strategies 
The following management techniques can be used to minimize the transport of mercury-contaminated sediment:

  • Stabilizing sediment deposits (terraces and fans) to minimize erosion
  • Sealing tunnels that drain hydraulic mine pits to reduce sediment discharges
  • Stabilizing the walls of hydraulic mine pits, which contribute eroded material during storm events
  • Re-routing drainage around contaminated mine pits to further reduce mobilization

Northern California was the birthplace of hydraulic gold mining in 1853. Over the next 30 years a remarkable amount (>one billion cubic meters) of hydraulic mining sediment was produced in the northern Sierra Nevada, before the 1884 Sawyer Decision largely halted hydraulic mining. Regulations were enacted to keep hydraulic mining debris from flooding downstream farms in the Sacramento Valley. A period of licensed hydraulic mining followed from 1893 to ~1950, during which debris control dams were permitted and constructed to capture the material washing off of hydraulic mines. Today some debris control dams remain although they contain a relatively small proportion of the total volume of mercury-contaminated sediment stored locally in terraces, tailings fans and channel beds. Managing mercury-contaminated sediment is critical to limit human and wildlife exposure to mercury, which can result in serious and irreversible health impacts.

One of the many debris control dams scattered across Tahoe National Forest.

The Sierra Fund would like to thank the Tahoe National Forest for generously allowing the authors access to the LiDAR data for the Yuba and Bear River watersheds. Without our partners at the U.S. Forest Service this groundbreaking work would not have been possible.