Combie Sediment and Mercury Removal Project

This project consisted of sediment removal from the mercury-impaired Combie Reservoir while implementing an innovative sediment recovery and water treatment process. Best-management practices and recommendations developed during the life of this project can be applied and serve as a roadmap to addressing other similarly impacted reservoirs throughout the Sierra Nevada. 

NOTE: This is a project led and funded by the Nevada Irrigation District (NID) and the California Department of Water Resources (DWR) Proposition 13 Bay-Delta Multipurpose Water Management Program.  The Sierra Fund supports this project for its great potential to be a model for restoring water storage capacity in the Sierra’s existing reservoirs, while remediating mercury left in our watersheds from legacy mining.

“Reservoirs are areas where mercury contaminated sediments can accumulate in the Gold Country, resulting in elevated fish mercury concentrations, lost water storage capacity and impaired water quality. Reservoir facilities present a unique opportunity to address legacy mercury contamination with innovative maintenance activities.”

Carrie Monohan, Ph.D., Program Director

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Rollins Reservoir and Camp Far West Reservoir. The Bear River Watershed is one the Sierra Nevada’s most impacted by historical gold mining practices in the state (May et al., 1999). Lake Combie is on the State Water Resource Control Board (SWRCB) Clean Water Act (CWA) 303(d) List as impaired for mercury and also has a site specific fish consumption advisory for mercury issued by the California Office of Environmental Health Hazard Assessment (OEHHA, 2009). Combie Reservoir’s original storage capacity was 5,555 acre-feet. Until 2003 NID performed routine dredging maintenance operations to maintain the water storage capacity, however, these activities were found to re-suspend mercury-impacted sediments, which resulted in a CWA violation. The suspension of maintenance dredging impacted the reservoir’s storage capacity, because sediments started to accumulate every year. This precipitated the need for a new and innovative approach to abate sediment and mercury from the reservoir.


This project included sediment removal in both wet (dredging) and dry (excavation) conditions from the mercury-impaired Combie Reservoir while implementing an innovative centrifugal mercury extraction process and water treatment in a manner that was protective of the environment and downstream ecosystems within the Bear River watershed. This project conducted extensive testing and monitoring to evaluate system performance and compliance with environmental regulations. Additional environmental and real-time monitoring within the reservoir and system effluent and receiving waters were performed in an effort to improve understanding of mercury cycling and to advance the science that protects the quality of our water.

Next Steps

The Nevada Irrigation District completed all removal actions at Combie Reservoir in August 2019. The project results and evaluation were compiled as part of the Combie Reservoir Sediment and Mercury Removal Project Completion Report. Report findings are invaluable to state regulators and water managers in the Sierra Nevada hoping to address similarly impacted reservoirs. Monitoring reservoir biota, sediment, and water quality before during and after sediment removal remains a critical project performance measure and is currently ongoing by the United States Geological Survey (USGS). 

Project Funders and Funders

California Department of Water Resources
Nevada Irrigation District
Great Lakes Environmental and Infrastructure