Combie Reservoir Sediment & Mercury Removal Project

NOTE:  This is a project of the Nevada Irrigation District.  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 accumulate in the Gold Country, resulting in elevated fish mercury concentrations, lost water storage space and impaired water quality. Reservoirs as man-made facilities present unique opportunities to address legacy mercury contamination with innovative maintenance activities.

The Nevada Irrigation District reservoirs on the Bear River are 303(d) listed as impaired for mercury. Private aggregate mining companies have removed sediments that migrate into the reservoirs. At Combie Reservoir, dredging operations were halted in 2003 as a result of mercury levels, affecting NID efforts to maintain reservoir storage capacity.


This project removes sediment from Combie Reservoir while introducing an innovative recovery process to reduce sediment and mercury in the Bear River watershed. If successful the process can be applied at other reservoirs throughout the Sierra Nevada. In time, there could be a beneficial effect toward remediation and reduction of mercury contamination.

Mercury in sediment at the bottom of reservoirs can methylate and be incorporated into the aquatic food web, biomagnifying to dangerously high levels in top predatory fish. Figure from SFEI Mercury report.


Reservoir sedimentation in Gold Country reservoirs presents unique challenges to ensure that mercury in the aquatic environment is removed safely and effectively. Basic principles are to avoid turbid water and to dispose material out of the 100 year floodplain. Project benefits include; water supply reliability, water quality protection and improvement, ecosystem restoration and enhanced recreation.

Next Steps

The Nevada Irrigation District begins full scale removal of accumulated sediments in 2018 and will continue through 2020 under a DWR grant. Sediment will be removed in the dry and by dredging. The effectiveness of sediment and water treatment steps will be monitored so that an adaptive management approach can be used to improve the engineering processes throughout the pilot project. Monitoring reservoir biota before during and after sediment removal remains a critical project performance measure. Real time monitoring of effluent will ensure that only clean water be returned back into the reservoir. Project outcomes will help inform reservoir best management practices in similarly affected water bodies within the Sierra Nevada.

Project Funders

Past and present funders include: Sierra Nevada Conservancy, California Department of Water Resources through the CABY IRWMG and Nevada Irrigation District.

See also:

Mercury Removal Project information on NID website – This page on the Nevada Irrigation District website includes a video and public documents pertaining to the Combie Reservoir project.

SNC Video on Combie Reservoir Project – This video, produced by the Sierra Nevada Conservancy and featuring The Sierra Fund Science Director Dr. Carrie Monohan, highlights the ongoing discharge from upstream legacy mine sites and the Combie Reservoir project as a key opportunity to clean up mercury.

NID’s Video on Combie Reservoir Mercury Removal Project – Another short video about Nevada Irrigation District’s pilot project to remove mercury-contaminated sediment from their reservoir.