Walker Creek Mercury TMDL

On September 29, 2008, the U.S. Environmental Protection Agency approved the Basin Plan amendment incorporating a TMDL for mercury in the Walker Creek and Soulajule Reservoir watersheds. This amendment had been adopted by the Regional Water Board in January 2007 and became effective on September 11, 2008 when it was approved by California's Office of Administrative Law.

TMDL Implementation Status (updated June 2019)

Tomales Bay Grazing Waiver Program
The Tomales Bay Grazing Waiver Program helps to reduce erosion along Walker Creek, which reduces mercury discharges downstream. This program also implements the Tomales Bay Pathogens TMDL and the Tomales Bay Mercury. For more information, please refer to the program page for the grazing waiver program website or the Tomales Bay Pathogens TMDL website.

Water Quality Monitoring in Walker Creek
Staff of the San Francisco Bay Water Board have collected storm samples from Walker Creek to evaluate progress towards meeting the TMDL target for mercury concentrations in suspended sediment, as illustrated in the Water Quality Report Card for Mercury in Walker Creek Watershed. The drought constrained this storm sampling effort, but staff plans to resume sampling in winter 2019-2020.

Soulajule Reservoir
Marin Municipal Water District is continuing its work to address methylmercury in Soulajule Reservoir. In 2018, the District produced the Synthesis of Phase 2 Pilot Studies and Prioritization of Reservoir Management Measures. This report describes and prioritizes a set of potential management methods for controlling methylmercury in Soulajule Reservoir. The District’s next steps are to evaluate these methods for effectiveness and develop a long-term reservoir management plan. The selected management methods must be consistent with water supply management objectives for Soulajule Reservoir, which currently include the following:

  1. Provide storage and supply for municipal drinking water;
  2. Achieve flows in downstream Walker Creek to support native anadromous fish (coho salmon and steelhead) required by the Department of Fish and Wildlife; and
  3. Support appropriate water quality objectives within Soulajule Reservoir and in downstream Arroyo Sausal.


Walker Creek, located in Western Marin County, California, is a 21-kilometer stream that flows through a former mercury mining district into Tomales Bay. The Walker Creek watershed supports steelhead and coho salmon, a diverse array of wildlife, and recreational fishing in Soulajule Reservoir. Mercury-laden sediment originating from inactive mercury mine sites threatens wildlife such as fish-eating birds, and also humans who consume fish from the reservoir.

The TMDL for mercury in the watershed includes a broad-based implementation strategy for reducing both mercury discharges and methylmercury production. Before developing the TMDL, the Water Board and U.S. EPA collaborated in highly successful cleanup efforts at the Gambonini Mine site (the greatest source of mercury in the watershed). Reductions already achieved in the amount of mercury coming off the mine site indicate that when parties in the watershed work together, water quality in Walker Creek will once again support a healthy fishery and the wildlife that depend upon it.

downstream depositional area in Walker Creek

For more information, contact:
Gerardo Martinez
Water Resource Control Engineer
San Francisco Bay Regional Water Quality Control Board
1515 Clay St., Suite 1400
Phone: (510) 622-1015
Fax: (510) 622-2460
E-mail: Gerardo.Martinez@waterboards.ca.gov