South Fork Tolt Hydroelectric Project

The South Fork Tolt River Hydroelectric Project was completed in 1995 and is City Light's newest hydro facility. The Project leverages the Seattle Public Utilities (SPU) Tolt Reservoir and Dam that has been providing about 30% - 40% of the drinking water to the metropolitan Seattle area since 1964. The Tolt is an earthen dam located on the South Fork Tolt River in the foothills of the Cascades in east King County upstream of a water fall that is a natural barrier to fish passage. Water from the reservoir is routed a few miles downstream via a penstock (pipeline) to the hydroelectric powerhouse where it generates electricity before sending the water to SPU's Tolt Water Treatment Facility.

While Seattle City Light's South Fork Tolt hydroelectric facility supplies less than 1% of Seattle City Light's power requirements, it is an important source of renewable energy for the utility.

Tolt Project License

The current license (FERC Project No. 2959) for the South Fork Tolt River Hydroelectric Project was initially issued in 1984 by the Federal Energy Regulatory Commission. Federal and state agencies and the Tulalip Tribes filed motions with FERC to intervene in the relicensing procedure focusing primarily on the need to provide adequate long-term provisions for flows, habitat restoration and monitoring. City Light filed a settlement agreement in 1988 which included fish and wildlife protection measures and mitigation jointly signed by the City, state and federal fish and wildlife agencies, and Tulalip Tribes. The agreement was accepted by the FERC, incorporated into the license and the stay was lifted in 1989. The forty-year license will expire in July 2029.

The Tolt Fish Advisory Committee (TFAC), which is comprised of the parties that negotiated the Settlement meets on an as-needed basis to oversee implementation of and compliance with license required mitigation and related operations.

Fish Protection

City Light works in partnership with Seattle Public Utilities and the TFAC to ensure protection, mitigation and monitoring of fish located downstream of the dam. This work includes:

• Adjusting flows from the dam on a seasonal and monthly basis to provide the right amount of water for fish to access habitat, spawn, and rear in the South Fork Tolt River
• Decreasing flows at the Dam or at the project slowly, so fish can sense the change and move safely to deeper water preventing them from getting trapped
• Ensuring water is continuously passed to the river, even when the project trips offline
• Monitoring water quality
• Conducting studies and research with a focus on salmon and steelhead to guide decisions on their protection, to better understand the South Fork Tolt River as their home, and assist in defining restoration actions
• Implementing fish habitat projects with a focus on improving river process, like placing large wood in the South Fork Tolt in the form of jams

Watch a video highlighting our research

Read one of our published reports

Watch a video highlighting one of our fish habitat projects

Recreational Opportunities

The Tolt Project is located inside the City of Seattle's Municipal Watershed that supplies Seattle and surrounding communities with drinking water. To maintain high-quality drinking water the area is closed to the public and recreation. To make up for this, City Light provided financial support to King County Parks to assist in the development of the Moss Lake Natural Area and to the USFS for the development of the Middle Fork Snoqualmie Trailhead and parking lot.

Cultural Site Protection

We actively protect cultural resources within the project area per the terms of the Tolt FERC License in partnership with Seattle Public Utilities, local Tribes, and federal, state, and county land managers.

Land and Habitat Protection

We are committed to protecting and enhancing our land and habitat in the Tolt Basin. Above the reservoir, the City of Seattle successfully exchanged lands within the South Fork Tolt Watershed in 1977 with Weyerhaeuser Company, giving Seattle 70% ownership (approximately 8,400 acres) of the land that supplies the water. The eastern 30% lies in the Mt. Baker-Snoqualmie National Forest. Seattle Public Utilities (SPU) manages these lands. You can learn about SPU's South Fork Tolt Municipal Watershed Management Plan for the Tolt watershed upstream of the dam.

In the Tolt/Snoqualmie Watershed downstream of the dam, City Light's Endangered Species Act Land Program purchases land that supports quality Chinook and steelhead habitat. The lands in the program were purchased with City Light and grant funds. Working with partners we also actively restore habitat that benefits both fish and wildlife in the basin.