Tolt River Watershed

The South Fork Tolt River is the smaller and lesser known (than the Cedar River) but still essential second supply watershed in SPU’s freshwater supply system. Located in the foothills of the Cascades in east King County, it supplies about 30% of the drinking water for 1.3 million people in and around Seattle. It first came on-line in 1964, and since 1989 has also supported a small Seattle City Light hydro-electric facility. In 1997, the City of Seattle successfully exchanged lands within the South Fork Tolt watershed with Weyerhaeuser Company, giving Seattle 70% ownership (approx. 8400 acres) of the land that supplies the water. The eastern 30% lies in the Mt. Baker-Snoqualmie National Forest.

About the South Fork Tolt River Watershed:

  • Is 12,500 acres in size (including 8400 acres of City-owned land)
  • Can provide up to 100 million gallons of drinking water per day
  • Ranges from 760 feet in elevation at the regulating basin, to 5,535 feet at McLain Peak.
  • Receives between 90 and 160 inches of precipitation a year

The South Fork Tolt Municipal Watershed (SFTMW) Management Plan

SPU began collecting data and evaluating needs for a landscape management plan for the Tolt watershed upstream of the Tolt dam in 2006. Eight technical staff work groups were formed to identify issues of concern and evaluate opportunities for management actions. These groups worked together to craft the South Fork Tolt Watershed Management Plan. The final plan was approved by SPU in 2011. Implementation of the plan is now underway, largely focusing on near-term road system improvements, small aquatic habitat restoration projects, and initiating the plan’s long-term forest habitat management program.

The final SFTMW Management Plan and all map appendices may be downloaded from the links below:

South Fork Tolt Municipal Watershed Management Plan (pdf)

Maps for the Plan:

1 - Tolt Water Supply System Facility Components (pdf)
2 -Water Delivery Infrastructure Map (pdf)
3 - Security Infrastructure (pdf)
4 - Neighboring Landownership (pdf)
5 - Shared Road Use Rights (pdf)
6 - U.S. Forest Service Use Allocation (pdf)
7 - Forest Age Classification (pdf)
8 - Forest Habitat Classification (pdf)
9 - Primary Forest Management Zones (pdf)
10 - Planned Forest Restoration Activities (pdf)
11 - Streams and Wetlands (pdf)
12 - Sediment from Roads and Landslide Potential (pdf)
13 - Rain-on-snow Zones and Sub-basin Boundaries (pdf)
14 - Peak Flow Sensitivity (pdf)
15 - Fish Distribution (pdf)
16 - Fish & Wildlife Habitat Classification (pdf)
17 - Invasive Plants—Yellow Hawkweed Infestations (pdf)
18 - Cultural Resources Site Potential (pdf)
19 - Road System Classification (pdf)

Further information about the plan, or requests for CD or hard copy versions of the plan and maps should be directed to Brent Lackey, Major Watersheds Strategic Advisor, at (206) 684-7890 or

Related links

Tolt Treatment Facility
Cedar River Watershed

Cedar Access Permit System (CAPS)

Watershed contractors, consultants, and vendors access permit system log-in. This system is not used for public tours or field trips. For information about tours, see Education Center.