Tolt Dam and Reservoir
Tolt Dam and Reservoir, looking east.
The City of Seattle owns and operates the Tolt Dam and Reservoir located on the South Fork Tolt River in unincorporated King County, Washington, approximately 35 miles east of Seattle. The dam was completed in 1962 and is a 200-foot high earthfill dam with a Morning Glory Spillway. The reservoir stores up to 57,900 acre-feet of water. South Fork Tolt River water makes up 30 to 40 percent of the drinking water supply provided by Seattle Public Utilities (SPU) to 1.6 million people in the greater Seattle area.
Seattle City Light (SCL) operates the South Fork Tolt Hydroelectric Project (Project No. 2959) under a license administered by the Federal Energy Regulatory Commission (FERC) and generates electricity from water released from the reservoir.
Tolt Reservoir and Morning Glory Spillway (foreground).
Dam Safety
Tolt Dam is regulated by FERC for dam safety. The dam is operated and inspected in accordance with federal regulations and guidelines.
SPU and SCL maintain and implement dam safety programs to reduce the likelihood and consequences of a Tolt Dam failure. The dam safety programs include a surveillance and monitoring plan to monitor the performance of the dam and an emergency action plan to document predefined actions that the City of Seattle will take if there is an emergency associated with the dam.
SPU operators monitor Tolt Dam continuously 24 hours a day, 7 days a week via instrumentation and cameras. SPU crews visually inspect the dam daily. SPU engineers inspect the dam quarterly. FERC engineers inspect the dam annually. Independent consultant engineers inspect and evaluate the dam every 5 years.
Downstream Communities
The South Fork Tolt Dam is located on the South Fork Tolt River, in the foothills of the Cascade Mountains. The South Fork Tolt River merges with the North Fork Tolt River below the dam to form the Tolt River, a tributary to the Snoqualmie River. The nearest incorporated communities downstream of Tolt Dam are the City of Carnation (14 miles downstream) and the City of Duvall (28 miles downstream).
Potential Inundation Areas and Times
In the unlikely event of a dam failure, water from the reservoir may inundate downstream communities. Maps of potential inundation areas and arrival times are located on the King County Dams Emergency Preparedness page.
Emergency Notifications
The City of Seattle’s Emergency Action Plan (EAP) for the Tolt Project includes predefined notification procedures to alert agencies and local governments of dam emergencies. Local authorities will then notify downstream residents to evacuate if necessary.
As of April 2024, King County Office of Emergency Management is the primary authority who will notify residents of dam emergencies using Wireless Emergency Alerts (WEAs) and ALERT King County messaging.
The National Weather Service (NWS) also issues flood warning notifications for flood scenarios, including dam break scenarios: NWS Flood Related Products.
The City of Seattle has historically operated an early warning system, including sirens located in and near Carnation which included weekly audible tests at noon on Wednesdays. As of April 2024, the sirens have been deactivated while a third-party assessment of the siren system is conducted.
For more information see the Tolt Dam Early Warning System page.
Be prepared
For more information on how you and your family can prepare, respond, and recover from disasters, please visit King County’s Office of Emergency Management.
To stay informed about potential hazards and threats that impact your area, including dam emergencies, sign up for ALERT King County.
Local governments maintain Comprehensive Emergency Management Plans (CEMPs) that document community specific planning for hazard mitigation, preparedness, response, and recovery.
Partners in safety
The City of Seattle works with the following agencies and local governments in public communication and preparation of the Emergency Action Plan (EAP) for the South Fork Tolt River Reservoir and Dam:
- King County
- City of Carnation
- City of Duvall
- Riverview School District
- Eastside Fire and Rescue
- Carnation / Duvall Police Department
- National Weather Service
- Snohomish County
- Indian Tribes
- Washington State Emergency Management
- Federal Energy Regulatory Commission