Regulating Basin West Dam Reservoir Report

Last Updated: December 15, 2025

Regulating Basin West Dam, also known as Tolt Dam, serves as a critical water supply structure in King County, Washington.


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Summary

Built in 1962 by Carey & Kramer Consulting Engineers, this earth dam stands at a height of 32 feet and has a storage capacity of 882 acre-feet. The primary purpose of the dam is to provide water supply, with secondary purposes including hydroelectric power generation.

Located on the North & South Fork Tolt River, this dam falls under the jurisdiction of the Federal Energy Regulatory Commission (FERC) and is operated by the local government. Despite its high hazard potential, with a moderate risk assessment rating, the dam has not undergone recent condition assessments. Emergency action plans are in place, although there is room for improvement in meeting guidelines and preparedness for potential risks.

Enthusiasts of water resources and climate in the region would find Regulating Basin West Dam to be a fascinating structure with a significant impact on the surrounding ecosystem. With its crucial role in water supply and potential for hydroelectric power generation, the dam represents a balance between resource management and environmental conservation. Monitoring and maintenance efforts are essential to ensure the safety and sustainability of this vital infrastructure in the face of changing climate patterns and water resource demands.

Year Completed

1962

Dam Length

320

Dam Height

32

River Or Stream

North & South Fork Tolt River

Primary Dam Type

Earth

Hydraulic Height

24

Drainage Area

1

Nid Storage

882

Hazard Potential

High

Foundations

Soil

Nid Height

32
       
Seasonal Comparison

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.
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