Mountain Lake Dam Reservoir Report

Last Updated: December 15, 2025

Mountain Lake Dam, situated in Olga, Washington, is a state-owned structure managed by the Washington Department of Ecology.


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Summary

Completed in 1905, this earth dam serves as a critical water supply source, with a maximum storage capacity of 3010 acre-feet and a normal storage of 2870 acre-feet. The dam stands at a hydraulic height of 17 feet and a structural height of 16 feet, providing essential water resources for the surrounding community.

This multi-arch dam on Cascade Creek in San Juan County not only fulfills its primary purpose of water supply but also caters to recreational activities in the area. With a surface area of 180 acres and a drainage area of 2.27 square miles, Mountain Lake Dam offers a picturesque setting for outdoor enthusiasts and water resource aficionados alike. Despite its high hazard potential, the dam has been deemed to be in satisfactory condition, as indicated by a recent inspection report in April 2019.

With its serene location and historical significance, Mountain Lake Dam stands as a testament to the enduring importance of water resource management in the face of climate challenges. As a vital infrastructure for water supply and recreation, this earth dam continues to play a crucial role in sustaining the ecosystem and meeting the needs of the local community in the beautiful landscapes of Washington State.

Year Completed

1905

Dam Length

172

River Or Stream

Cascade Creek

Primary Dam Type

Earth

Surface Area

180

Hydraulic Height

17

Drainage Area

2.27

Nid Storage

3010

Structural Height

16

Hazard Potential

High

Foundations

Soil

Nid Height

17
       
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.