Milles Lake Dam Reservoir Report

Last Updated: December 13, 2025

Milles Lake Dam in Twisp, Washington, stands as a vital structure for fish and wildlife conservation and irrigation purposes.


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Summary

Completed in 1950, this earth dam boasts a hydraulic height of 13 feet and a structural height of 11 feet, providing a crucial 24-acre-feet of storage capacity for the surrounding area. Situated on the Unnamed Tributary of the Twisp River, the dam serves as a key feature within the Okanogan County landscape, regulated and inspected by the Washington Department of Ecology to ensure its safety and functionality.

With a Hazard Potential rated as High and a Condition Assessment deemed Satisfactory during the last inspection in June 2021, Milles Lake Dam remains a significant contributor to the region's water resource management. Its location in the Congressional District 04 of Washington underlines its importance within the state's infrastructure. As a state-regulated and state-permitted dam, it plays a crucial role in maintaining the ecological balance of the area, serving as a lifeline for the local fish and wildlife populations while also supporting agricultural activities through irrigation provisions. The dam's capacity to store 20 acre-feet of water under normal conditions highlights its essential role in sustaining the surrounding environment.

Year Completed

1950

Dam Length

1150

River Or Stream

Unnamed Tr. -Twisp R.

Primary Dam Type

Earth

Surface Area

27

Hydraulic Height

13

Drainage Area

1.32

Nid Storage

24

Structural Height

11

Hazard Potential

High

Foundations

Unlisted/Unknown

Nid Height

13
       
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.