Sinlahekin Dam No 3 Reservoir Report

Last Updated: December 15, 2025

Sinlahekin Dam No 3, also known as Conners Lake, is a state-owned structure located in Loomis, Washington, along the Sinlahekin Creek.


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Summary

Built in 1950 by URS-Woodward Clyde, this earth dam stands at a hydraulic height of 24 feet and a structural height of 22 feet, with a length of 285 feet. The dam serves primarily for recreational purposes, providing a storage capacity of 593 acre-feet and a surface area of 211 acres.

The dam is regulated by the Washington Department of Ecology and undergoes regular inspections, with the last assessment conducted in 2016. Despite its high hazard potential, the condition of Sinlahekin Dam No 3 is deemed satisfactory. Emergency action plans are in place but may require updates to meet current guidelines. The dam's risk assessment and management measures are currently unspecified, highlighting the importance of ongoing monitoring and maintenance to ensure its safety and functionality for water resource and climate enthusiasts in the region.

Overall, Sinlahekin Dam No 3 plays a vital role in providing recreational opportunities while also serving as a crucial infrastructure for water management in Okanogan County, Washington. Its historical significance, coupled with its ongoing regulatory oversight and maintenance efforts, underscore the importance of sustainable practices in safeguarding water resources and addressing climate-related challenges in the region.

Year Completed

1950

Dam Length

285

River Or Stream

Sinlahekin Creek

Primary Dam Type

Earth

Surface Area

211

Hydraulic Height

24

Drainage Area

70

Nid Storage

593

Structural Height

22

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

24
       
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.