Antilon Lake Dam Reservoir Report

Last Updated: December 13, 2025

Antilon Lake Dam, located in Manson, Washington, is a public utility dam primarily used for recreation purposes.


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Summary

Completed in 1913, this earth dam stands at a hydraulic height of 62 feet, with a structural height and NID height also at 62 feet. The dam has a length of 300 feet and a storage capacity of 2475 acre-feet, providing ample space for recreational activities on its 118-acre surface area.

Managed by the Washington Department of Ecology, Antilon Lake Dam is state-regulated and inspected regularly to ensure its safety and satisfactory condition. With a high hazard potential, the dam is equipped with emergency action plans to mitigate any risk of failure. The last inspection in November 2018 deemed the dam to be in satisfactory condition, with an inspection frequency of every 5 years. In case of an emergency, the dam is equipped with updated emergency contacts and guidelines to meet safety standards.

The dam's location along Tr-Johnson Creek in Chelan County, Washington, makes it a vital water resource for the region. With a maximum discharge of 360 cubic feet per second, the dam plays a crucial role in managing water flow and storage in the area. As a popular spot for outdoor enthusiasts, Antilon Lake Dam serves as a recreational oasis while also serving as a critical infrastructure for water resource management in the region.

Year Completed

1913

Dam Length

300

River Or Stream

Tr-Johnson Creek

Primary Dam Type

Earth

Surface Area

118

Hydraulic Height

62

Drainage Area

2.46

Nid Storage

2475

Structural Height

62

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

62
       
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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.