Mayfield Forebay Dam Reservoir Report

Last Updated: December 13, 2025

Mayfield Forebay Dam, located on the Cowlitz River in Lewis, Washington, stands as a testament to engineering excellence with its primary purpose of hydroelectric power generation.


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Summary

Built in 1963 by the Harza Engineering Co., this concrete gravity dam reaches a height of 47 feet and serves as a vital component in the region's water resource management system. With a normal storage capacity of 133,718 acre-feet and a maximum storage of 184,194 acre-feet, the dam plays a crucial role in supplying water for both hydroelectric power generation and recreational activities.

The dam, with a multi-arch core type and resting on a rock foundation, boasts a hydraulic height of 44 feet and a structural height of 63 feet. Though classified as having a low hazard potential, the risk assessment for the dam indicates a high risk level (2). Despite this, the condition assessment is currently not available, prompting the need for updated risk management measures. Managed by the Federal Energy Regulatory Commission, the dam undergoes regular inspections, with the last one conducted in September 2020. Additionally, emergency action plans are in place, ensuring preparedness in case of unforeseen events.

Mayfield Forebay Dam's strategic location and design make it a vital component in the water resource infrastructure of the region, contributing significantly to both power generation and water supply. As a key feature along the Cowlitz River, the dam continues to play a crucial role in supporting various activities while ensuring the safety and stability of the surrounding areas. With its rich history and impressive specifications, the Mayfield Forebay Dam stands as a symbol of sustainable water management practices in the face of changing climate conditions.

Year Completed

1963

Dam Length

112

Dam Height

47

River Or Stream

Cowlitz River

Primary Dam Type

Concrete

Surface Area

2250

Hydraulic Height

44

Drainage Area

1392

Nid Storage

184194

Structural Height

63

Hazard Potential

Low

Foundations

Rock

Nid Height

63
       
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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.