Thorald Dam Reservoir Report

Last Updated: December 15, 2025

Thorald Dam, located in Washington's San Juan County, serves as an essential irrigation source with a primary purpose of supplying water for agricultural needs.


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Summary

Completed in 1988, this earth dam stands at a hydraulic height of 19 feet and a structural height of 16 feet, with a length of 400 feet. The dam boasts a storage capacity of 98 acre-feet, providing water for irrigation and recreation purposes in the region.

Managed by a private entity, Thorald Dam is regulated by the Washington Department of Ecology, ensuring state jurisdiction, permitting, inspection, and enforcement. With a low hazard potential and a condition assessment of "Not Rated," the dam poses minimal risk to the surrounding area. Despite not being rated, the dam's emergency action plan (EAP) status, risk assessment, and risk management measures are currently unspecified, indicating a potential area for improvement in terms of disaster preparedness.

Nestled along the Tr-Parks Bay river or stream, Thorald Dam plays a crucial role in water resource management in the region. With its strategic location and solid construction, this dam stands as a testament to human ingenuity in harnessing water resources for the benefit of local communities while maintaining a low risk profile. For water resource and climate enthusiasts, Thorald Dam offers a fascinating case study in dam engineering and management within the picturesque landscape of San Juan County, Washington.

Year Completed

1988

Dam Length

400

River Or Stream

Tr-Parks Bay

Primary Dam Type

Earth

Surface Area

7.5

Hydraulic Height

19

Drainage Area

0.16

Nid Storage

98

Structural Height

16

Hazard Potential

Low

Foundations

Unlisted/Unknown

Nid Height

19
       
Seasonal Comparison

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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.