Mackay Reservoir Report

Last Updated: December 5, 2025

Mackay, a privately owned dam located in Idaho, was completed in 1918 and sits on the Big Lost River Sinks.


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Summary

This rockfill dam stands at a height of 75.2 feet with a storage capacity of 45,000 acre-feet and a surface area of 1,392 acres. With a drainage area of 788 square miles, the dam has a maximum discharge rate of 7,000 cubic feet per second.

Despite being classified as having a high hazard potential, Mackay is currently in fair condition following its last inspection in October 2019. The dam features an uncontrolled spillway and a slide (sluice gate) outlet gate. Its emergency action plan (EAP) status is not specified, and there have been no inundation maps prepared. The risk assessment for Mackay indicates a moderate risk level (3), suggesting a need for ongoing monitoring and risk management measures to ensure the safety and integrity of the structure.

For water resource and climate enthusiasts, Mackay presents an intriguing case study in dam infrastructure management and risk assessment. Its location in the scenic Custer County, Idaho, surrounded by the natural beauty of the Big Lost River Sinks, underscores the importance of balancing water resource development with environmental conservation and safety considerations. As discussions around climate change and water scarcity continue to evolve, understanding the condition and risk profile of dams like Mackay becomes increasingly vital for sustainable water management practices in the region.

Year Completed

1918

Dam Length

1430

Dam Height

75.2

River Or Stream

BIG LOST RIVER SINKS

Surface Area

1392

Hydraulic Height

67

Drainage Area

788

Nid Storage

45000

Outlet Gates

Slide (sluice gate)

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

75
       
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.