Yellowtail Afterbay Reservoir Report

Last Updated: December 5, 2025

Yellowtail Afterbay, located in Montana along the Bighorn River, is a federal-owned hydroelectric facility managed by the Bureau of Reclamation.


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Summary

Completed in 1965, this concrete dam stands at a height of 59 feet and has a maximum storage capacity of 3,141 acre-feet. With a high hazard potential and very high risk assessment rating, the dam is equipped with a controlled spillway and has a maximum discharge capacity of 15,500 cubic feet per second.

The primary purpose of Yellowtail Afterbay is hydroelectric power generation, serving as a crucial infrastructure for water resource management in the region. Its location in Big Horn County, Montana, plays a key role in regulating water flow and providing storage for the surrounding area. Despite its high hazard potential, the dam's condition assessment is currently not available, highlighting the importance of regular inspections and risk management measures to ensure the safety and functionality of the facility.

In the event of emergencies, the dam's Emergency Action Plan (EAP) is crucial for mitigating risks and ensuring the safety of downstream communities. While the EAP preparation and risk assessment are in place, there is a need for ongoing monitoring and updates to meet guidelines and address any potential vulnerabilities. Overall, Yellowtail Afterbay represents a significant asset in water resource management and climate resilience efforts, showcasing the critical role of infrastructure in supporting sustainable development in the region.

Year Completed

1965

Dam Length

1360

Dam Height

59

River Or Stream

BIGHORN RIVER

Primary Dam Type

Concrete

Surface Area

181

Hydraulic Height

46

Drainage Area

19848

Nid Storage

3141

Structural Height

72

Hazard Potential

High

Foundations

Unlisted/Unknown

Nid Height

72
       
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.