East Boulder Tailings Dam Reservoir Report

Last Updated: December 27, 2025

The East Boulder Tailings Dam in Sweet Grass, Montana, is a privately owned structure regulated by the state's Department of Environmental Quality.


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Summary

The dam, with a maximum storage capacity of 460 acre-feet and a normal storage of 153 acre-feet, serves primarily for tailings disposal. Located in a high hazard potential area, the dam has a satisfactory condition assessment as of June 2018.

Constructed with a surface area of 103 acres, the East Boulder Tailings Dam poses a potential risk to the surrounding area in case of failure, prompting the need for regular inspections. The last inspection was conducted in February 2021, with an inspection frequency of every 5 years. Despite its high hazard potential, the dam meets regulatory guidelines and is equipped with emergency action plans to mitigate risks.

Climate and water resource enthusiasts can appreciate the significance of the East Boulder Tailings Dam as a critical structure for managing mine waste in the region. With its location in a picturesque setting in Sweet Grass, Montana, the dam serves as a reminder of the importance of maintaining and regulating such structures to ensure the safety of nearby communities and the environment.

Surface Area

103

Nid Storage

460

Hazard Potential

High
       
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.