Mill Lake Reservoir Report

Last Updated: December 5, 2025

Mill Lake, located in Ravalli, Montana, is a private reservoir primarily used for irrigation purposes.


°F

°F

mph

Wind

%

Humidity

Summary

The dam, constructed in 1895 by the USDA NRCS, is an earth dam standing at 25 feet tall and stretching 400 feet in length. With a storage capacity of 670 acre-feet and a surface area of 42 acres, Mill Lake plays a crucial role in providing water for agricultural activities in the region. The reservoir is regulated by the Montana Department of Natural Resources and Conservation (DNRC) and is considered to have a high hazard potential.

Despite being over a century old, the condition of the dam at Mill Lake has not been rated, and it undergoes inspections every five years. Although the risk assessment indicates a moderate level of risk, there are no specific risk management measures or emergency action plans in place for the reservoir. The spillway, with a width of 30 feet, is uncontrolled, which adds to the potential hazards associated with the dam. While the reservoir holds historical significance in the region, there is a need for increased monitoring and maintenance to ensure the safety of the structure and surrounding communities in the event of an emergency.

Year Completed

1895

Dam Length

400

Dam Height

25

River Or Stream

MILL CREEK

Primary Dam Type

Earth

Surface Area

42

Hydraulic Height

25

Drainage Area

1

Nid Storage

670

Structural Height

25

Hazard Potential

High

Nid Height

25
       
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.