Big Lake Reservoir Report

Last Updated: December 5, 2025

Big Lake, located in Missoula, Montana, is a significant earth dam constructed in 1915 primarily for water supply purposes.


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Summary

The dam stands at a height of 30 feet and has a storage capacity of 621 acre-feet, with a normal storage capacity of 143 acre-feet. The reservoir covers a surface area of 38 acres and drains a 4.1 square mile watershed, with a maximum discharge of 95 cubic feet per second.

Managed by the local government and regulated by the Montana Department of Natural Resources and Conservation (DNRC), Big Lake is classified as having a significant hazard potential and a moderate risk level. The dam features an uncontrolled spillway with a width of 25 feet and outlet gates that are also uncontrolled. The last inspection of the dam was conducted in August 2017, with its condition currently rated as "Not Rated" in terms of assessment.

Water resource and climate enthusiasts will find Big Lake an intriguing structure due to its historical significance, design, and operational details. Situated in a picturesque location in the Seattle District, the dam serves as a vital resource for water supply in the region. With its unique features and regulatory oversight, Big Lake presents an opportunity for further exploration and study in the realm of water resource management and climate resilience.

Year Completed

1915

Dam Length

440

Dam Height

30

River Or Stream

TR-LAKE CREEK

Primary Dam Type

Earth

Surface Area

38

Hydraulic Height

18

Drainage Area

4.1

Nid Storage

621

Structural Height

19

Outlet Gates

Uncontrolled

Hazard Potential

Significant

Nid Height

30
       
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.