Flathead Lake At Polson Mt Reservoir Report

Last Updated: December 5, 2025

Flathead Lake is the largest freshwater lake in Montana, located in the town of Polson.


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Summary

The lake was formed by glaciers during the last ice age and has a surface area of 191.5 square miles. Its hydrology is dominated by the Flathead River, which is the primary surface flow provider, and snowpack, which provides the majority of the lake's water. The lake serves as a vital source of irrigation water for local agriculture and supports a variety of recreational activities, including fishing, boating, and swimming. The lake is also home to several species of fish, including trout, salmon, and bass. Its history includes being a sacred site for the Salish and Kootenai tribes and being explored by Lewis and Clark in 1805. Today, Flathead Lake is considered one of Montana's most treasured natural resources.

Reservoir Details

Elevation Of Reservoir Water Surface Above Datum, Ft 24hr Change 0.0%
Percent of Normal 100%
Minimum 2,883.11 ft
2008-04-10
Maximum 2,893.59 ft
2022-06-25
Average 2,890 ft
       
Seasonal Comparison
Elevation Of Reservoir Water Surface Above Datum, Ft Levels

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.