Thornton Lake Dam Reservoir Report

Last Updated: December 5, 2025

Thornton Lake Dam, located in Deer Lodge, Montana, serves the primary purpose of irrigation and is privately owned.


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Summary

The dam, completed in 1904, is a rockfill structure with a height of 11 feet and a hydraulic height of 8 feet. It impounds Thornton Creek and has a normal storage capacity of 243 acre-feet, covering a surface area of 31 acres.

Despite being privately owned, Thornton Lake Dam is state-regulated and subject to inspection, permitting, and enforcement by the Montana Department of Natural Resources and Conservation (DNRC). The dam has a significant hazard potential and is classified as having a very high risk, requiring attention to risk management measures. The outlet gates consist of a slide (sluice gate) and the spillway type is controlled, ensuring controlled release of water in case of excess storage.

Water resource and climate enthusiasts will be intrigued by Thornton Lake Dam's historical significance, engineering design, and management practices. Its location near the Thornton Creek and its impact on irrigation in the area make it a notable structure in Montana. With its risk assessment indicating a high level of risk, efforts to ensure the safety and integrity of the dam are crucial for water resource sustainability and climate resilience in the region.

Year Completed

1904

Dam Length

315

Dam Height

11

River Or Stream

THORTON CREEK

Primary Dam Type

Rockfill

Surface Area

31

Hydraulic Height

8

Drainage Area

11

Nid Storage

243

Structural Height

11

Outlet Gates

Slide (sluice gate)

Hazard Potential

Significant

Nid Height

11
       
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.