Underdal #1 Reservoir Report

Last Updated: December 5, 2025

Underdal #1 is a privately owned earth dam located in Toole County, Montana, along the Timber Coulee river.


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Summary

Constructed in 1963 for recreational purposes, the dam stands at a height of 19 feet and has a storage capacity of 84 acre-feet. With a surface area of 14.5 acres, the dam serves both irrigation and recreational purposes, making it a vital water resource for the local community.

Managed by the Montana Department of Natural Resources and Conservation (DNRC), Underdal #1 has been designated as a low hazard potential structure with a condition assessment currently not rated. Despite its age, the dam remains in good operational condition with state-permitted inspections and enforcement ensuring its safety and functionality. The surrounding area benefits from the recreational opportunities provided by the dam and its reservoir, contributing to the overall water resource management in the region.

Located in Congressional District 00, Montana, Underdal #1 plays a crucial role in water management and climate resilience efforts in the region. With its historical significance dating back to the 1960s, the dam continues to serve as a valuable asset for the community, highlighting the importance of sustainable water resource management and infrastructure maintenance in the face of changing climate conditions.

Year Completed

1963

Dam Length

400

Dam Height

19

River Or Stream

TIMBER COULEE

Primary Dam Type

Earth

Surface Area

14.5

Hydraulic Height

15

Nid Storage

84

Hazard Potential

Low

Nid Height

19
       
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.