Wills Reservoir Report

Last Updated: December 5, 2025

Wills, R.


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Summary

is a private irrigation dam located in Missoula, Montana, along the TR-UNION CREEK. Built in 1966 by the USDA NRCS, this Earth-type dam stands at a height of 20 feet and has a length of 300 feet. It serves the primary purpose of irrigation and has a storage capacity of 51 acre-feet, with a normal storage level of 26 acre-feet. The dam is regulated by the DNRC and is subject to state permitting, inspection, and enforcement.

Despite its low hazard potential and current "Not Rated" condition assessment, Wills, R. does not have a detailed emergency action plan (EAP) in place. The dam's spillway width is 18 feet, and it has a maximum discharge capacity of 15 cubic feet per second. The structure is not owned or funded by any federal agency but is under the jurisdiction of the state of Montana. Congressman Greg Gianforte represents the Congressional District in which the dam is located.

Overall, Wills, R. is a significant water resource infrastructure in Montana that plays a crucial role in supporting agricultural activities through irrigation. While it currently poses a low hazard potential, the lack of a detailed EAP and condition assessment raises some concerns about its long-term safety and maintenance. Water resource and climate enthusiasts can appreciate the importance of monitoring and managing dams like Wills, R. to ensure the sustainable use of water resources in the region.

Year Completed

1966

Dam Length

300

Dam Height

20

River Or Stream

TR-UNION CREEK

Primary Dam Type

Earth

Hydraulic Height

20

Drainage Area

1.63

Nid Storage

51

Hazard Potential

Low

Nid Height

20
       
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.