Mannix Reservoir Report

Last Updated: December 5, 2025

Mannix is a privately owned irrigation dam located in Helmville, Montana, along the Madison Gulch.


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Summary

Completed in 1962, this earth dam stands at a height of 30 feet and has a storage capacity of 53 acre-feet. Its primary purpose is irrigation, serving the agricultural needs of the region. Despite its age, Mannix has a low hazard potential and is currently not rated for its condition.

Managed by the Montana Department of Natural Resources and Conservation (DNRC), Mannix is regulated, permitted, inspected, and enforced by the state. The dam is situated in Powell County, within Congressional District 00 of Montana, represented by Greg Gianforte. The location of Mannix is at S3 12N 9W, with a latitude of 46.8167 and a longitude of -112.708. The dam is not part of the US Army Corps of Engineers (USACE) and has not been assessed for risk management measures.

With a focus on water resource and climate issues, enthusiasts will find Mannix to be an interesting case study in private dam ownership and state regulation. As a crucial piece of infrastructure for irrigation in the area, Mannix highlights the importance of proper maintenance and oversight to ensure the safety and effectiveness of water management systems. Its location in the scenic landscape of Montana adds to the significance of its role in sustainable water resource utilization.

Year Completed

1962

Dam Length

150

Dam Height

30

River Or Stream

MADISON GULCH

Primary Dam Type

Earth

Hydraulic Height

30

Nid Storage

53

Hazard Potential

Low

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.