John Ball Reservoir Reservoir Report

Last Updated: December 5, 2025

John Ball Reservoir, located in McCone County, Montana, is a privately owned earth dam constructed in 1935 for fire protection, stock, and small fish pond purposes.


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Summary

With a dam height of 16 feet and a storage capacity of 145 acre-feet, this reservoir on the TR-South Fork Rock Creek serves as a vital water resource in the region. The reservoir has a maximum discharge of 1000 cubic feet per second and features an uncontrolled spillway with a width of 100 feet.

Despite being classified as having a significant hazard potential and a moderate risk assessment, the condition of John Ball Reservoir is currently not rated. The dam is regulated by the Montana Department of Natural Resources and Conservation (DNRC) and undergoes regular state inspections and enforcement. While the reservoir has not been modified in recent years, it continues to provide essential water supply and storage for the surrounding area, highlighting the importance of proper maintenance and monitoring in ensuring its safety and functionality for both agricultural and ecological purposes.

Year Completed

1935

Dam Length

360

Dam Height

16

River Or Stream

TR-SOUTH FORK ROCK CREEK

Primary Dam Type

Earth

Hydraulic Height

16

Drainage Area

1.09

Nid Storage

145

Hazard Potential

Significant

Nid Height

16
       
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.