Sun River Diversion Reservoir Report

Last Updated: January 11, 2026

The Sun River Diversion, also known as Diversion Lake, is a federal water management structure located in Simms, Montana.


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Summary

Completed in 1914 by the Bureau of Reclamation, this concrete dam serves primarily for irrigation purposes along the North Fork Sun River. With a height of 132 feet and a storage capacity of 6,395 acre-feet, the dam spans 261 feet in length and covers a surface area of 202 acres.

Managed by the Bureau of Reclamation, the Sun River Diversion has a low hazard potential and a moderate risk assessment rating. The uncontrolled spillway, with a width of 140 feet, can handle a maximum discharge of 34,000 cubic feet per second. The dam is situated in a picturesque location in Teton County, Montana, and plays a crucial role in regulating water flow for agricultural needs in the region.

Despite its age, the Sun River Diversion is a reliable and essential water resource infrastructure in the area. With regular inspections and a well-maintained structure, this dam continues to support the irrigation needs of the local community while preserving the natural environment of the North Fork Sun River.

Year Completed

1914

Dam Length

261

Dam Height

132

River Or Stream

NORTH FORK SUN RIVER

Primary Dam Type

Concrete

Surface Area

202

Hydraulic Height

114

Drainage Area

600

Nid Storage

6395

Structural Height

132

Hazard Potential

Low

Foundations

Rock

Nid Height

132
       
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.