Rainbow Reservoir Report

Last Updated: December 5, 2025

Rainbow is a privately owned hydroelectric dam located on the Missouri River in Cascade County, Montana.


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Summary

Constructed in 1910, this historic dam stands at 44 feet tall and has a hydraulic height of 42 feet. With a storage capacity of 1237 acre-feet and a maximum discharge of 125,000 cubic feet per second, Rainbow plays a crucial role in water resource management in the area.

Managed by the Federal Energy Regulatory Commission, Rainbow's controlled spillway, with a width of 1026 feet, helps regulate water flow and prevent flooding in the region. Despite its high hazard potential, the dam's condition assessment is not available, highlighting the need for ongoing monitoring and maintenance. With an emergency action plan last revised in 2020, Rainbow is equipped to handle potential risks and ensure the safety of surrounding communities.

As a significant piece of infrastructure in the Omaha District, Rainbow serves as a key source of renewable energy and water supply for the local area. Its rich history and engineering design by Charles T. Main make it a fascinating site for water resource and climate enthusiasts interested in the intersection of human ingenuity and environmental stewardship.

Year Completed

1910

Dam Length

1405

Dam Height

44

River Or Stream

Missouri River

Primary Dam Type

Other

Surface Area

126

Hydraulic Height

42

Drainage Area

22928

Nid Storage

1237

Structural Height

44

Outlet Gates

Other Controlled - 30, Slide (sluice gate) - 5

Hazard Potential

High

Foundations

Rock

Nid Height

44
       
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.