Rainbow
Key Takeaway
Rainbow is classified as high hazard in Montana. It was completed in 1910 and is 116 years old. Its primary use is hydroelectric. High hazard means loss of life is likely if the dam fails — it does not indicate the dam's current condition. Learn more.
Physical Details
| Dam Height | 44 ft (taller than 95.7% in MT) |
| Dam Length | 1,405 ft |
| Dam Type | Other |
| Max Storage | 1.2K acre-ft |
| Normal Storage | 1.2K acre-ft |
| Surface Area | 126 acres |
| Drainage Area | 22,928 sq mi |
| Max Discharge | 125,000 cfs |
| Year Completed | 1910 (116 years old) |
| NID ID | MT00557 |
Safety Information
Loss of human life is likely if the dam fails.
Hazard potential describes downstream consequences of failure, not the dam's current condition. What does this mean?
Ownership
NorthWestern Energy
Private
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Low Hazard 36 ftFrequently Asked Questions
What is the hazard classification of Rainbow?
Rainbow is classified as High Hazard. Loss of human life is likely if the dam fails. This classification refers to the potential downstream consequences of a failure, not the dam's current structural condition.
Who owns and operates Rainbow?
Rainbow is owned by NorthWestern Energy (Private). The owner is responsible for maintenance, inspections, and compliance with dam safety regulations.
When was Rainbow built?
Rainbow was completed in 1910, making it 116 years old. Many U.S. dams were built between the 1930s and 1970s during a period of major infrastructure investment.
What is the primary purpose of Rainbow?
Rainbow serves the following purposes: Hydroelectric. Many dams in the U.S. are multi-purpose, supporting flood control, water supply, recreation, and other uses.
How tall is Rainbow?
Rainbow has a dam height of 44 ft. Dam height is measured from the natural streambed at the downstream toe to the top of the dam, and may differ from the visible height.
When was Rainbow last inspected?
Rainbow was last inspected on August 10, 2022. Inspection dates indicate when a formal review occurred, not the results of that inspection.