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Draw-down of reservoir above Rainbow Dam

Draw-down of reservoir above Rainbow Dam (September 2022)
Posted at 5:41 PM, Sep 27, 2022
and last updated 2022-09-28 09:42:45-04

NorthWestern Energy is doing maintenance to Rainbow Dam along the Missouri River in Great Falls.

The dam host wood flashboards that are the actual water retaining feature and a steel fixture. The lifespan of the boards sits at around 20 years and were last changed in 2004.

The dam has a steel structure that is embedded into the concrete portion on the dam. The wooden flashboards rest against the steel, holding the water. Rainbow Dam has 12, 12-inch flashboards stacked on top of each other creating a 12-foot-tall spillway.

The boards are referred to as flashboards, because in the event of a flood situation, the steel behind the boards can be released and the boards are washed down downstream.

For the maintenance to take place, the dam has had to drop the pool level of the reservoir 11 feet lower than its previous fill level. This poses a danger to recreators along the river.

“…it's just very important to caution people to stay out of the river in that area. While that pool is lowered. There'll be unexpected hazards that they can't see. And we want everyone to be safe." Said Jo Dee Black, Spokeswoman for NorthWestern Energy.

The pool level will remain low until around October 6, 2022, to allow for maintenance crews to finish the repairs.

Black said that along trails and areas near the river and dam are posted to signs to stay advised to surrounds. They ask to keep pets and your person out of the revealed mud as it can be unpredictable. Signage also explains what happens if you hear a siren and water levels rise.


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