Cascade County Commissioners approved a $44,000 contract to TD&H Engineering for the Rainbow Dam Road slide during their April 28 meeting.

A roughly 150-foot section of the road has shifted significantly and is beginning to slide toward the river, creating a safety concern, particularly for heavy vehicles that travel the road regularly, according to county public works.

“The failure is associated with an improperly installed retaining wall located at an existing culvert crossing for a large unnamed ephemeral drainage,” according to the county.

The road has been monitored over time, due to instability along the riverbanks, according to public works.

During the April 22 work session, Les Payne, county public works director, said the guard rail had caved in and the road had shifted across the river from Giant Springs Park.

Giant Springs Road slide repair begins this week [2025]

During a recent assessment, county staff determined the road movement required immediate attention and contacted multiple engineering firms to evaluate the safety of continued use of the roadway.

TD&H Engineering was the only firm to provide a cost estimate for the work and the total contract is under the threshold requiring public bids under state law.

Temporary barriers were installed to restrict heavy traffic from the compromised edge of the roadway while a long-term plan is developed based on those evaluations.

County approves contract for Giant Springs Road repairs [2024]

Rainbow Dam Road is the primary access route for NorthWestern Energy to reach Rainbow Dam and heavy equipment and materials are routinely transported for dam operations, according to county public works.

Payne said during the April 22 work session that he’d contacted a NWE employee at the dam asking if the company would be willing to cost-share. Payne said the answer was no.

County staff anticipates the repair will include a “retaining wall system, likely a mechanically stabilized earth wall with geogrid reinforcement extending toward the roadway centerline,” according to the scope of work.

Shallow bedrock in the area will likely influence the design and may limit excavation depth and wall embedment, according to the county.

The area is within an unmapped floodplain so floodplain permitting isn’t anticipated, according to staff.

The contract scope includes survey, geotechnical engineering, civil design, construction document preparation, and some bidding and construction phase services.