Check out this overhead drone footage of the flooding of Interstate 90 at Crow Agency.
The Montana Department of Transportation is providing an alternate route for travelers in the affected area of I-90:
For westbound travelers, take S338 in Wyoming, which changes to S314 in Montana. Go north on S314 to US 212 in the Busby area, and turn left. When you get to the Little Big Horn Battlefield interchange on I-90, take a right over the top of the interchange and continue west to Billings.
Eastbound travelers from Billings need to take I-94 to Highway 39 west of Forsyth (Colstrip cut across). Then go to US 212 at Lame Deer, turn west to Busby, then south on S314 to Wyoming S338.