Just a general rule about rain in the intermountain west--Mont., Idaho, Colorado, New Mexico: Unless it's torrential, rain doesn't fill up creeks like it does in the east. It does however, make the bigger drainages rise which can bring in playspots on rivers like the Colorado (South Canyon) and Salmon (Blackhawk, Cat's Paw,).
For the fall in Montana, hit the Kootenai if it's between 10,000 and 20,000 (dam release). You can surf your brains out at Super Wave and then hit the park and huck waterfall at Kootenai Falls until your spine compresses. Also, Idaho's Salmon has sick play and the North Fork of the Payette (dam release) runs in the fall at 2,000--better than Gauley fest with less people and more continuous hair. Also, keep an eye on the Little White Salmon near Hood River. It's close to the coast and gets going with rain.......peace...........