A quick little hike will get you down to the bottom of the Painted Rocks Dam. This is the begining of the West Fork of the Bitterroot River. There is some of the best fishing here, but if you catch a Bull Trout, make sure you put it back!
You're looking for a Bison Tube hidden with local materials.
Bring a writing stick and be sure of your footing, some of these rocks are really loose.
Be careful around the Spillway and waters edge. Water levels can rise rapidly, and without warning.... because of this, the cache is far back in a safe area.