I drifted about 160 miles of the Missouri river a few years ago in Montana . all along the river were metal markers with the camp number from the journals on them. It was amazing how close the camps were, sometime you could see between them. They seemed to alternate N and S of the river... (I say drifted,, we took two drift boats and rowed down stream for 2 weeks. Silly river has no current to push you and the wind blows the wrong direction... )
One thing I'm sure of, those guys had to be about as dirty as you can get, that mud in the river sticks to everything like glue. 5 years later my hunting jacket still bleeds the color of the mud when I wash it..
If it rains and your off pavement be prepared to just sit it out. You will not be riding. You cant even walk in the stuff, it sticks to your boots so bad you gain about 2" in height each step. It needs to be dry or frozen to drive on.
While I was there they had to use a D8 cat to pull a skid trailer down to the river and load a 4x4 Pickup on it and drag it back out... Cost the guy like 2500 bucks to retrieve his truck..
There are roads paralleling the river pretty much all the way, but your usually about 1 mile from the river. Lots of small roads that drop into the river though and dead end.. Mostly at old deserted ranches.