canadianflyguy, First of all, welcome to FAOL from SoCal. New guys split the firewood and bring donuts.
When I planned a trip to Montana, I picked July because of the runoff. Then I watched the reports on the internet as that July developed into one of the hardest runoff in years for the Yellowstone and a few other rivers. It was still raging when I was there. It's a crapshoot so if you have your dates set in stone just go and make the best of it. Heck, it's Montana. Go, fish, have fun, sightsee,eat and photograph wildlife and scenery.
That time of the year I would make campground reservations now. It's easier to cancel reservations later than it is to get them when the time is short.
I like the Missouri river from Craig/Wolf creek to Cascade. There are several streams in Yellowstone NP that should be fishable then, too. Jim
I'm either going to, coming from or thinking about fishing. Jim