You should check out the Big Horn River in Montana just below the dam (its just north of the southern Montana border). It is a fabulous river with tons of fish and clear water. It is on the Crow Indian reservation or at least in the middle of it. Remember "Dances with Wolves". That is what the terrain looks like (prairie). The fly shops are very knowledgeable about the good flies for the day and if you are going to use a guide they are good. I just put my pontoon boat in and float it. There is a guide, tall lanky character who wears a big floppy white hat whom I watched with his customers and was very impressed with him. He took a lot of time teaching the guys who were with him. They caught a lot of big fish. I don't know his name.
I stop along the way and wade in. You should be pretty good at nymphing with a strike indicator though because it is the best technique for the river. The float I do is about three miles long and covers great water. There is a pull out parking lot there to leave your vehicle. I like to fish it in June. It is so much fun and the water is easy to float on a pontoon boat, pretty flat with riffles and runs.