Runoff conditions in late spring/early summer can be problematic; dependent on snow pack/precip and temps. I was in MT early July 2010, a year of moderate snowfall and spring temps; rivers were in great shape as was the fishing. In 2011 heavy snowpack and heavy spring rains led to flooding and high river levels all around the state; I was there in late July/early August and had never seen the rivers that high that time of year (fishing was great). Tailwaters are not as effected by runoff, but even they were crazy high in 2011. Not trying to scare you off, just think you should be aware.
Regards,
Scott