Beaverhead River is located near Dillon, Montana. The trail is primarily used for fishing and fly fishing and is accessible from May until September.
My favorite BIG TROUT river in MT. Ridiculously huge fish everywhere. Use light tippets - 6X and small nymphs in the day then move to big streamers and stout tippets - 15lbs in evening or early. You wil be amazed.