The Yellowstone is the longest undammed river in the Lower 48, and it’s one of the most challenging and unpredictable trout fisheries in the world. One day it can be flyfishing nirvana, and the very next day the trout can shut down completely. Hardcore guides and aficionados view this as a supreme challenge. The best times to fish can be late summer or early fall, when trout key on terrestrials and streamer patterns. The Yellowstone has 200-plus miles of trout water, and some of the best runs are near downtown Livingston. More info: Dan Bailey’s Fly Shop, 800-356-4052;

Next Best: Roaring Fork River, Colorado The Roaring Fork exudes that same wild character as it pounds an unimpeded 70-mile path to the Colorado River, dropping over 4,000 vertical feet along the way. More info: Alpine Angling, 800-781-8120;