The longest undammed river in North America exits Yellowstone Park in Gardiner and for the next 60 miles flows northward to Livingston through the most beautiful river valley on earth. Okay, beauty is subjective. No doubt the spectacular trout fishing in the Yellowstone and the spring creeks that feed it would color anybody's judgment. In fact, the angler's challenge is to tear his eyes from the mesmerizing rings of rising trout long enough to look around. The jagged, snowcapped Absarokas march northward along the east bank, and the vast Montana sky forms a blue bowl overhead. Wildlife abounds in the lush and fertile valley. It's worth pausing to look at. Three spring creeks-Nelson, DePuy, and Armstrong-rise on private ranchland and feed the river. They are loaded with fat (14- to 20-inch) brown, rainbow, and cutthroat trout that feed all day on lavish hatches of midges and pale morning dun, sulfur, and blue-wing olive mayflies. It's year-round fishing, though July and August are prime time. The ranchers preserve the quality of the angling by enforcing fly-rod-only, catch-and-release regulations. They charge a modest fee and restrict the number of anglers. You can book a day of fishing through most of the many outfitters in the Livingston and Bozeman area. The best way to experience the valley is from a drift boat on the Yellowstone with a fly rod in hand any time from the end of runoff in early July until the snow flies in October. Throw streamers or hoppers against the banks or climb out and bounce dry flies through a riffle. Just remember to look up once in a while. It's heavenly. For more information, go to www.bozemannet.com/busdir/fishing.htm.