Free flowing along its entire length, the Yellowstone carries high mountain water 670 miles across the plains of Montana to its junction with the Missouri, just across the North Dakota line. At the peak of spring snowmelt, the upper Yellowstone is an awesome sight. Banks collapse, cottonwoods topple, willows are torn away. New channels appear, old ones become sloughs or oxbows where waterfowl breed and big hookjawed cutthroats and brown trout hunt silently. It is an ancient equation of destruction and renewal.