Yellowstone River in the Hayden Valley, Yellowstone National Park. This, the longest undammed river in the contiguous United States at 692 miles long. Within Yellowstone National Park the river has many faces. As it flows out of Yellowstone Lake it is a wide smooth flowing calm river. As the river passes through the Hayden Valley it makes gentle meandering curves. Along this stretch, it is common to spot Bald Eagles flying overhead or perched on the river’s edge. Wolves frequent the valley as well, likely attracted by the elk and bison who also frequent the lush grasslands that cover the valley floor. Further downstream are the iconic Upper and Lower Falls of the Yellowstone River as it enters the Grand Canyon of Yellowstone. The river in and out of the national park is known to be one of the best trout streams in the country.