What is the longest undammed river in the USA?

The Yellowstone River is a tributary of the Missouri River, approximately 692 miles (1,114 km) long, in the western United States. Considered the principal tributary of the upper Missouri, the river and its tributaries drain a wide area stretching from the Rocky Mountains in the vicinity of Yellowstone National Park across the mountains and high plains of southern Montana and northern Wyoming.

There are no storage dams located on the mainstem of the Yellowstone River. However, the watershed contains five major reservoirs: Bull Lake, Boysen, Buffalo Bill, Big Horn, Tongue River, and Lake De Smet reservoirs.

The river rises in northwestern Wyoming in the Absaroka Range, on the Continental Divide in southwestern Park County. The river starts where the North Fork and the South Fork of the Yellowstone River converge. The Yellowstone River flows northward through Yellowstone National Park, feeding and draining Yellowstone Lake, then dropping over the Upper and Lower Yellowstone Falls at the head of the Grand Canyon of the Yellowstone within the confines of the park. After passing through the Black Canyon of the Yellowstone downstream of the Grand Canyon, the river flows northward into Montana between the northern Absaroka Range and the Gallatin Range in Paradise Valley. The river emerges from the mountains near the town of Livingston, where it turns eastward and northeastward, flowing across the northern Great Plains past the city of Billings.