A view of the Yellowstone River in Hayden Valley. The Yellowstone River is the longest undammed river in the continental United States. NPS/Neal Herbert. The Yellowstone River is the last major undammed river in the lower 48 states, flowing 671 miles (1080 km) from its source southeast of Yellowstone into the Missouri River and then, eventually, into the Atlantic Ocean. It begins in the Absaroka Mountain Range on Yount Peak. The river enters the park and meanders through the Thorofare region into Yellowstone Lake. It leaves the lake at Fishing Bridge and flows north over LeHardy Rapids and through Hayden Valley.
Yellowstone River
Yellowstone River