The Yellowstone River flows from Yellowstone National Park to its confluence with the Missouri River, changing from a swift mountain river with rapids into a slower plains river that meanders and has many islands. William Clark first encountered the river at Bozeman Pass and traveled downstream, making stops along the way. Pompeys Pillar stands near the beginning of the river’s braided portion, where the channel twists and turns around islands, and from the top of the Pillar several sandbars and islands can be seen woven into the Yellowstone.