In 1806, Captain William Clark and his party of explorers drove 50 horses east to follow the Yellowstone River to its confluence with the Missouri River. The horses helped carry provisions when navigable waters were not available. Clark needed a cottonwood tree large enough to make a dugout canoe so they could travel by river, but he could not find one in this area because the cottonwood species on the eastern side of the Continental Divide is not as large as that on the western slope.