During the construction of their winter camp in west-central North Dakota, Lewis and Clark noted ice flows on the Missouri by mid November (1804) and reported that the river had iced over in early December. Breakup did not commence until March, when ice flows were accompanied by numerous bison carcasses, the large herbivores having fallen through the winter ice and drowned.
Finally able to leave Fort Mandan on April 7, 1805, the explorers headed WNW through western North Dakota and then westward across the High Plains of Montana beneath migrant flocks of waterfowl and the smoke of prairie wildfires. Bison carcasses littered the river banks, clearly fed on by wolves and grizzlies; a large number of bald eagles were also observed in this region (likely feasting on the carcasses as well). Beyond the mouth of the Yellowstone River, massive herds of bison, elk and pronghorns were encountered, accompanied by packs of wolves and lone grizzlies that fed on the young, old and sick; beaver were also reported to be especially large and abundant along this stretch of the Missouri. Porcupines and mountain lions were documented for the first time and, nearing the Musselshell River, the travelers sighted outlying ranges of the Rocky Mountains.
Beyond the mouth of the Musselshell, the waters of the Missouri were noticeably clearer, its flow speed had increased and cliffs rose above the river; bighorn sheep, often in sizable herds, were first encountered in this area. By the beginning of June, the party had reached the mouth of Maria's River, a tributary named by Captain Lewis; their journey would soon become more difficult.