In the spring of 1805, it took the Corps of Discovery about two weeks to travel through the stretch of the Missouri River that today lies within the Charles M. Russell National Wildlife Refuge. This remote prairie grassland marked the beginning of their journey from mostly flat open country into rougher "breaks" with steep coulees and rocky river bottoms. The land here was covered with native grasses, scattered sagebrush, and patches of trees such as cottonwoods and some pines growing along the draws and cliffs. Wildlife was plentiful, and the expedition had many chances to hunt for food, including bison, elk, deer, and pronghorn.

In this region they also saw bighorn sheep on the high bluffs and met the powerful grizzly bear, which many Native people had already warned them about. In their journals, the captains often called the grizzly a "white bear" because its long, pale guard hairs can give its fur a lighter, sometimes silvery look. Later, near today's Great Falls of the Missouri, they named places such as White Bear Island in honor of these fierce bears they both respected and feared.