 Once , tens of millions of American bison ( commonly called buffalo ) , one of the keystone species of the Great Plains and the Ohio Valley , roamed the plains of the Missouri River basin . Most Native American groups in the basin relied heavily on the bison as a food source , and their hides and bones served to create other household items . In time , the species came to benefit from the indigenous peoples ' periodic controlled <unk> of the grasslands surrounding the Missouri to clear out old and dead growth . The large bison population of the region gave rise to the term great bison belt , an area of rich annual grasslands that extended from Alaska to Mexico along the eastern flank of the Continental Divide . However , after the arrival of Europeans in North America , both the bison and the Native Americans saw a rapid decline in population . Hunting eliminated bison populations east of the Mississippi River by 1833 and reduced the numbers in the Missouri basin to a mere few hundred . Foreign diseases such as smallpox raged across the land , <unk> Native American populations . Left without their primary source of sustenance , many of the remaining indigenous people were amalgamated into <unk> areas and reservations . 
