On the way west, on May 8, 1805, Lewis and Clark found a big river flowing in from the north and named it the Milk River because its water looked pale and cloudy, like tea with a little milk in it. Lewis guessed that this river might lead toward the Saskatchewan River and help the United States join the fur trade that British and Canadian traders already controlled.

Beyond Fort Mandan, their maps were still mostly blank or had only rough outlines, so much of this country was new to them. As they traveled toward the Great Falls, they kept finding new rivers and bends they did not expect. Native nations shared helpful information based on their own lands and trails, but each tribe knew best about its own area, so their stories did not always match. Because of this, Lewis and Clark sometimes had to stop, look carefully at the land, compare what they had been told, and decide which river to follow next.?

Updated: August 7, 2020