John Evans' Maps Guided Lewis And Clark

From the University of Nebraska - Lincoln, The John Evans 1796-97 Map Of The Missouri River [See maps in the link]:

"One of the curious twists of Great Plains history is that the first accurate eyewitness map of the Missouri River in what is now North and South Dakota...was produced by a Welshman who had come to the United States to seek evidence for something that never existed: the illusory 'welsh Indians.'"  

Meriwether Lewis and William Clark [see portraits below; Clark on the left, Lewis on the right] used Evans' map(s) as a guide.