Can you clarify or expand on your question a bit? The French explorers of the 17th century described and mapped the Great Lakes as summarized here.

They believed they had found very large lakes in the middle of the continent that were the source of the St. Lawrence River—as indeed they had. There was also the possibility that the lakes fed another river (or a short portage led to a river system) that flowed southwestward to the Pacific, but once Marquette and Jolliet descended the Mississippi all the way to present-day Arkansas, they realized that it must instead flow into the Gulf of Mexico.