Although what is now Missouri and all of "Louisiana" (which is much larger than as we now know it) was nominally under Spanish control there were in fact few Spanish people. Nearly all white people in Missouri were French. Ste. Genevieve was a major French town as were the towns of Cahokia and Kaskaskia across the Mississippi River in the Illinois Country. Migration from Quebec was common and done generally by canoe up the St. Lawrence , portage to Great Lakes and ascend a tributary of the Mississippi to that great river. Travel on down to Missouri was the easy part. There is a vast literature on the subject.