I just finished a unit on early American history with my class, and we learned a little about this too! I was especially fascinated by the log boats used in Lewis and Clark's journey. They actually used dugout canoes in their trip into the American Northwest. The carved them from cottonwoods and pine trees that could carry four or five men each, plus baggage. I still have a hard time wrapping my head around the fact that a single tree could carry five grown men plus baggage!