Why didn't he just build another boat?
The whole time he was trying to shove that boat up the hill I was wondering "why the heck didn't he just build another boat in the other river?"
I'm not thinking of a nice boat like the one he had, but rather like a giant raft with a bunch of logs tied together.
They had enough equipment to create the whole pulley system, so building a giant raft to transport all that rubber he wanted seems much more plausible.
Of course, he could have still created a nice walkway between the two rivers to help transport the rubber by foot after acquiring it.