All good points. Perhaps my choice of a river as an exploitable commons was clumsy?

But it's important to understand that your last paragraph is missing my point entirely. Gatehouse operators aren't competitors; their services are not substitutable without additional costs. It's not like a barge operator can say "I will use gates 1, 2, and 4, but 3 is too pricey so I'll just not go through that gate." Maybe 2 and 4 could collaborate to build a portage, and this could be used to control 3's price, but to enforce the cartel price upon 3, 2 and 4 would have to operate the portage at a loss. All of this is avoidable economic inefficiency.

Thanks for humoring me in all this - all this writing is a great way for me to distill my thoughts on the subject.