I was wondering why rivers were so frequently made colonial and state boundaries in America. Of course they used them to ship goods and provided ocean access for them but why make them a state boundary when a state can just claim a big river for themselves?
Two reasons:
Rivers are easy to survey and simplify issues of uncertainty about the location of a border, at least initially. Those straight-line4 borders you see on a map are usually made by people who haven't any familiarity with the lands they are preparing to administer.
Because rivers are natural avenues for trade, as many states as possible want to have direct access to major rivers. Making the river a border between states makes this easier.