How did explorers discover the source of a river? As you follow a river upstream, it keeps branching off again and again and again. And at each branching, it’s not always obvious which is the main river and which is a tributary. So how did they know which to follow, to find the true source of the river? Or was it simply a matter of very time-consuming trial and error?
And discovering the “continental divide” presents an even greater problem. Don’t you have to create a whole lot of maps, in order to say “the rivers east of here wind up in a different ocean than the rivers west of here”?