Continental mountain divides are not in the same places they were five or ten or twenty million years ago. Heck, continents are not even in the same place they were then.
And lakes which are landlocked now, have not always been so.
For example what is now the Salt Lake in Utah, as recently as 15,000 years ago stretched into Idaho in the north, to the Southern border of Utah, and into Eastern Nevada. {Google Lake Bonneville}
We visited Ogden Utah a few years ago. It is simply mind boggling to look up at the mountains around Salt Lake and observe high water marks remaining fifty feet, and a hundred feet, and higher upon the mountain sides.
Hmmm, there might be a pretty good chance one would find the same species of fishes in all the tributaries which drained into this once massive lake.
Unless of course, one simply refuses to believe the Earth existed a million years ago.
People who choose to brew up their own storms bitch loudest about the rain.
