Here's some wild river history for you:
The great lakes are super big, have huge flow rates, Superior is famously super deep since it's a continental-rift lake that was widened by glacial retreat .... But they only formed like 14,000 years ago when the glaciers retreated...
The river Tyne in England is 30 million years old, just when Antarctica was separating from Australia and South America.
The river Thames is 58 million years old, that's just after the meteor that wiped out the dinosaurs.
The Rhine is at least 240 million years old ... From the Triassic era if not earlier.
And then there's 3 rivers in Appalachia that are ~ 320 million years old... The French Broad river, the Susquehanna river, and (ironically) the New river. They've been continuously flowing since the carboniferous period, literally when Pangea first started forming and before any bacteria or enzymes could break down trees (which eventually compacted and became all the coal in the mountains that formed alongside them).