I've rowed the Thames from Lechlade to Teddington, years ago now. The upper reaches are a bit hairy in terms of under/overgrowth but you're into something that's navigable and not over-managed quite quickly, from memory.

There are very built-up stretches, but there are also parts where it's just... water. Going through green fields. For hours. The biggest difference I can imagine with the stone age isn't the river itself, it's what's on the banks. It would have been far more wooded than it is now; lots that's currently farm would have been forest, and from that point of view foraging would probably have been a lot easier.