I reckon the importance of waterways (especially navigable ones) at that time can't be underestimated. They were by far the quickest way to travel any distance before roads. Both Wye and Severn cover large areas of what is now Wales, providing the nearest thing to a highway across the country. And their sources must have been known, Pumlumon is remote to our minds, but following the rivers upstream was hardly like trying to reach the source of the Nile or something. Besides which, a major river is a source of food and water, easy to obtain and seemingly inexhaustible.

As to what the "canoes" represents, who knows, particularly as that seems an archaic form of water craft for the BA (didn't they have coracles of some sort by then?). It's so difficult to get hold of these things, perhaps the people buried were fishers or even descendants of seafaring settlers, but it's only speculation (fun though).

One thing I do bet is that there's a whole load more to be discovered under all that peat and bog up in those mountains though.