Okay, here is another actual answer to your question - maybe. This is a seed from an old game of mine. It has several straights (a waterway connecting two bodies of water), not actually a river but they look like one. They are narrow channels that run right across the island. Find the first and biggest one in this seed by heading South from the beginning spawn location. It isn't too far.

5qSu9iTy2J

https://valheim-map.world/?seed=5qSu9iTy2J&offset=-768%2C-731&zoom=0.174&view=0&ver=0.202.19

The bad news is that Valheim Map World more or less says that seeds got shuffled after H&H and the cave update. You may not get the same map with the same seed. So head South when you get there and if you don't find it within ten minutes of travel, it isn't the right seed because there is no way you'd miss it. It also has very high and steep banks it's entire length.