Had a quick search of the forums and I think I have another river bug to add to the list (If it was posted beofre I missed it).
I took a screenshot of a river running into a lake to show a friend (I know it's a known issue) as seen here:
The issue is a few turns later the river took on a completely different appearance.
This starts just before the river reaches the town, it is actualy quite normal in appearance to the west where it is unfortunately covered by the sovereigns portrait.
Debug.err (renamed it for hosting purposes):
I guess the logical assumption would be the townsfolk dammed the river. Which I think owuld be a neat feature. It would be great if you could build damns in rivers near your towns to increase the output of farms there by irrigating them. It could have tactical uses as well. Build a town up river to a rival race. Damn the river and dry out their lands (while making yours more fertile). It would just add another interesting layer to the game.