What you can do is basically "clip" the rivers out (I usually make them in photoshop on their own layer) and then add them back one by one. See which one is causing the issue. I had something similar happen with the localization of my Atlantis mod.Hm. graphics.log is complaining about circular rivers where there are no rivers at all, and rivers that are too short everywhere there's a blue pixel. I'm going to put that aside for now and come back to it later.
EDIT: Your post hadn't appeared when I'd posted this. Looks really good. I'm thinking that we should create a wiki of some sort to keep track of all this information, given how fragmented it can get at times.
Thanks, and I agree. It might be useful.
The only four mods I think are essential are:
Better Rebels (because it is a good idea)
Climate (It will take some time to adapt, but will add a layer of depth to the mod)
Improved CoAs (vanilla's look... way too colorful)
More clothing (so different groups look different)