Oh yeah, first thing I did was resize it. Well that's not true. First thing I did was copy it to the heightmap and notice it had a 60 pixel border all the way around lol. THEN I resized it!
And I resized then 5000 x 5000's on my end because World Painter works internally with 128x128 pixel tiles. After importing I'd end up with a water border 8 pixels wide on the right and bottom of the maps that need to be removed before exporting. Instead, I just upsize each 5k tile to 5120 x 5120 before importing and it comes out perfectly to 40x40 World Painter tiles, no border to get rid of.
Anyway, check this out. I turned the rivers blue (to aid visibility) and laid them on top of the heightmap, same tile. x=2 y=3.
You can see from the terrain that the rivers don't have any correlation. I tried flipping and mirroring the rivers but no luck matching them up.
I just got home so I'm going to check some of the other images out.
EDIT: Ok, I got lucky. Found the problem on the next image. It's offset by one in the y direction. I'll upload to show you....
Base x=2 y=3 tile.
River x=2 y=4 tile.
Combined.
So the rivers for x2y4 match up with the heightmap for x2y3!
Any ideas? I'll verify a few more and let you know.
EDIT: Ok, bleh. This is just weird. So far RIVER x0y3 fits with HEIGHTMAP x0y4, and RIVER x0y4 goes with HEIGHTMAP x0y3!
Got it. The y values are backwards. So while the heightmap tiles go from x0y0 to x0y7 and then x1y0 to x1y7 etc., the river maps go from x0y7 to x0y0 and then x1y7 to x1y0 etc.
So no problem. I can just rename the files and I'm good to go! Should work out well. I don't know what this means for your exporting workflow though. Heh heh?