Please ignore the "PL2 developements" thread - something went awry with it's logging.
Anyhow, the thing is when you see a 'distance' river branch tile it is sometimes rotated wrongly.
When you get close it flips into the correct orientation.
I modified the river branch bmp's in all the PL2 stuff but the problem has been fixed by "Zsolt" and "1MARK66" at SWWISA
http://forums.delphiforums.com/swwisa/messages?msg=4251.19
It fixes all seasons and all maps (including MMP) in RB3D regardless of what UOP or terrain may be used.
This means that STOCK and all other river branches now display properly!
You won't have to D/L any PL2 patches again but you will need to get the "DML_river_fix" patch and place the files in Simpatch.
The patch is currently available at SWWISA in the "river orientation" thread but should be available soon from other sources.

The river fix can now be downloaded from Swordfish's upload page.
Many thanks to Zsolt Fazekaszs and to Mark_66 at SWWISA for finding the root cause of the problem and for providing the fix!