Hi All
Here are a couple of test renders of my water masking and fake water surface. I've stuffed up my image map adjustments along the way and will have to go back a step to fix these up (banks are too sharp and the water distribution is a little off... up close)
The white surface below the river bed is mainly for me to preview the mask at the moment. There's a fake stone shader above it which provides texture for the banks. My new mask creation technique is making life easier although I had to choose an angle that picked out a flaw I missed
Render times aren't too bad yet. The lake took 4 1/2 hours, a fair proportion of which was taken up populating and rendering the trees (2.5 million in each image over 20x20km, medium quality).