I've finally got all of the data and will have a go at producing the extra files tonight.
Here's a close up of my river. I've been tossing up adding a slope restriction to help avoid glitches like those on the left bank (water surface going up rocks left of centre), although it also helps to see them on the first render to see if the mask really should be cleaned up more or if you can be lazy and try and hide it
I guess any flat areas of water away from the river could be passed of as puddles.
You'll also notice on the right shore that the water surface extends onto the bank in the "fuzzy zone" of the image mask. This was not deliberate, but I think it's pretty cool. All it needs is some fractal breakup in a reversed mask to make it less even.
The slope of the bank in this region is controlled by the difference between the two colour adjust shaders of the image mask and the difference in displacement between the river bed and water surface (0.5m in this case).... The river bed is displaced downwards first in the terrain section and then upwards for the river/lake surface.
