Darkfire, here is a cap of the set up I am using, I pretty much have 2 Image Map Shaders (labelled IMS, and then whatever masks it uses, as I switch back and forth to see what works best). All the masks are generated in Saga Gis from the heightfield/ter, and then tweaked in PS (all those steps are in the tutorial linked above at ME-DEM Project).
The river mask is simply a greyscale image with the river channels towards white, terrain black, this is what the watershader goes into, and is the same mask used to apply the displacement shader to the ter.
The Image map shader (IMS stream power channels 3) runs to both a merge shader and a color adjust shader.
I use a inverted version of the channel mask to control the rest of the surfacing, I think I also gave it a little gaussian blur to soften it..It runs into a color adjust shader and then into the base color shader.
Then the output from the last surface shader is run into the input on the merge shader which has the water shader, IMS stream power channels 3 (connected to the merge shaders A input), and color adjust ( connected to the mix controller) flow going into it.
The color adjust shader is used to tweak the mask and the settings in the merge shader are used to set the color and displacement merge type.
I will try to post the tgd tommorw, once I give the different nodes sensible names.
One thing I have noticed with the image shader is that in order to get it to overlay on the terrain right, so the small thin channels get water in the bottom and not the sides, is that I have to offset the x and y placement by the meter spacing of my terrain. For example this terrain uses 12.5 meters per pixel so the lower left corner had to be set at 12.5 0 12.5 in the Imae map shader setup to match the terrain
SeerBlue