Trouble with the mud. Not too bad, but I did some tests to get the best mud, and it seems that a reflective shader with RT reflections ON is the best, though the slowest. You get the trees to cast their sheen on the mud, which is missing in the default shader way. And the 'double' reflectivity of mud and the remaining trickle of water (water shader) interacts, which gives these whitish areas that look like foam, but are not supposed to be there.
I think the best way to speed up render is to crop the mud part and set RT on, and do the rest without reflections at all.