Nice go!

Joe gave the suggestion I would have. Select Alpha from the river layer, invert and feather it. Then move to the forest colour layer's mask (and many the texture, too) and fill the selection with black (or grey, depending on the intensity of the effect. This will put a "space" around the rivers. Alternately, create a bevel effect (explained somewhere - stf for my post) around the rivers that will make then look set into the land. The stop looking like they are painted on top of the forest that way.

-Rob A>