Try one of Jezelf's Tutorials, I believe I remember seeing him use his Alpha layer (with Rivers) and did a posterize to blur it way out and to get definition to the rivers...

Here

This might help out but I can't recall any other tutorials using the filter but Jezelf... "4 ways to give your map a relief using height maps - using Wilbur for one of them."

But that might be Poster Edges... eh I can't remember