Morteza Ahmadi shows how you can mask flow maps using a height map of the terrain in EasyTerrain for Maya.
EasyTerrain is a tool for Autodesk Maya that lets you create and texture terrains quickly. The latest release EasyTerrain has some new features and modifications along with it, notably texturing tools and the ability to generate flow maps.
One new feature in EasyTerrain 2.5 lets you mask Flow Maps using the Height Map of a terrain. Check out this tutorial from Morteza Ahmadi that shows how it works. The demo shows how you can mask flow map textures for your terrain by using the originating height map.