Are you using it as a Blend Shader? Blend Shader essentially acts like an alpha, so it will mask out your power fractal terrain so it only shows where the white (river) is. What you should probably do is just put the Image Map shader after the Power Fractal in the terrain network, before Compute Terrain, turn off Apply Color and turn on Apply Displacement, then adjust to negative displacement. That ought to work.
- Oshyan