If you want a river, start with a soft simple shape (long, and about twice as wide as you want your river to be). Warp it (you can even use the fractal warper), play with the softness through a color adjust shader. Build the terrain outside using the same simple shape mask, but inverted, again using color adjust shaders to adjust sharpness of edges. Or use a larger simple shape for a river valley, warped the same way of course (copy warper, double line). That's the basis.
