procedural terrain generating

any good tutorials about procedural terrain generating /prog/? I am currently particularly interested how rivers can be generating, how do the affect the terrain.

I created something like this using Perlin noise (what most people use for procedural generation). I basically had one Perlin noise function generate the terrain bumpmap, and another to generate sparse, connected lines that were rivers. Then I interpolated between the two to create riverbanks.
Of course they didn't look or act like real rivers, but it's a start. I guess you'd have to read up on how rivers work in the real world.