I'm using Photoshop CS4.

Is there a way to draw the basic shape of a river, and then use some method to get it all nicely organic/squiggly like so?
http://www.current-river.com/index_files/Map.jpg

I don't have a height map/etc, so I'm drawing the rivers manually, but all my efforts have it looking a little unnatural, and I was hoping some filter might add the right bit of randomness to it.