Recently I've been working on a planet-scale scene, but studying the earth-like planets made by others I found they lacked good mountain ranges and rivers. To alleviate that, I came up with a method for making narrow, twisty, planet-scale lines using vorronois. The exact method can be found in the attached file. Do note that you'll need a high detail to get accurate rivers. Lower detail settings will cause them to become broken up.
What I've done is, by using a distorted voronoi cell function I've made different tectonic plates and using the voronoi diff function I've got a blending mask detailing the edges between the plates. This I use to make deep troughs and mountains. The rivers are made again by using a distorted voronoi diff function.
As for the attached image, that's a proof of concept and to give you an idea of the effect. You can see some artifacts on the seas, which are because the river water has an altitude of 0, which usually causes artifacts. Some extra masking should fix that, by avoiding generating rivers on seas. Also, you'll notice that rivers often seem to flow from one point in a mountain range. I'm going to try to fix that by altering the distortion of the rivers based upon the plate "colour" as given by the voronoi 3d vector near the mountains.
