Quote Originally Posted by tmilktoast View Post
But, clearly the elevation (and rockiness?) of the land has something to with that.
Exactly.... really, it's the elevation. Just like any liquid, it will always flow toward the lowest point. This is why higher elevations tend to be somewhat straight(ish) watercourses, while flatlands tend make more meandering watercourses. Rockiness does not matter in the short term for slow moving rivers, but if water is flowing equally over an area with hard soil/rock and soft soil(ie, exact same height and slope), eventually erosion will shift the current toward the softer soil and the harder will be above the water line. Simple physics at work here.