course of a river-bed entirely depends on the lie of the land. If this changes in course of time, the course of the river will change. The water runs downwards to the sea, pulled by the Earth’s attraction as near as it can get to the center of the Earth. It must run just as a ball would run on an irregular surface. Thus, sometimes, where the Earth falls evenly, like a tilted table, a river will run quite straight, but if there is a little hillock in the way the river will run round it.
When we notice the ordinary curves and twists in the course of a river, we may see no good reason for them, for all the land may look equally flat. But that is only because we can not, with out unaided eyes, see accurately enough. If we use a special instrument to show us how the land lies at any point, we shall find that the river is really doing the only thing it can – running downwards all the time.