The course of rivers cannot be properly understood if regarded as objects complete in themselves. In reality, a river is only a part of a system of channels serving to drain the whole of the area over which it extends, the breadth as well as the length. Of this system the tributaries are an essential part. So, too, valleys and combes which still exist but no longer serve for streams, and others altogether effaced, have, in former times, been included in the system. I hold to my belief, recorded nearly twenty-six years ago, that only by taking into account the influence of tributary streams can river-windings be explained, and that the subject is important as “bearing on the formation of warths and the maintenance of navigation, channels.”