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Each of the main land-masses of the world is largely divided piecemeal into catchment areas of drainage systems supplying major rivers, the example of Africa being illustrated in Fig. i. A river may be defined as a mixture of water, rock detritus, and other materials, flowing in a channel having pro-gressively lower levels. Strahler (1951) quotes from John Playfair’s Law (published in 1802): ‘Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together forming a system of valleys connecting with one another, and having such a nice adjustment of their declivities that none of them joins the principal valley either on too high or too low a level; a circumstance which would be infinitely improbable if each of these valleys were not the work of the stream which flows in it.’ This statement comprises three main points: (1) valleys are proportioned in size to streams flowing in them, (2) stream junctions are accordant in level, and (3) therefore, valleys are carved by streams flowing in them.