he best
footing for the ascent.[650] Wadis also determine the line of caravan
routes across the highlands of the Sahara. In the desert of Southwest
Africa, the Khiuseb Is the first river north of the Orange to reach the
Atlantic through the barrier dunes of the coast. Hence it has drawn to
its valley the trade routes from a wide circle of inland points from
Ottawe to Windhoek and Rehobeth, and given added importance to the
British coast of Walfish Bay, into which it debouches.[651] But just to the
north, the broad dry bed of the Swakop offered a natural wagon route
into the interior, and has been utilized for the railroad of German
Southwest Africa.

[Sidenote: Increasing historical importance from source to mouth.]

The historical importance of a river increases from its source toward
its mouth. Its head springs, gushing from the ground, and the ramifying
brooks of its highland course yield a widely distributed water supply
and thereby exercise a strong influence in locating the dwellings of
men; but they play no part in the great movements and larger activities
of peoples. Only when minor affluents unite to form the main stream,
enlarge it in its lower course by an increasing tribute of water, and
extend constantly its tributary area, does a river assume real
historical importance. It reaches its fullest significance at its mouth,
where it joins the world's highway of the ocean. Here are combined the
best geographical advantages--participation in the cosmopolitan
civilization characteristic of coastal regions, opportunity for inland
and maritime commerce, and a fertile alluvial soil yielding support for
dense populations. The predominant importance of the debouchment stretch
of a river is indicated by the presence of such cities as London,
Rotterdam, Hamburg, Bremen, Bordeaux, Odessa, Alexandria, Calcutta,
Rangoon, Bangkok, Hongkong, Canton, Nanking and Shanghai, Montreal and
Quebec, New York, Philadelphia, New Orleans, Buenos Ayres and
Montevideo. This debouchment stretch gains in practical value and hence
in permanent historical importance if it is swept by a scouring tide,
which enables the junction of inland and maritime routes to penetrate
into the land. Even Strabo recognized this value of tidal reaches.[652]
Hence in tideless basins like the Baltic and Caribbean, the great river
ports have to advance coastward to meet the sea; and the lower course of
even mighty streams like the Volga and Nile achieve a restri