Even where such rivers have a volume too scanty to float a raft, they yet point the highway, because they alone supply water for man and beast across the desert tract.  The Oxus and Sir Daria have from time immemorial determined the great trade routes through Turkestan to Central Asia.  The Platte, Arkansas, Cimarron and Canadian rivers fixed the course of our early western trails across the arid plains to the foot of the Rockies; and beyond this barrier the California Trail followed the long-drawn oasis formed by the Humboldt River across the Nevada Desert, the Gila River guided the first American fur-trapping explorers across the burning deserts of Arizona to the Pacific, and the succession of water-holes in the dry bed of the Mohave River gave direction to the Spanish Trail across the Mohave Desert towards Los Angeles.  In the same way, Livingstone’s route from the Orange River in South Africa to Lake Ngami, under the direction of native guides, ran along the margin of the Kalahari Desert up the dry bed of the Mokoko River, which still retained an irregular succession of permanent wells.[648]

[Sidenote:  Wadi routes in arid lands.]

In the trade-wind regions of the world, which are characterized by seasons of intense drought, we find rivers carrying a scant and variable amount of water but an abundance of gravel and sand; they are known in different localities as wadis, fiumares and arroyos.  Their beds, dry for long periods of the year, become natural roads, paved with the gravel which the stream regularly deposits in the wet season.  Local travel in Sicily, Italy[649] and other Mediterranean countries uses such natural roads extensively.  Trade routes across the plateau of Judea and Samaria follow the wadis, because these give the best gradient and the 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