constituted the main line of
intercourse between the Mediterranean and Equatorial Africa. The Tigris,
Euphrates, Indus, and the Niger where it makes its great northern bend
into the Sahara near Timbuctoo,[646] attest the value to local fertility
and commerce inherent in these rivers of the deserts and steppes. Such
rivers are always oasis-makers, whether on their way to the sea they
periodically cover a narrow flood-plain like that of the Nile, or one
ninety miles wide, like that of the Niger's inland delta above
Timbuctoo;[647] or whether they emerge into a silent sea of sand, like
the Murghab of Russian Turkestan, which spreads itself out to water the
gardens of Merv.

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 t