Note 700 — Chapter XI · —The Anthropo-Geography Of Rivers

Where navigation is peculiarly backward, a river may present a barrier. An instructive instance is afforded by the River Yo, which flows eastward through northern Bornu into Lake Chad, and serves at once as boundary and protection to the agricultural tribes of the Kanuri against the depredations of the Tibbu robbers living in the Sahara or the northern grassland. But during the dry season from April to August, when the trickling stream is sucked up by the thirsty land and thirstier air, the Tibbu horsemen sweep down on the unprotected Kanuri and retreat with their booty across the vanished barrier. The primitive navigation by reed or brushwood rafts, practiced in this almost streamless district, affords no means of retreat for mounted robbers; so the raiding season opens with the fall of the river.700