There are no navigable rivers in Algeria. The most important river is the Chelif, which takes its origin in the Saharan Atlas, crosses the High Plateau, breaks through the Atlas of the Tell, and, turning westward, traverses obliquely the Tell for a distance of about 108 miles before discharging into the sea. The Chelif is the only stream which rises in the Saharan Atlas and pierces the northern range. To the south of the Saharan Atlas are several important oueds. One, the Oued Djedi, rises near Aflou and goes easterly, past Laghouat, until it reaches the Chott Melrir, south of Biskra; two others, the M’Zab and the Nessa, drain the region of the Chebka, and taking an easterly or a southeasterly direction reach the Oued Rirh or its upper extension. The Oued Rirh constitutes the northern portion of the great Oued Igharghar, or a tributary of this oued, and extends about 60 miles north from Touggourt to the Chott Merouan. The region of the Oued Rirh is of great economic importance from the production of dates, made possible through the development of artesian wells by the French government. The oueds as a whole are very like the arroyos of the southwestern portion of the United States, in that they carry water for a small portion of the year only, when the torrential rains fill them with a muddy, rushing flood. In the plains adjacent to the oueds, at Ghardaia especially, the natives usually dig their wells, from which water for domestic as well as for irrigating purposes is obtained. The flood-water of the oueds is also diverted into ditches, or impounded for later use, although the latter has not met with uniform success. The depths at which water has been found vary greatly. At Ouargla the water lies within 3 feet of the surface, although there are also very deep artesian wells, and at Ghardaia it varies from 10 to 50 feet or more. At the daya of Tilrempt the water in the deepest wells stands as deep as 300 feet; it is drawn in a very primitive fashion for purposes of watering flocks and for the bordj. The heaviest vegetation is to be found along the oueds and the nearby flood-plains. Here the water relations are the most favorable and the oueds constitute highways along which plants venture into the desert from the more humid regions.