12 [ No. 99 GEOGRAPHY narrow, while their valleys are more or less deeply incised. Exceptions to this general rule are , however, found in the Welle district, where the tendency is for broad navigable reaches to alternate with falls or rapids, and in the Katanga, where the rift -valleys of Upemba Lualaba and Mweru-Luapula have a structure more or less independent of the rivers which traverse them .

On reaching the central basin the velocity of the rivers is checked ; they become wide and sluggish , and in many places overflow at certain seasons of the year. The débris which they have carried down with them forms numerous submerged sandbanks, but with each flood the rivers once more pick up this material only to redeposit it elsewhere. Their channels are conse quently never the same for two years in succession. In addition to this the rivers are frequently blocked by weeds and other vegetation, and in their efforts to over come this obstacle the main currents are constantly taking new directions.

After it has collected the drainage of its vast basin the Congo carries it down from the sandstone plateau through the gorge which it has cut in the Crystal Mountains to the lowlands bordering the coast. As the difference in level is considerable its course is again interrupted by rapids and falls.

The Congo, as will be seen from the map, has count less tributaries. The largest on the right bank are the Ubanghi, which forms for a considerable distance the boundary between Belgian and French Congo, and has as its main affluent the Welle with its system of smaller rivers ; the Aruwimi (or Nowelle ), which joins the Congo at Basoko ; the Lukuga, which connects Lake Tanganyika with the main stream ; and the Luvua or Luapula, which connects the Congo with Lake Mweru .

On the left bank the main tributaries are the Lomami, which pursues a course almost parallel to that of the