north of the lunda divide the kwango and many other streams flow north from the tableland to join the kasai river (one of the largest affluents of the congo), which in its upper course forms for fully the boundary between angola and the congo. south of the divide some rivers flow into the zambezi river system and thence to the indian ocean, others to the okavango river (as the cubango river is called along the border with namibia and in botswana) and thence to lake ngami and the okavango swamp in botswana. the tributaries of the cubango river and several of the southern rivers flowing to the atlantic are seasonal, completely dry much of the year.