evidence: Upon leaving the Chenal, the Congo divides into two branches, forming Malebo Pool, a vast lacustrine area about 15 by 17 miles (24 by 27 km), which marks the end of the middle Congo. Immediately downstream occur the first waterfalls of the final section of the river’s course. Cataracts and rapids are grouped into two series, separated by a fairly calm central reach, in which the elevation drops from a little less than 900 feet (275 metres) to a few yards above sea level. The Congo’s estuary begins at Matadi, downstream from the rapids that close off the interior Congo; 83 miles (134 km) in length, it forms the border between Angola and the Democratic Republic of the Congo. At first the estuary is narrow—less than half a mile to about a mile and a half in width—with a central channel 65 to 80 feet (20 to 24 metres) deep, but it widens downstream of Boma. There the river, obstructed by islands, divides into several arms, and in some places the depth does not exceed 20 to 25 feet (6 to 7.5 metres), which makes dredging necessary to allow oceangoing vessels to reach Matadi.