Physiography of the Congo basin

 The Congo basin has a large
depression in the central portion. Referred to as a "cuvette", it is a
large, shallow, saucer-shaped area. This depression contains Quaternary
alluvial deposits which rest on thick sand and sandstone sediment of
continental origin. Along the eastern edge of the cuvette outcrops of
sandstone have formed. The cuvette has a filling that dates to
Precambrian times (570 million years ago). Studies have shown that the
sediment has built up over time from the erosion of the formations that
surround the cuvette.

The Congo River system is composed of three distinct sections – the upper Congo, the middle Congo and the lower Congo.

The upper Congo contains
confluences, lakes, waterfalls, and rapids. The Congo River has its
beginnings in the Democratic Republic of the Congo (Zaire) where
several small rivers unite. The Lualaba River and Luvua River join
together and then continue on to the Boyoma (Stanley) Falls. Navigation
is possible in the upper Congo by large vessels with low tonnage. Due
to the rapids at headstreams and at various locations along the river
navigation is interrupted and other means of transportation must be
used.

Kisangani is situated
downstream from the Boyoma Falls and is at the beginning of where the
Congo River becomes navigable. For 1,000 miles the river flows towards
Kinshasa. At first the river is narrow but soon widens as it enters the
alluvial plain. From the point where the river widens, strings of
islands occur which divide the river into different arms. The width of
the Congo River can vary from 3.5 miles to 7 miles, reaching up to 8
miles at the mouth of the Mongala River. Along the banks of the river
are natural levees which have been formed by deposits of silt. When the
river floods these levees are washed away and the river boundaries are
increased.

The middle Congo is
characterized by the narrowing of the river. The banks are a half-mile
to a mile apart, the river is much deeper and its current is faster.
This section of the Congo is referred to as the Chenal (Channel) or
Couloir (Corridor). It is along this stretch of the river that its
principal tributaries flow into the Congo. They include the Ubangi
River, Sangha River and the Kwa River. This results in a tremendous
increase in the flow of water from 250,000 cubic feet per second at
Kisangani to its maximum when it reaches Kinshasa.

From the middle Congo (Chenal)
the river divides into two. One branch forms Malebo Pool, which is 15
miles by 17 miles large. This is the end of the middle Congo. Just
downstream are the first of 30 waterfalls as the river continues to
flow towards Matadi. At Matadi, the Congo's estuary begins in a narrow
channel only half a mile to a mile wide. Eventually it widens below
Boma but islands are once again a factor, dividing the river into
several arms. The Congo now flows freely into the Atlantic Ocean.