The Congo basin represents one of the most marked depressions to be found between the Sahara to its north, and the Atlantic Ocean to the south and west. It also includes the East African lakes to the east. Tributaries flow down slopes that vary from 900 to 1,500 feet into a central depression that forms the basin. It measures more than 1,200 miles north to south - from the Congo-Lake Chad watershed to the Angolan plateaus. West to east - from the Atlantic to the Nile-Congo watershed - it also measures 1,200 miles.
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.
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