The second longest river of Africa and one of the longest in the world, the Congo River drains c.1,425,000 sq mi (3,690,750 sq km) including all of Congo (Kinshasa) and parts of Congo (Brazzaville), Cameroon, Central African Republic, Burundi, Tanzania, Zambia, and Angola. The Lualaba River, considered to be the upper Congo River, rises in SE Congo (Kinshasa), flows north over rapids and falls to Bukama, and thence across a vast plain and through a series of marshy lakes (Kabwe, Kabele, Upemba) to receive the Luvua River at Ankoro. The Luvua River has its most remote source in the Chambeshi River, which rises in N Zambia and flows southwest into swamps around Lake Bangweulu; it emerges from the swamps as the Luapula River, continues N along the Congo (Kinshasa)–Zambia border into Lake Mweru, exits from there as the Luvua River, and continues NW to the Lualaba River. A third major headstream is the Lukuga River, which drains from Lake Tanganyika and joins the Lualaba River near Kabalo. From Kabalo, the Lualaba River flows N to Kisangani in a varied course marked by a deep and narrow gorge (the Gates of Hell) below Kongolo, a navigable stretch from Kasongo to Kibombo, a section of rapids and falls from Kibombo to Kindu, a shallow but navigable section from Kindu to Ubundu, and a section of seven cataracts—known as Boyoma Falls—between Ubundu and Kisangani that marks the end of the Lualaba and the beginning of the Congo River proper.
Below Kisangani, the Congo flows west and southwest, in a great curve unbroken by falls or rapids for about 1,090 mi (1,750 km) to Kinshasa. For most of its middle section the Congo is from 4 to 10 mi (6.4–16.1 km) wide, with many islands and sandbars. Because its many large tributaries (including the Lomami, Kasai, Lulonga, Ubangi, Aruwimi, Itimbiri, and Mongala rivers) drain areas with alternating rainy seasons on either side of the equator, the Congo has a fairly constant flow throughout the year. Between Bolobo and Kwamouth the Congo narrows in width to between 1 mi and 11/2 mi (1.6–2.4 km) but, c.350 mi (560 km) from its mouth, widens to form lakelike Pool Malebo, on which Kinshasa and Brazzaville are located. From the western end of Pool Malebo, the Congo descends 876 ft (267 m) in a series of 32 rapids, known as Livingstone Falls, to the port of Matadi.
Below Matadi (83 mi/134 km inland) the Congo is navigable by oceangoing vessels and, despite such hazards as the whirlpools of the Devil's Cauldron, shifting sandbars, and sharp bends in the river, forms one of the largest natural harbors in Africa. The river is tidal to Boma, c.60 mi (100 km) upstream. The Congo River enters the Atlantic Ocean between Banana Point, Congo (Kinshasa), and Sharks Point, Angola, and dredging is required to keep a navigable channel open. The river is continued offshore by a c.500-mi-long (800-km) submarine canyon that is c.4,000 ft (1,220 m) deep.