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 cataractsknown as Boyoma Fallsbetween 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 (6.416.1 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 1 1/2 mi (1.62.4 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.

With railroads to bypass major falls (Matadi-Kinshasa; Kisangani-Ubundu; Kindu-Kongolo), the Congo River and its tributaries form a system of navigable waterways c.9,000 mi (14,480 km) long, along which move much of central Africa's copper, palm-oil kernels, cotton, sugar, and coffee. The chief ocean port is Matadi, with its associated oil port, Ango Ango; the chief river ports are Kinshasa and Kisangani. River steamers operate throughout the year between Kinshasa and Kisangani. The Congo River is Africa's largest potential source of hydroelectric power; the most valuable site is along Livingstone Falls, where the first phase of the Inga Power Project has been completed. In spite of government initiatives, hydroelectric power is underdeveloped.

The Congo river basin encompasses the world's second largest contiguous rain forest, surpassed only by that of the Amazon. The region is biologically diverse, and a huge watershed. The forest is threatened by illegal logging and the poaching of large mammals (especially for the bushmeat trade), but two summits (1999, 2005) that brought together the nations of the basin have committed its participants to forest conservation and have led to the establishment of wildlife preserves.

The mouth of the Congo River was visited (1482) by Diogo C㯬 the Portuguese navigator. It became known as the Zaﲥ River (a corruption of the local name Mzadi meaning great water) and was later referred to as the Congo River (for the Kongo kingdom located near its mouth); it was called Zaﲥ River by the government of Zaﲥ (now Congo [Kinshasa]) from 1971 to 1997. The Congo's lower course was traced upstream as far as Isangila by a British force under Capt. J. K. Tuckey in 1816, and its upper headwaters by the missionary David Livingstone, who followed the Lualaba River to Nyangwe in 1871. The journalist Henry Stanley traveled from Nyangwe to Isangila and on to Boma during his great transcontinental journey (187477), thus proving the headwaters to be tributaries of the Congo River, and not sources of the Nile as hypothesized by Livingstone.