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The Congo River and Flooded Forests ecoregion (also known as the Congo Basin) contains the most diverse and distinctive group of animals adapted to a large-river environment in tropical Africa. An exceptional group of endemic species is adapted to large rapids in the lower Congo. Most rivers in the world flow from north to south. Some go in the opposite direction. The Congo does both. Sometimes called Central Africa's "highway," this mighty river is a major transportation route for people and their goods. It is also one of the longest in the world and, within Africa, second only to the Nile.
In the Pliocene Age (12 million to 2 million years ago), the Congo Basin was a large lake that had no outlet to the sea. Over time, water broke through the rim of this huge lake and became a river that flowed down a series of rocky rapids to enter the Atlantic Ocean. The basin still retains the shape of a giant shallow saucer. Except for the beginning and end of the river’s course, the Congo River is of a uniform elevation for much of its length. Rain falls nearly year-round in the basin, averaging an incredible 5 to 8 feet (1.5 to 2.5 m) per year. This supports a lush equatorial rain forest that’s the second largest in the world after the Amazon. Floodwaters originate from the northern tributaries between August and November and from the southern tributaries between May and June. During flooding, the river spills out from the main channel over the floodplain and into the rain forest. The spawning of many fish species is synchronized with flooding. Most species move upstream with the onset of flooding and then onto the floodplain to spawn, where they and their young take advantage of the abundant food sources. The phenomenon of flooding allows nutrients from the terrestrial environment to enter the aquatic food chain and support the high diversity of fish in the Congo Basin.
With more than 700 fish species, 500 of which are endemic, the Congo Basin ranks second in the world in fish diversity, after the Amazon. These fish are found not only in the river itself but also in its associated habitats -- swamps, nearby lakes, and headwater streams. Among the many feeding habits of this diverse group, there are fish that glean their food from the mud, "fin-biters" that eat the fins of living fish, and piscivores that eat other fishes. Endemic aquatic mammals include the aquatic genet, swamp otter, Ruwenzori otter shrew, giant otter shrew, and Allen’s swamp monkey. A highly specialized, rheophilic (current-loving) species of fish has evolved to survive in the highly oxygenated, swiftly moving rapids of the lower Congo River. Thirty-four endemic fish inhabit the lower Congo rapids. These fish have special adaptations for surviving in the rapids. Many have reduced eye size or no eyes at all because of the muddy waters, and many have a modified body shape for easy maneuverability in the swift current. Several also have tubelike mouths for feeding in the spaces between the rocks of the rapids.
Several urban centers in the Congo Basin are growing, and with their growth comes the potential for an increase in untreated sewage and other sources of pollution that could harm nearby freshwater systems. Sedimentation and erosion also occur near logging operations. A few large-scale mining operations exist in the Congo, but they probably affect only localized areas.
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