Lake Victoria

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Lore & Background

Lake Victoria occupies a shallow depression in Africa. Photojournalist John Reader, writing in his Alan Paton Literary Award-winning *Africa: A Biography of a Continent*, describes Lake Victoria as being relatively geologically young at about 400,000 years old—having been formed as westward-flowing rivers were backed up 'when a fractured block of the Earth's crust tilted along the line of the Great Rift Valley, raising its western edge.' Geological cores taken from its bottom show Lake Victoria has dried up completely at least three times since it formed. According to another primary source, Lake Victoria last dried out about 17,300 years ago, and it refilled 14,700 years ago—as the African humid period began. The lake receives 80 percent of its water from direct rainfall. Average evaporation on the lake is between 2.0 and 2.2 metres per year, almost double the precipitation of riparian areas. Lake Victoria is drained solely by the Nile River near Jinja, Uganda, on the lake's northern shore. In terms of contributed water, this makes Lake Victoria the principal source of the longest branch of the Nile.

Reader's Guide

Lake Victoria is Africa's largest lake by area and the world's largest tropical lake. It is a critical water source for the Nile River and supports a vast ecosystem, including many endemic fish species, especially cichlids. The lake is home to many species of fish which live nowhere else, especially cichlids. Invasive fish, such as the Nile perch, have driven many endemic species to extinction. The lake exhibits eutrophic conditions. In 1990–1991, oxygen concentrations in the mixed layer were higher than in 1960–1961, with nearly continuous oxygen super saturation in surface waters. Oxygen concentrations in hypolimnetic waters were lower in 1990–1991 for a longer period than in 1960–1961, with values of less than 1 mg per litre occurring in water as shallow as 40 metres compared with a shallowest occurrence of greater than 50 metres in 1961. These changes have arisen for multiple reasons: successive burning within its basin, soot and ash from which has been deposited over the lake's wide area; from increased nutrient inflows via rivers, and from increased pollution associated with settlement along its shores. Between 2010 and 2022, the surface area of Lake Victoria increased by 15% flooding lakeside communities. The main group of fish in Lake Victoria is the haplochromine cichlids, with more than 500 species, almost all endemic, and including an estimated 300 that still are undescribed. Their extraordinary diversity and speed of evolution have been the subjects for many scientists studying the forces that drive the richness of life everywhere.

Did You Know?

• Lake Victoria has dried up completely at least three times since it formed, with the last drying about 17,300 years ago.
• The lake receives 80% of its water from direct rainfall, and average evaporation is between 2.0 and 2.2 metres per year.
• Between 2010 and 2022, the surface area of Lake Victoria increased by 15%, flooding lakeside communities.
• The lake is home to more than 500 species of haplochromine cichlids, almost all endemic, with an estimated 300 still undescribed.
• The only outflow from Lake Victoria is the Nile River, which exits near Jinja, Uganda.