Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after Lake Superior in North America. In terms of volume, Lake Victoria is the world's ninth-largest continental lake, containing about of water. Lake Victoria occupies a shallow depression in Africa. The lake has an average depth of and a maximum depth of .United Nations, Development and Harmonisation of Environmental Laws Volume 1: Report on the Legal and Institutional Issues in the Lake Victoria Basin, United Nations, 1999, page 17 Its catchment area covers . The lake has a shoreline of when digitized at the 1:25,000 level, with islands constituting 3.7% of this length.
The lake's area is divided among three countries: Tanzania occupies 49% (), Uganda 45% (), and Kenya 6% ().J. Prado, R.J. Beare, J. Siwo Mbuga & L.E. Oluka, 1991. A catalogue of fishing methods and gear used in Lake Victoria. UNDP/FAO Regional Project for Inland Fisheries Development (IFIP), FAO RAF/87/099-TD/19/91 (En). Rome, Food and Agricultural Organization.
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.
Names Though having multiple local language names (; ; ; ), the lake was renamed after Queen Victoria by the explorer John Hanning Speke, the first Briton to document it in 1858, while on an expedition with Richard Francis Burton. The expedition is detailed in Part One, Chapters 1–5 (pp. 9–98).
Geology
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". A primary study, attempting "fluvial differentiation of the basin of Lake Victoria", draws several relevant tentative conclusions. First, during the Miocene era, what is now the catchment area of the lake was on the western side of an uplifted area that functioned as a continental divide, with streams on the western side flowing into the Congo River basin and streams on the eastern side flowing to the Indian Ocean. Second, as the East African Rift System formed, the eastern wall of the Albertine Rift (or Western Rift) rose gradually reversing the drainage towards what is now Lake Victoria. Third, the opening of the main East African Rift and the Albertine Rift downwarped the area between them as the rift walls rose, creating the current Lake Victoria basin.
During its geological history, Lake Victoria went through changes ranging from its present shallow depression, through to what may have been a series of much smaller lakes. Geological cores taken from its bottom show Lake Victoria has dried up completely at least three times since it formed. These drying cycles are probably related to past ice ages, which were times when precipitation declined globally. 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.
Hydrology and limnology Lake Victoria receives 80 percent of its water from direct rainfall. Average evaporation on the lake is between per year, almost double the precipitation of riparian areas. Lake Victoria receives its water additionally from rivers, and thousands of small streams. The Kagera River is the largest river flowing into this lake, with its mouth on the lake's western shore. Lake Victoria is drained solely by the Nile River near Jinja, Uganda, on the lake's northern shore.
In the Kenya sector, the main influent rivers are the Sio, Nzoia, Yala, Nyando, Sondu Miriu, Mogusi, and Migori. The only outflow from Lake Victoria is the Nile River, which exits the lake near Jinja, Uganda. In terms of contributed water, this makes Lake Victoria the principal source of the longest branch of the Nile. However, the most distal source of the Nile Basin, and therefore the ultimate source of the Nile, is more often considered to be one of the tributary rivers of the Kagera River (the exact tributary remains undetermined), and which originates in either Rwanda or Burundi. The uppermost section of the Nile is generally known as the Victoria Nile until it reaches Lake Albert. Although it is a part of the same river system known as the White Nile and is occasionally referred to as such, strictly speaking this name does not apply until after the river crosses the Uganda border into South Sudan to the north.
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 (i.e., the layer of water that lies below the thermocline, is non-circulating, and remains perpetually cold) were lower in 1990–1991 for a longer period than in 1960–1961, with values of less than 1 mg per litre (< 0.4 g/ft³)occurring in water as shallow as compared with a shallowest occurrence of greater than in 1961. The changes in oxygenation are considered consistent with measurements of higher algal biomass and productivity. 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.
Bathymetry
The lake is a shallow lake considering its large geographic area with a maximum depth of approximately and an average depth of . A 2016 project digitized ten-thousand points and created the first true bathymetric map of the lake. The deepest part of the lake is offset to the east of the lake near Kenya and the lake is generally shallower in the west along the Ugandan shoreline and the south along the Tanzanian shoreline.
Native wildlife Mammals Many mammal species live in the region of Lake Victoria, and some of these are closely associated with the lake itself and the nearby wetlands. Among these are the hippopotamus, African clawless otter, spotted-necked otter, marsh mongoose, sitatunga, bohor reedbuck, defassa waterbuck, cane rats, and giant otter shrew.
Reptiles Lake Victoria and its wetlands has a large population of Nile crocodiles, as well as African helmeted turtles, variable mud turtles, and Williams' mud turtle. The Williams' mud turtle is restricted to Lake Victoria and other lakes, rivers, and swamps in the upper Nile basin.
Cichlid fish
Lake Victoria formerly was very rich in fish, including many endemics, but a high percentage of these became extinct since the 1940s. The main group in Lake Victoria is the haplochromine cichlids (Haplochromis sensu lato) with more than 500 species, almost all endemic,DeWeerdt, S. (28 February 2004). Dark secret of the lake. New Scientist. Retrieved 26 March 2017. and including an estimated 300 that still are undescribed.Sayer, C.A., L. Máiz-Tomé, and W.R.T. Darwall (2018). Freshwater biodiversity in the Lake Victoria Basin: Guidance for species conservation, site protection, climate resilience and sustainable livelihoods. Cambridge, UK and Gland, Switzerland: IUCN. This is far more species of fish than any other lake in the world, except Lake Malawi. These are the result of a rapid adaptive radiation in the last circa 15,000 years. Their extraordinary diversity and speed of evolution have been the subjects for many scientists studying the forces that drive the richness of life everywhere. The Victoria haplochromines are part of an older group of more than 700 closely related species, also including those of several smaller lakes in the region, notably Kyoga, Edward–George, Albert, and Kivu.
Most of these lakes are relatively shallow (like Victoria) and part of the present-day upper Nile basin. The exception is Lake Kivu, which is part of the present-day Congo River basin, but is believed to have been connected to Lakes Edward and Victoria by rivers until the uplifting of parts of the East African Rift. This deep lake may have functioned as an "evolutionary reservoir" for this haplochromine group in periods where other shallower lakes in the region dried out, as happened to Lake Victoria about 15,000 years ago. In recent history only Lake Kyoga was easily accessible to Victoria cichlids, as further downstream movement by the Victoria Nile (to Lake Albert) is prevented by a series of waterfalls, notably Murchison. In contrast, the Owen Falls (now flooded by a dam) between Victoria and Kyoga were essentially a series of rapids that did not effectively block fish movements between the two lakes.McClanahan, T. and T.P. Young (1996). East African Ecosystems and Their Conservation. pp. 201–06.