Lake Victoria

• 1. Introduction

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  Lake Victoria covers approximately 59,947 square kilometres, which makes it the largest lake in Africa by area. It is also the largest tropical lake in the world. Only Lake Superior in North America holds a bigger surface of fresh water. Three countries share its shores. Tanzania holds 49 percent of the lake, Uganda holds 45 percent, and Kenya holds the remaining 6 percent. This enormous sheet of water is geologically young, and it has emptied completely more than once. So how did the basin form, and where does all that water go? Who first looked across it and claimed it was the source of the Nile? And what happened to the fish? Lake Victoria was once home to a stunning variety of species found nowhere else. Their disappearance has been called the "most dramatic example of human-caused extinctions within an ecosystem".

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• 2. A Basin Made by Rifts

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  The photojournalist John Reader, in his Alan Paton Literary Award-winning book Africa: A Biography of a Continent, puts the lake's age at about 400,000 years. That is young for a lake of this size. He says 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 scientific study of the basin's river history adds detail in three tentative steps. During the Miocene, the land now draining into the lake sat on the western side of an uplifted continental divide. Streams to the west ran toward the Congo River basin. Streams to the east ran toward the Indian Ocean. Then the East African Rift System formed, and the eastern wall of the Albertine Rift, also called the Western Rift, rose. That slowly reversed the drainage toward what is now Lake Victoria. Finally, as the main East African Rift and the Albertine Rift opened, the ground between them sagged while the rift walls climbed. That sagging ground is the present basin.

  The lake has not always looked like this. Geological cores from its bed show that it dried up completely at least three times. These dry spells probably followed past ice ages, when rainfall dropped across the globe. One source says the most recent drying came about 17,300 years ago. The lake refilled 14,700 years ago, as the African humid period began. At other times it may have shrunk into a series of much smaller lakes.

  Today, about 80 percent of the lake's water falls straight onto it as rain. Evaporation removes between 2.0 and 2.2 metres a year, almost double the rainfall on the surrounding shores. Thousands of small streams and several rivers supply the rest. The Kagera River is the largest, entering on the western shore. In Kenya, the main feeders are the Sio, Nzoia, Yala, Nyando, Sondu Miriu, Mogusi, and Migori.

  Only one river carries water out. Near Jinja, on the northern shore in Uganda, the Nile leaves the lake. By water contributed, that makes Victoria the main source of the longest branch of the Nile. Yet the river's most distant source is usually placed in a tributary of the Kagera, rising in either Rwanda or Burundi. Which tributary remains undetermined. The uppermost Nile is known as the Victoria Nile until it reaches Lake Albert. Strictly, the name White Nile applies only after the river crosses into South Sudan.

  The lake's measurements are huge, yet its shape is modest. Its volume of about 2,424 cubic kilometres ranks ninth among continental lakes. Its catchment covers 169,858 square kilometres, and its digitized shoreline measures 7,142 kilometres, with islands making up 3.7 percent of that length. The average depth is only 40 metres, and the maximum is roughly 80 metres. A 2016 project digitized ten thousand points to make the first true bathymetric map. It shows the deepest water offset toward Kenya, with shallower water along the Ugandan west and the Tanzanian south.

  A lake this dependent on rain can swell. Between 2010 and 2022, the surface area grew by 15 percent, flooding lakeside communities.

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• 3. Cichlid Explosion

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  More than 500 species of haplochromine cichlids have been recorded in Lake Victoria, almost all of them found nowhere else. An estimated 300 of them are still undescribed. Only Lake Malawi has more fish species in a single lake. All this variety arose in a burst of rapid adaptive radiation over roughly the last 15,000 years. Scientists studying what drives the richness of life have found the lake's diversity and speed of evolution endlessly useful.

  The Victoria fish belong to an older group of more than 700 close relatives. That group also fills several smaller lakes, notably Kyoga, Edward-George, Albert, and Kivu. Most are shallow and sit in the upper Nile basin. Kivu is the exception, because it belongs to the Congo basin. Rivers are believed to have linked it to Edward and Victoria until parts of the East African Rift were uplifted. Being deep, Kivu may have served as an "evolutionary reservoir" when the shallower lakes dried out. Victoria itself dried about 15,000 years ago. Waterfalls shaped later movement. Murchison Falls stops fish heading downstream toward Lake Albert. The Owen Falls, now flooded by a dam, were only rapids, so fish could pass between Victoria and Kyoga.

  Within the lake, the haplochromines filled at least 16 ecological roles. There were eaters of detritus, zooplankton, insects, prawns, and molluscs, and there were fish-eating hunters. Males were brightly coloured and females dull. Before the mass extinction of recent decades, about 90 percent of the native fish were haplochromines. The only other native cichlids were two critically endangered tilapia, the Singida tilapia, or ngege, and the Victoria tilapia.

  Many of these fish are now gone. A high percentage became extinct from the 1940s onward. Estimates say at least 200 species, about 40 percent, have been lost, including more than 100 undescribed ones. Early fears were worse, with some estimates reaching 65 percent. Then Nile perch numbers began to fall, and several species feared extinct were rediscovered.

  Some survived in small satellite lakes. Others held on in refuges among rocks or papyrus, where the perch could not follow. A few adapted to the changed lake itself, developing a larger gill area for oxygen-poor water, altered feeding apparatus, better sight in murky water, and a smaller head with a larger tail base for faster swimming. Some species vanished in their pure form but live on as hybrids of close relatives. Fish-eaters, mollusc-eaters, and insect-eaters were hit hardest. The plankton-eaters fared best, and by the late 1990s their densities matched or exceeded earlier levels, though with fewer species. A few threatened species live in zoos, public aquaria, and private tanks as insurance, and some survive only there.

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• 4. The Perch Decision

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  In 1927-1928, Michael Graham carried out the first systematic fisheries survey of Lake Victoria. His report called the ngege "the most important food fish of the lake" and added that "No other fish equals it in the quality of the flesh". He also warned that the European flax gill net, with its 5-inch mesh, had undoubtedly reduced ngege numbers near markets. That included the Kavirondo Gulf, the northern shore, the Sesse Islands, and Smith's Sound. His survey catches included several Haplochromis species now thought extinct, such as Haplochromis flavipinnis, Haplochromis gowersii, and Haplochromis longirostris.

  Overfishing had already reduced tilapia and the ningu, Labeo victorianus, in the first half of the twentieth century. As of 1997, the lake still supported Africa's largest inland fishery. To boost it, managers introduced several non-native tilapias from the 1950s: redbreast, redbelly, Nile, and blue-spotted. The most infamous addition was the large, predatory Nile perch.

  The idea dates to the 1920s, when people proposed stocking a big open-water predator. Warnings came at the same time. They asked for extensive research first, and they focused mainly on the native tilapia. The smaller haplochromines, despite their role in local fisheries, were dismissed by the colonial government as "trash fish".

  The first Nile perch release in the region came right after the Owen Falls Dam was completed in 1954. The Uganda Game and Fisheries Department and local African fish guards put them in above Murchison Falls. More were released in Lake Kyoga in 1955, but not in Victoria. Scientists asked that further introductions wait for results from Kyoga. By the late 1950s, though, Nile perch were turning up in Victoria's catches. With the species already present, few objected when more were moved there in 1962-63.

  How the first fish reached Victoria is disputed. Department officials denied involvement. They said the perch must have passed through the dam while it was shut for maintenance, which many scientists consider highly unlikely. Africans employed by the department say they released the fish in 1954-55 on the orders of senior officials. By 1970 the perch had spread through the whole lake.

  The numbers rose slowly at first, then drastically, peaking in the 1980s before declining from the 1990s. Native species dominated catches until the 1970s. After that, the catch was roughly 60 percent Nile perch, 30 percent native Lake Victoria sardine, and 7 percent Nile tilapia. The sardine, small and abundant in open water, had supported only minor fisheries until other natives declined. At the early 1990s peak, 500,000 tonnes of Nile perch were landed every year.

  Restoring the native ngege is now a goal. A restocking effort for Oreochromis esculentus involves Conservation International Kenya, Victory Farms, and the Kenya Marine and Fisheries Research Institute.

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• 5. Green Water

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  Oxygen readings from 1990-1991 told a troubling story. In the mixed upper layer, concentrations were higher than in 1960-1961, with surface waters nearly always supersaturated. Deep, cold water below the thermocline held less oxygen for longer. Levels under 1 milligram per litre reached waters as shallow as 40 metres, whereas in 1961 the shallowest such reading was deeper than 50 metres. These changes fit with more algae and higher productivity, signs of a eutrophic lake. Three causes are named. Repeated burning in the basin has dropped soot and ash across the lake. Rivers bring more nutrients. And settlements along the shore add pollution.

  The loss of the haplochromines may have made it worse. Tropical waters stay fertile only if nutrients keep dissolving. Rivers supply few relative to the lake's size, so most are thought to be locked in bottom deposits. Plant matter there decays slowly, but animal flesh decays far faster. About 80 percent of Haplochromis species ate detritus, and they also ate one another. Together they formed a tight recycling system, moving nutrients and biomass up, down, and sideways through the water. Some even left the lake when people and land animals caught them. Without them, algal blooms became more frequent, and these may be behind mass fish kills.

  Pollution comes mainly from raw sewage, domestic and industrial waste, and farm fertiliser and chemicals. The basin is mostly rural, yet it holds major towns. Kisumu, Kisii, and Homa Bay lie in Kenya. Kampala, Jinja, and Entebbe are in Uganda. Bukoba, Mwanza, and Musoma are in Tanzania. Factories there discharge some chemicals directly into the lake or its rivers. Small beaches and local authorities often lack proper sewage treatment. Logging and deforestation reduce how much polluting chemical the land can absorb.

  The flood of nutrients fed a plant. Water hyacinth colonised the lake in the late 1990s. Tracking began in 1993, and biomass reached its maximum in 1997. It had declined again by the end of 2001. Growth was strongest in the sheltered north, near the large urban areas of Uganda. The weed starves the water of oxygen, raising toxicity and disease risk for fish and people. Its mats block boats and ferries, cut off the shoreline, interfere with hydroelectric power, and clog industrial water intakes. It even offers snails an ideal habitat. Two snail genera, Biomphalaria and Bulinus, host the parasite behind bilharzia, which is already common among people at the lake. The loss of snail-eating cichlids adds to the risk.

  The mats had one possible upside. They shield fish from overfishing, and some species thought extinct have reappeared. The overall effect is still unknown. Pulling the weed out by hand failed, since it grew back fast. Public awareness campaigns followed. Later control used two kinds of water hyacinth weevil and big harvesting and chopping boats, which work much better. The weeds may also have been dislodged by El Niño, which raised water levels in late 1997 and early 1998. Heavy winds, rain, and waves may have helped. The weed is now described as in remission, and that may last if control continues.

  In 2013, a green power plant using harvested hyacinth began operating in Kisumu County. It produces biogas, and its by-product can be used as fertiliser.

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• 6. A Disputed Discovery

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  The lake carries several local names: Ukerewe, Nam Lolwe, 'Nnalubaale, and Nyanza. Long before any European arrived, Arab traders crossed the inland routes in search of gold, ivory, other precious goods, and slaves. Many African peoples lived around the lake.

  In 1858, John Hanning Speke reached the southern shore. He was travelling with Richard Francis Burton to explore central Africa and find the Great Lakes. Speke was the first Briton to document the lake, and the first European to sight it. Looking out over what he called a "vast expanse of open water", he believed he had found the source of the Nile. He named it after Queen Victoria.

  Burton had been ill and was resting farther south in Kazeh, near present-day Tabora. When he heard that Speke claimed to have proved the Nile's source, he was outraged. In his view, the question was still unsettled. A very public quarrel followed. It stirred fierce debate among scientists of the day and drew other explorers keen to confirm or refute Speke.

  The famous Scottish explorer and missionary David Livingstone tried to verify the claim in the late 1860s. He pushed too far west and wound up in the Congo River system instead. The matter was finally settled by the British-American explorer Henry Morton Stanley. His expedition was funded by the New York Herald newspaper. Over 1875-76 he circumnavigated the lake and reported the great outflow at Ripon Falls on the northern shore.

  That northern outflow, where the Nile begins, would later be reshaped by engineers.

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• 7. Harnessing the Lake

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  For at least 12,000 years, the lake's water drained over a natural rock weir at Jinja. In 1952, engineers acting for the government of colonial Uganda blasted out the weir and its reservoir. In its place they built an artificial barrage to control the lake level and slow the erosion of the rock. An arrangement called the "agreed curve" was set up to mimic the old outflow. It capped the flow at between 300 and 1,700 cubic metres per second, depending on the lake level.

  In 2002, Uganda completed a second hydroelectric complex nearby, the Kiira Hydroelectric Power Station, with World Bank help. By 2006, the lake had fallen to an 80-year low. Daniel Kull, an independent hydrologist living in Nairobi, calculated that Uganda was releasing about twice the permitted amount of water. He held it primarily responsible for the recent drops.

  Many towns and cities rely on the lake for drinking water and farming. The Lamadi water scheme serves Mwanza and satellite towns including Lamadi, Misungwi, Magu, Bukoba, and Musoma. The European Investment Bank started it in 2013 to protect the lake's health by cleaning up the towns' water and sanitation. It aims to supply safe drinking water to an estimated one million people and better sanitation to 100,000. Sand filtration removes sediment and suspended solids before chlorination, and it relies on the local environment.

  Since the 1900s, ferries have linked Uganda, Tanzania, and Kenya. The main ports are Kisumu, Mwanza, Bukoba, Entebbe, Port Bell, and Jinja. MV Victoria, the fastest and newest ferry, was designated a Royal Mail Ship until 1963. In 1966, MV Uhuru and MV Umoja began train ferry services between Kenya and Tanzania.

  The lake has also seen disasters. The ferry MV Bukoba sank on the 21st of May 1996, killing between 800 and 1,000 people. It is among Africa's worst maritime disasters. On the 20th of September 2018, the passenger ferry MV Nyerere sank in Tanzanian waters, and over 200 people died. On the 6th of November 2022, Precision Air Flight 494, an ATR 42-500 carrying 39 passengers and four crew, crashed while approaching Bukoba Airport. Nineteen people died.

  An environmental data repository has existed since 2016, holding shoreline, bathymetry, pollution, temperature, and wind data. The East African Community and the Lake Victoria Basin Commission expanded it with the inaugural State of the Basin Report 2025, announced at the first Lake Victoria Day in Mwanza. The report draws on a €60 million Water Information System to monitor ecological pressures and climate resilience for the 45 million people living in the catchment.

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