Lake Victoria

July 14, 2012, 2:35 pm
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Content Cover Image

Lake Victoria viewed just to the north of west across the lake. Source: NASA

Lake Victoria (also known as Victoria Nyanza, Ukerewe and Nalubaale) is the world's second largest fresh water lake by area. It is located in eastern central Africa, straddling the equator, and is shared between the nations of Uganda, Tanzania and Kenya. It is generally considered to be the source of the Nile River, the world's longest river.

Geography

caption Lake Victoria's location in Africa. Source: NASA

 

Size: 250 miles (400 km) long; 200 miles (320 km) wide;

Area: 26,828 miles2 (69,485 km2) - second largest after Lake Superior

Geographic coordinates: The lake extends between latitudes 0o30' N and 2o30' S and between longitudes 31o50' E 34o10' E

Surface altitude: 3,720 feet (1,135 m)

Maximum Depth: 265 feet (81 m) - average 132 feet (40m)

Shoreline: 3,440 km (2,138 mi)

Ports: Kisumu in Kenya; Mwanza and Bukoba in Tanzania; and Entebbe, Port Bell, and Jinja in Uganda.

Lake Victoria is a relatively shallow lake lying in a depression of the plateau between two branches of the Great Rift System, the eastern Great Rift Valley and the western Albertine Rift.

caption Lake Victoria Basin. Source: Robert Simmonin, based on data provided by the USDA Foreign Agricultural Service

Rather unusually, water inflow and outflow to lake Victoria is dominated by rainfall and evaporation rather than rivers. The lake is fed by several small streams, most notably the Kagera River that bring water from the lake's basin (see image). However, it receives 85% of its inflow from rain over the lake itself. The only outlet for Lake Victoria is the Victoria Nile west of Jinja which is part of the complex White Nile system and is commonly viewed as the "source" of the Nile River. The Victoria Nile however, accounts for just 15% of the outflow, while the remaining 85% occurs through evaporation from the surface of the lake. These facts, combined with the relative shallowness of the lake, makes it very sensitive to rainfall and climate fluctutations. The lake's level has varied by up to three meters over the past century in response to rainfall, and more recently, managed outflows for generation of electricity. The residence time of water in Lake Victoria is 23 years.

Lake Victoria has numerous shallow bays and swamps, including extensive papyrus swamps. There are a number of small "satellite" lakes  that connect to Lake Victoria, including lakes Kanyaboli, Sare, and Namboyo in Kenya; lakes Nabugabo, Gigati and Agu in Uganda; and, lakes Ikimba and Burigi in Tanzania.

Lake Victoria is relatively young; its current basin was formed only 400,000 years ago, when westward-flowing rivers were dammed by an upthrown crustal block. The lake's shallowness, limited river inflow, and large surface area relative to its volume make it vulnerable to climate changes; cores taken from its bottom show that Lake Victoria has dried up completely three times since it formed. These drying cycles are probably related to past ice ages, which are times when precipitation declined globally. The lake last dried out 17,300 years ago, and filled again beginning 14,700 years ago.

The lake has more than 3,000 islands, many of which are inhabited. These include the Ssese Islands in Uganda, a large group of islands in the northwest of the lake that are becoming a popular destination for tourists.

Zoomable satallite map of Lake Victoria

History

The Baganda, the native people on the north of the lake, called the lake Nalubaale or "Home of the Spirit".

Long before European exploration of the region, Arab traders were active along the east-African coast line and along inland routes in search of gold, ivory, other precious commodities and slaves. An excellent map, known as the Al Idrisi map from the calligrapher who developed it and dated from the 1160s, clearly depicts an accurate representation of Lake Victoria, and attributes it as the source of the Nile. The Arab name for the lake was Ukerewe.

The first European to see the lake was the British explorer John Hanning Speke in 1858. Hanning embarked on a expedition with Richard Francis Burton to discover the source of the Nile River.

The caravan…began winding up a long but gradually inclined hill until it reached its summit, when the vast expanse of the pale-blue waters of the N’yanza burst suddenly upon my gaze. It was early morning. The distant sea-line of the north horizon was defined in the calm atmosphere between the north and west points of the compass; but even this did not afford me any idea of the breadth of the lake….This view was one which, even in a well-known and explored country, would have arrested the traveller by its peaceful beauty. - John Hanning Speke, August 3,

Believing he had found the source of the Nile on seeing this vast expanse of open water for the first time, Speke named the lake after Queen Victoria. Burton, who had been recovering from illness at the time and resting further south on the shores of Lake Tanganyika, was outraged that Speke claimed to have proved his discovery to have been the true source of the Nile, which Burton regarded as still unsettled. A very public quarrel ensued, which not only sparked a great deal of intense debate within the scientific community of the day, but much interest by other explorers keen to either confirm or refute Speke's discovery.

The British explorer and missionary David Livingstone failed in his attempt to verify Speke's discovery, instead pushing too far west and entering the River Congo system instead. In 1875, explorer Henry Morton Stanley, on an expedition funded by the New York Herald newspaper, confirmed Speke's discovery by circumnavigating the lake and reporting the great outflow at Ripon Falls on the lake's northern shore. William Garstin performed a comprehensive survey of the lake in 1901.

Water Level and Energy

Until the 1950s, the Victoria Nile began as it flowed out of Lake Victoria over Rippon Falls whose outflow from the lake increased and decreased according to the lake level. Rippon Falls are now submerged as a result of the building of Nalubaale (Owens) Dam (1949-54) a few miles down stream. (The Victoria Nile is now considered to begin at the dam). Hydroelectricity from Nalubaale (Owens) Dam is the primary source of electric power in Uganda. In the 1990s, a second hydroelectric facility was added next to the first to meet growing energy needs in Uganda, and fed by additional flows from the lake.

caption Owen Falls Dam/Nalubaale Power Station. The facility provides most of Uganda's electricity, regulates the river outflow of the lake and may be adversely impacting lake level. Source: USDA Foreign Agricultural Service

In late 1961-62, above-normal rainfall raised the lake by 2.5 meters following a sixty-year period of fluctuations around near constant level. As a result, the operating rule for managed outflow was adjusted to maintain “equilibrium” water level for the lake at higher level. However, recent operating practices have exceeded past sustainable operating water levels because of the additional power station.

Since late 2005, the water level of Lake Victoria has lower than it has been since before 1960. The graph below shows that the water level is quite variable because of its dependence on rain immediately over the lake itself. The reasons for the current lake level is debated. While rainfall has been low, the current lower water levels on the lake may be the norm and the levels seen since 1960 unusually high.

caption Lake Victoria height variation reative to a fixed point at Jinja. Source: Robert Simmonin, based on data provided by the USDA Foreign Agricultural Service

However, some have blamed the managed flows to the hydroelectric facilities, which though moderated, are likely exacerbating the low water levels in the lake. Further, it has been argued that the outflows violated a 1950's agreement with Egypt governing acceptable water use from Lake Victoria. The Ugandan Sunday Vision newspaper reported:

However, it now seems that the dams themselves are as much to blame as the recent drought. Daniel Kull, a hydrologist with the UN's International Strategy for Disaster Reduction in Nairobi, Kenya, calculates that if the dams had been operated according to the agreed curve during the past two years, the drought would have caused only half the water loss actually seen. "Today's lake levels would be around 45 centimetres higher," he writes in a report released this week by the California-based environmental lobby group, International Rivers Network.

Others in the Ugandan government have pointed to the low rainfall, the greater (but uncontrollable) role played by evaporation, and reduced outflows which have caused a wave of power outages that impacted a country almost totally dependent on power from this source. Uganda has rapidly opened three new power plants running on diesel at to diversify its power supply and is exploring new hydroelectric facilities at other locations. As of early 2009, Lake Victoria's water level remained low.

The lower water level has also impacted transportation, trade on the lake (forcing boats to dock farther from shore), fishing, and tourism

Fisheries

Lake Victoria fisheries have traditionally been dominated by a large number cichlid fish species (particularly Haplochromine cichlids and Tilapiine cichlids) which have evolved rapidly from a few species that were present in the rivers that helped fill the lake 14,700 years ago. It is estimated that the number of Haplochromine cichlids species may have been in excess of 500 while current estimates are at least 150 species (out of over 200 cichlid species overall), most endemic to the lake.  A 1971 UN survey reported that Haplochromine cichlids constituted about 80% of the total fish biomass of the lake. Cichlids were also a significant food source for the rapidly expanding population living around the lake.

By the 1950s, growing lakeside populations began to have a noticeable impact on the water quality and fisheries of Lake Victoria. Changes in land use resulted in increased flows of silt, and chemicals into the lake, which, combined to reduce fishing capacity, thus, leading to a decline in catch, even as the demand for fish was rapidly increasing.

In the 1950s, the Nile perch (Lates niloticus) was first introduced into the lake's ecosystem in an attempt to improve fishery yields of the lake. Introduction efforts intensified during the very early-1960s. However, the species was present in small numbers until the early to mid-1980s, when it underwent a massive population expansion and came to dominate the fish community and ecology of the world's largest tropical lake. Also introduced was the Nile tilapia (Oreochromis niloticus), now an important food fish for local consumption.

The most recent data for fish catch show the dramatic change in the Lake Victoria fishery:

Lates niloticus (Nile Perch)

 63%

 Rastrineobola argentea (Silver Cyprinid, Dagaa, Mukene, Omena)

 19%

 Oreochromis niloticus ( Nile tilapia)

 9%

 Haplochromine cichlids

 <1%

 

The abrupt change in fisheries is rather startling. The Nile perch is a predator and has devastated Haplochromine cichlid populations to a point where many species are becoming extinct in the wild or extinct totaly. Some species are being maintained  in zoos and aquaria, e.g. as part of the Association of Zoos and Aquarium's Species Survival Plan for these species. Some species which were extirpated from Lake Victoria itself, are known to survive in nearby smaller so-called satellite lakes, such as Lake Koyota, Lake Edward and Lake Albert. The damage is not limited to  Haplochromine cichlids. For example, the Singidia tilapia (also called ngege - Oreochromis esculentus) is also assessed as critically endangered and is found only in a few satellite lakes.

The Nile perch has become the most important commercial species and is exported for consumption worldwide. The impact of perch fishing upon the local economy is the subject of the documentary Darwin's Nightmare. However, Nile perch catches have diminished dramatically in recent years due to overfishing and poor enforcement of fisheries regulations. The division of catch among the three nations sharing Lake Victoria is estimated to be:

The overfishing of the Nile perch maybe having the side effect of allowing the populations of a few endemic cichlid species to increase again, particularly one to three species of zooplankton-eating, herring-like cichlids (Yssichromis) that school with the abundant native Rastrineobola argentea.

In 1996, The World Bank funded a project to restore and sustain the ecology of Lake Victoria and its fisheries, called LVEMP (Lake Victoria Environmental Management Project). Meanwhile, the European Union invested another large sum in fisheries infrastructure and monitoring. One product of these foreign aid programmes has been the training of a new generation of East African aquatic ecologists, conservation professionals, and fisheries scientists. There has also been an increase in the fishery research institutes of the lake.

Water Hyacinth

Another exotic species introduced to lake Victoria has also caused major environmental problems, the water hyacinth (Eichhornia crassipes), a plant native to the tropics of South America. It appears to have been been introduced to the region by Belgian colonists to Rwanda in the nineteenth century to beautify their holdings and then advanced by natural means to Lake Victoria where it was first sighted in 1988.  In ten years, it had spread along the shorelines in thick mats that covered an estimated 77 square miles (200 km2) impacting fishers, transportation and hydroelectric power production. Agressive efforts to halt its spread included the introduction of the mottled water hyacinth weevil (Neochetina eichhorniae) which ate the plant. After initial succes, when the plants coverage was dramatically reduced, it has returned as shown in the NASA images below.

caption Water Hyacinth in Winam Gulf, in the northeast corner of Lake Victoria in Kenya. Source: NASA

NASA reports:

These images show the Winam Gulf, in the northeast corner of Lake Victoria in Kenya. The gulf was the most severely affected region during the first hyacinth outbreak in 1998, with as much as 17,231 hectares (67 square miles) of the plant growing on its surface.

By 2000, the area covered by water hyacinth was down to about 500 hectares (2 square miles), and in December 2005, when the right image was taken, the lake appeared to be clear.

In November and December 2006, however, unusually heavy rains flooded the rivers that feed into the Winam Gulf. The rain and floods raised water levels on the lake and swept agricultural run-off and nutrient-rich sediment into the water. As a result, the Winam Gulf was brown when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite took the top left photo-like image on December 18, 2006. Vegetation around the lake was dramatically greener due to the rains.

The influx of fertilizer and sediments not only turned the water brown, but it also fed a fresh outbreak of water hyacinth. Bright green plants cover much of the Winam Gulf in the top left image. Though other plants such as algae may be contributing, water hyacinth is almost certainly one component of the soupy mass. As the photo shows, water hyacinth was growing along the shoreline, particularly in Kisumu Bayand Nyakach Bay.

A comparison between December 2005 and December 2006 shows that Kisumu Bay was entirely covered by water hyacinth in 2006, and the shoreline of Nyakach Bayalso appeared to change shape as the plant grew out from the shore.

Surrounding Ecoregions

The lands surrounding Lake Victoria represent three ecoregions.The rolling hills and plateaus of the Victorian Basin Forest-Savanna Mosaic ecoregion extend from the lake north and west. It is most noted for its high species diversity and endemism resulting from the mixture of habitat types and species from both western and eastern Africa. Southern Acacia-Commiphora bushlands and thickets cover the lands east and southeast of the lake with wide-sweeping grasslands and associated woodlands dominated by species of acacia and commiphora trees. Southwest of the lake, densely forested Central Zambezian Miombo Woodlands include trees 50 to 65 feet (15-20 m), high rising over a broadleaf shrub understory with grassland underneath.

caption Ecoregions around Lake Victoria. Source. World Wildlife Fund.

Further Reading

  1. Lake Victoria: Ecology, Resources, Environment by Joseph L. Awange and Obiero Ong, Springer, 2006 ISBN: 3540325743.
  2. Africa by John Reader, National Geographic Society, 2001 ISBN: 0792276817.
  3. Merriam-Webster's Geographical Dictionary, Third Edition, 2007 ISBN: 0877795460.
  4. Darwin's Dreampond: Drama on Lake Victoria by Tijs Goldschmidt, The MIT Press, 1998 ISBN: 0262571218.
  5. Lake Victoria Basin Cichlids by Mark Smith, Barrons Book, 2001 ASIN: B0006JKLOU
  6. Lake Victoria's Falling Waters by Holli Riebeek, design by Robert Simmon, NASA Earth Obervatory, March 13, 2006
  7. Uganda by Philip Briggs, Bradt Travel Guides; 5th edition, 2007 ISBN: 184162182X.
  8. Freshwater Ecoregions of Africa and Madagascar: A Conservation Assessment (World Wildlife Fund Ecoregion Assessments) by Michele L. Thieme, Robin Abell, Melanie L. J. S. Stiassny, Paul Skelton, Neil Burgess, Bernhard Lehner, Eric Dinerstein, David Olson, Guy Teugels, and Andre Kamdem-Toham, Island Press, 2005  ISBN: 1559633654.
  9. History and timing of human impact on Lake Victoria, East Africa by Dirk Verschuren, Thomas C. Johnson, Hedy J. Kling, David N. Edgington, Peter R. Leavitt, Erik T. Brown, Michael R. Talbot and Robert E. Hecky, Proc. R. Soc. Lond. B (2002) 269, 289–294
  10. Socio-economic effects of the evolution of Nile perch fisheries in Lake Victoria: a review by J. Eric Reynolds and D. F. Greboval, FAO Fishery Policy and Planning Division, 1998. 
  11. Ecological, environmental and socioeconomic aspects of the Lake Victoria's introduced Nile perch fishery in relation to the native fisheries and the species culture potential: lessons to learn by John S. Balirwa, African journal of Ecology, (2007) V.  45 Issue 2, 120 - 129
  12. The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria by A. P. Achieng, Journal of Fish Biology (2006) V. 37, Issue sA, 17-23.
  13. Decreasing levels of Lake Victoria Worry East African Countries by Gideon Munaabi, February 12, 2006
  14. Uganda pulls plug on Lake Victoria by Fred Pearce, New Scientist, 09 February 2006
  15. Troubled Waters: The Coming Calamity on Lake Victoria by Sarah Stuteville, CLPMag.org, June 24, 2008'
  16. East African Community
  17. Lake Victoria Fisheries Organisation
  18. Major rivers, lakes, mountains, and other terrestrial features of Uganda

 

 

 

 

 

 

 

Glossary

Citation

Saundry, P., & Fund, W. (2012). Lake Victoria. Retrieved from http://www.eoearth.org/view/article/154134

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