Environmental change in the Great Lakes Systems of the World: |
Lake Victoria has been experiencing a major and irreversible ecological transformation since the 1950s. The lake, indeed the largest fresh water lake in Africa, is now in a state of rapid flux and its fisheries are under increasing economic pressure. The introduction of the Nile Perch (Lates niloticus) in 1959 has contributed to the irreversible ecological change of the lake. It has also generated new social and economic activities, some of which are putting additional pressure on the already fragile ecosystem. These changes are taking place in a policy environment of economic liberalisation largely characterised by the rapid opening up of local economies to internationally competitive enterprises. Local social and economic institutions—mainly informal agencies in the form of households, fishermen, fishmongers, women groups, and local traders—have, however, considerably limited (and sometimes diminishing) capacities to engage in the exploitation of Lake Victoria's fisheries. Of urgent public policy concern is the impact of the growing export of fish to the international markets on the food security and the local livelihoods of the lake's riparian communities.
Lake Victoria, with a surface area of 68,800 km2 and an adjoining catchment of 184,000 km2, is the world's second largest body of fresh water, and the largest in the developing world, second only to Lake Superior in size. Lake Victoria touches the Equator in its northern reaches, and is relatively shallow, reaching a maximum depth of about 80 m, and an average depth of about 40 m. The lake's shoreline is long (about 3,500 km) and convoluted, enclosing innumerable small, shallow bays and inlets, many of which include swamps and wetlands which differ a great deal from one another and from the lake itself. Because the lake is shallow, its volume is substantially less than that of other large Eastern African lakes with much smaller surface area. Lake Victoria holds about 2,760 km3 of water, only 15 percent of the volume of Lake Tanganyika, even though the latter has less than half the surface area.
Some 85 percent of the water entering the lake does so from precipitation directly on the lake surface, the remainder coming from rivers which drain the surrounding catchment. The most significant of these rivers, the Kagera, contributes roughly 7 percent of the total inflow, or one half of that over and above direct precipitation. The Kagera River, which rises in the highlands of Burundi and Rwanda, forms the border between Rwanda and Tanzania before turning to the east, and flows for at least 150 km completely in Tanzania territory. It discharges into the lake just north of the border between Tanzania and Uganda. Some 85 percent of water leaving the lake does so through direct evaporation from its surface, and the remaining 15 percent largely by way of the Victoria Nile, which leaves the lake near Jinja in Uganda, and flows via the Owen Falls, and the Murchison Falls to join the outflow from Lake Albert; these outflows are the main sources of the 'White Nile'.
The lake's origins are still the subject of scientific dispute, but it seems likely that it is much more recent than the other great lakes of eastern Africa. Many of the rivers now flowing east into Victoria (including Kagera) once flowed west, at least in the Miocene, Pliocene, and part of the Pleistocene eras (within the past 2 million years), possibly eventually into the Nile system, and more recent upthrust of the western side of the basin is thought to have reversed these rivers, and caused Lake Victoria to form by flowing eastwards. It is possible that the lake could have formed as recently as 25,000 to 35,000 years ago, and recent evidence suggests it may have dried up completely between 10,000 and 14,000 years ago.
Next Section:2 The Biology of Lake Victoria
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed15/12/2005