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The Rift Valley, also known as the Great Rift Valley, extends over 7,000 kilometers from Syria through the Red Sea, the Gulf of Aden, and the African continent to Mozambique. Its width varies from 30-100 kilometers and depth from several hundred to several thousand meters (for example, Mount Kenya peaks at 5,200 meters, and the Danakil Depression is 155 meters below sea level). Formation of the Great Rift Valley along a tectonic rift began in the Eocene and became more active in the Mesozoic period, and included intensive volcanic activity and magma eruptions that formed large basaltic lava fields. Continuous uprising of the highlands and drifting apart of the African, Somali, and Arabic plates resulted in the breaking and sinking of the highlands, creating large lakes in the Great Rift Valley.
The main populated areas of the Great Rift Valley are in the middle elevations in Africa, where subsistence farming is the predominant land use. Cropping patterns are dependent on climatic conditions that are determined by elevation. High elevations (over 3,000 meters) are generally marginal for crop production and are used for grazing small stock. Intermediate elevations (2,500-3,000 meters) with sufficient average annual rainfall are suitable for rain-fed agriculture (such as root crops, legumes, barley, and wheat), while crop-livestock systems are most common. As rainfall varies considerably, small-scale farmers have to deal with recurrent droughts that threaten agricultural production. Vegetation is strongly shaped by anthropogenic influences. At lower elevations, pastoralism is the predominant land use. Where water is available, commercial agriculture (mainly cotton and sugar cane) is common. Growing population pressures are causing significant land cover changes throughout the Great Rift Valley.
Population densities and land use in the Great Rift Valley are generally determined by the availability of water. Some of the lakes, like Lake Victoria, are used for commercial fisheries, irrigation, and industrial purposes. Rivers are mainly used for domestic purposes, livestock, and irrigation. Changes in the quality and quantity of water resources are occurring due to population growth, urbanization, and large-scale agricultural development that rely on irrigation and other (potentially harmful) agricultural inputs. Soil erosion is a common problem and is largely caused by increasing rates of resource utilization, such as deforestation, inadequate agricultural practices, and overgrazing.
The Great Rift Valley Lakes, as a consequence, suffer from increased levels of sedimentation, as well as increased nutrient loads and eutrophication, causing die-off of fish and algal blooms. Large commercial irrigation schemes have considerable impacts on the water levels of lakes and rivers, particularly during times of low rainfall and high evaporation, and cause serious water scarcity. In addition, agricultural drainage water creates serious water quality problems. Fisheries are threatened by the decline in lake levels and pollution, as breeding grounds of fish in shallow waters are affected.
Rapid urbanization in the vicinity of the Great Rift Valley Lakes Region is reducing water quality and availability. Untreated domestic and industrial wastewaters are increasingly disposed into natural water bodies, resulting in health problems and the spread of waterborne diseases, declining drinking water quality, and poor sanitary conditions. Climate change adds to the variability of rainfall, increasing the vulnerability of livelihoods to drought.
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