Tuesday, June 10, 2008

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Lake Chivero

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In 1952, the Manyame River was dammed 40 km southwest of Harare, creating Lake Chivero. The Lake was intended primarily as a water supply for Harare, but it is also a source of water for irrigation and industry and serves as a local fishery.

One year after Lake Chivero was created, water hyacinth, an invasive wetland plant, made its first appearance, as a result of the influx of nutrients from nearby agricultural lands and municipal and industrial wastes from Harare. In 1955/1956, the first serious water hyacinth outbreak occurred and was successfully treated with chemical herbicides. The next outbreak in 1971/1972 covered approximately 25 per cent of the lake. Attempts to end a third outbreak in 1986 used mechanical and chemical controls until public concern about the chemicals brought an end to their use. By 1989, water hyacinth covered 20 per cent of the lake's surface (1989 image, yellow arrows); by 1990, it covered 35 per cent. Weevils that feed on water hyacinth were released as a biological control; mechanical and new chemical controls continued.

By 1997, it appeared that water hyacinth had been brought under control (2000 image, yellow arrows). By 2005, however, the invasive plants had returned again, reportedly covering as much as 40 per cent of the lake. In addition to water hyacinth, this most recent infestation includes massive amounts of another invasive plant, spaghetti weed (Hydrocotyle ranunculoides).
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