Zambezi River Basin

Aquatic weeds, mostly free-floating species such as Water Hyacinth ( Eichhornia crassipes ), Water Lettuce ( Pistia stratiotes ), RedWater Fern ( Azolla filiculoides ), and Kariba Weed ( Salvinia molesta ), are dominant in the Zambezi basin (Hirji et al. 2002). Although water hyacinth growth is a problem throughout the basin, areas that are particularly problematic are the Kafue Flats, Lower Shire, Lake Kariba, and Lake Chivero. In Lake Kariba water hyacinth and hippo grass ( Vossia cuspidate ) are found in the estuaries and along the shoreline. Invasive Alien Species Lake Chivero Lake Chivero was created in 1952 with the damming of the Manyame River, 37 kilometres southwest of Harare (Shekede et al. 2008). Water hyacinth appeared in the lake for the first time in 1953. Its proliferation was aided by the nutrient enrichment of the lake from nearby farms and

from municipal and industrial waste from Harare. By 1956, the first serious water hyacinth outbreak had been successfully controlled using chemical herbicides (UNEP 2008). The weed reappeared, and by 1976 occupied 42 per cent of the lake, before declining to 36 per cent in 1989 and later to 22 per cent in 2000. Chemical spraying and mechanical weed control methods used during the 1960s and 1970s also lowered levels of nutrient enrichment are believed to have contributed to the decline (Chikwenhere and Phiri 1999). The introduction of biological control in 1990 using the water-hyacinth weevils, Neochetina eichhorniae and N. bruchi , reduced most of the water hyacinth in the lake in the late 1990s (Chikwenhere and Phiri 1999). By 2005, the invasive plants had returned, covering as much as 40 per cent of the lake (UNEP 2008). Another widespread weed in the lake is the spaghetti weed ( Hydrocotyle ranunculoides ).

31 Aug 1986

15 May 1989

6 May 1990

23 Jun 1990

Created in 1952 to supply water to Harare, Lake Chivero became hypertrophic 15 years later. Nutrient removal and wastewater treatment reduced the trophic state in the 1970s. Urban growth in the 1980s resulted in increased discharge of partially treated wastewater into the lake.

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