Data on the hydrology, chemistry, and benthic invertebrates of the Great Fish River prior to the opening of the Orange River/Fish River interbasin.water transfer scheme in 1977, are compared with similar data for post transfer conditions. As a result of the transfer of water the once irregular seasonal flow of the Fish River has now become perennial. The mean annual runoff of the upper river has increased by between 500 and 800 per cent. Mainly because of abstractions for irrigation, the mean annual discharge of the lower river has changed little, but seasonal flow variation in the lower river has been considerably reduced. The inflow of low salinity water from the Orange River has diluted the highly mineralized Fish River water, reducing concentrations of sodium, magnesium, chloride, and sulphate, but not of calcium or total alkalinity. The invertebrate communities of riffles have changed substantially as a result of the water transfer, and only 33 per cent of taxa identified were common to both pre- and post-transfer surveys. In particular, the dominant chironomid, hydropsychid, and simuliid species have changed, although there is no evidence that overall invertebrate densities have been altered. The most striking change in the fauna has been the replacement of the pre-transfer dominant simuliids (Simulium adersi and S. nigritarse) by S. chutteri, a blood-feeding pest of livestock, which now causes considerable problems to farmers. The major changes in invertebrate species can be attributed to the more permanent flow and increased area of erosionaI habitats.
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