A biogeographical approach to interbasin
water transfers: implications for river
B.R. Davies, C.D. Snaddon, M.J. Wishart, M.C. Thoms and M. Meador
Water availability has placed important constraints on the location and size of early human settlements. However, rapid increases in technology and engineering over the past century or more, have reduced this dependence. Urban sprawls and agricultural developments now flourish in environments that, without major engineering and technological advances, could not possibly support them. Many examples spring to mind. In South Africa, Gauteng Province (essentially the cities of Johannesburg and Pretoria, the industrial heartland of the country) is entirely supported by an array of interbasin water transfers (IBTs) (Van Niekerk et al., 1996; Basson, 1997; Davies and Day, 1998). The desert cities of Phoenix (Reisner, 1986) and Las Vegas in the United Sates and Adelaide in Australia (Davies et al., 1992) all rely heavily on the importation of water from distant regions. Modern industrialized agriculture is also completely dependent upon irrigation technologies that now rely largely on IBT schemes (e.g. McCully, 1996; Wackernagel and Rees, 1996).
Not only have IBTs extended the limits of development of human society beyond the sustainability of the immediate environment, but their construction and use have serious implications for the river systems that are manipulated. Although impoundment is frequently a part of many IBTs, river regulation by IBTs leads to more complex problems than those caused by simple storage of water (Day, 1985; Snaddon, 1998). While the effects of river regulation by impoundment have been well documented (e.g. Ward and Stanford, 1979, 1983a, b; Lillehammer and Saltveit, 1984; Petts, 1984; Craig and Kemper, 1987; Petts et al., 1989), the ecological effects of IBTs are relatively unknown. In this chapter there is a focus on the implications of IBTs for river conservation in terms of the breakdown of biotic integrity that such schemes impose. The definition of an IBT adopted in this chapter follows that of Davies et al. (1992), viz: ‘the transfer of water from one geographically distinct river catchment or basin to another, or from one river reach to another’. Further, the term biotic integrity is used here as defined by Karr (1991), as ‘the ability to support and maintain a balanced, integrated, adaptive community of organisms having a species composition, diversity and functional organization comparable to that of the natural habitat of the region’.
This chapter addresses the biotic effects of IBTs at differing scales, from biogeographical regions and subregions to basin scales, and finally to that of specific river reaches. Temporal considerations associated with the operation of IBTs are also considered. These include, for instance, the evolutionary implications resulting from the breakdown in catchment integrity, long-term changes in discharge patterns, disruption of seasonal cycles and local short-term effects and recovery.