... whatever stresses are placed upon them (Hellawell, 1986). Biomonitoring is the use of the biological responses to assess changes in environment. Therefore, macroinvertebrates are most frequently used as indicator species. Cairns & Pratt (1993) conclude that biological surveillance of communities, with special emphasis on characterising taxonomic richness and composition, is perhaps the most sensitive tool now available for quickly and accurately detecting alterations in aquatic ecosystems. Conca and Wright (1992) states that ideally ecologists and managers should understand the processes which lead to the observed patterns of community structure in unstressed flowing-water systems and that this would provide a firm foundation from which to investigate the processes taking place when environmental stresses lead to community change, both structural and functional. Many parameters will determine the distribution pattern of macroinvertebrates in freshwaters. Numerous studies and publications have been produced to determine which are the most relevant factors affecting this distribution. Hynes (1960, ...

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