This report presents the results of a study of the aquatic macro invertebrate fauna of the River Murray and the lower reaches of its major tributaries between July 1980 and December 1985. Sampling was undertaken at 14 sites four times per year from 1980 to 1983 and then lvice per year during 1984 and 1985. Artificial substrate samplers were used, supplemented with sweep-net sampling to capture organisms unlikely to colonise the sampler units. The primary aim of the project was to record the distribution, diversity and abundance of the macroinvertebrates of the River Murray, in order to provide baseline data from which future comparisons could be made in relation to the effects of river development and changes in water quality. During the period covered by this study (1980-1985) , a total of 439 invertebrate taxa were recorded. The number of individual organisms recorded in each of these taxa from each site is provided on microfiche inside the back cover of this report (Appendix 4). Several new species have been discovered in the course of this study and the known range of other species has been extended. A reference collection of River Mur ray invertebrates has been established at the Murray-Darling Freshwater Research Centre, Albury, New South Wales. The data presented in Appendix 4 were analysed to compare the invertebrate communities at each of the 14 sites along the River. This analysis showed that the sites could be grouped into 5 discrete zones on the bas is of similarity of community structure. The most obvious difference between sites was a marked reduction in the numbers of invertebrates at sites in South Australia compared with those upstream. It seems likely that high river turbidity downstream of the Rufus River junction is one factor limiting invertebrate numbers at the South Australian monitoring sites and the change from a lotic to a lacustrine ecosystem resulting from the construction of regulating weirs along the river in South Australia is another likely cause. At sites 3 and 4, immediately downstream of the Hume Reservoir and Yarrawonga Weir respectively, the re a remarked reductions in macroinvertebrate abundance and diversity. This may be due to a combination of factors including changes to the pattern and seasonality of flow, reduced water temperature and dissolved oxygen and a reduction of s us pended particulate organic material which normally provides a food source for downstream communities. Grouping the animals into functional feeding groups revealed a gradual change along the length of the river, consistent with the general principles on which the "river continuum" concept is based. Community changes were observed to occur with increasing distance downstream as the source of food changed from being generated within the catchment (allochthonous) to within the river (autochthonous). These groupings were confirmed by numerical clustering techniques which identified ten community groups from the animals collected in the survey. Ttemporal changes at each site over the course of the survey were also examined. The factors which most affected the species diversity and abundance were the drought of 1982 and the high flows experienced in 1983. The changes were complex, with an increase in limnophilous species in the headwater sites during the drought and a temporal)' increase in species diversity in South Australian sites during the high flows of 1983, as animals were washed downstream. Larval chironomid (midge) populations sampled at Euston (River Murray site 8) and Burtundy (Darling River-site 10) exhibited a high incidence of mouthpart deformities of the type which have been linked to 'environmental stress ' (including pesticide pollution) in overseas studies. While analyses of water and sediment samples have not shown any recent evidence of organochlorine contamination at these sites, it is proposed to undertake further studies into the possible causes of the deformities in chironomid larvae. Having established this inventory' of aquatic invertebrate fauna and obtained preliminary information on the effects of droughts and floods on the invertebrate populations of the river, it is now proposed to monitor change in invertebrate community and population structure as an indicator of specific changes in water quality resulting from 'point source' discharges of salt, industrial effluent, treated sewage effluent and turbidity.