This research investigates the impact of tannery effluent on the self purification capacity of natural streams by biodegradation and on sustaining macroinvertebrates. Water samples were collected from upstream (US) and downstream (DS) of the discharge point of four tanneries along the course of the Tinishu Akaki river (Ethiopia) for physicochemical parameters and macroinvertebrate analysis. As the concentration of chromium and sulfide increases from US to DS of the river, the removal of SS (Suspended Solids), and reduction of BOD5 (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) by the self purification process decreases from 71%, 59% and 50%, respectively, to 10%, 30% and 28%, respectively, while also the nitrification capacity becomes severely hampered. The macroinvertebrate %EPT (ratio of total number of individuals classified as Ephemeroptera, Plecoptera, and Trichoptera to the total number of individuals), which is a measure of the presence of pollution sensitive macroinvertebrates decreased from 24% to zero, starting downstream from the third tannery, indicating severe pollution. The diversity index and correlation values between the concentration of physicochemical parameters, BOD5 reduction, the macroinvertebrate count and the chromium concentration further support these findings. The conclusion from this study is that, although the physicochemical parameters might indicate that, given enough distance between the tanneries, self recovery of the river can be gained for a while, the biodiversity and general health of the river is severely compromised such that aquatic life can hardly be sustained
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