The objectives of this study were characterization of slaughterhouses wastewater and evaluation of the performance of their treatment system, and their effluent impact on the receiving rivers water quality. Samples from untreated effluents of Kera and Luna slaughterhouses, treated effluent of Luna slaughterhouse and water samples of Little Akaki and Modjo Rivers were examined using standard procedure over the duration of two months period of dry season. It was found that except temperature and pH, the levels of all the parameters of the raw wastewater and both rivers water did not comply with effluent discharge limit and drinking water standards of the country, respectively. The values of most parameters were significantly higher for Kera slaughterhouse than Luna slaughterhouse, at 0.05 significant level of post Hoc ANOVA. Again the addition of discharges has shown many fold increase in most of the analyzed parameters of both rivers. The Luna slaughterhouse lagoon, overall percentage removal of color, turbidity, TS, TSS, BOD5, COD, NH3-N, NO2--N, NO3--N, S-2 , SO4-2, PO4-3, TP, FC, TC and FOG were 56.67%, 63.22%, 63.77%, 88.68%, 91.59%, 90.91%, -743.08%, 84.33%, 97.7%, 40.76%, 89.19%, -117.43%, -11.43%, 50.41% , 36.30%,and 66.18% respectively. It was found to be not complying with the Ethiopian environmental protection authority discharge limit. The study has revealed that there was an adverse impact on the physiochemical and bacteriological characteristics of the receiving rivers as a result of the discharge of untreated and inadequately treated effluents from these slaughterhouse industries. There is a need of establishment and improvement of slaughterhouse wastewater treatment system, as well as an intervention of regulatory bodies to ensure production of high quality treated final effluents by the slaughterhouse industries.rnKey words: Bacteriological loads, Lagoon, Physicochemical characteristics, River Water quality, Slaughterhouse wastewater.