In this study, some heavy metals (Pb, Cd, Cr, Cu, and Co) were analysed in the pioneer hydroelectric dam of South West Ethiopian’s of Gilgel Gibe I and its potential tributaries. Five water samples from the dam and four water samples from potential tributaries were collected by systematic random sampling and purposive sampling method respectively. The water Samples and blanks were digested prior to analysis. FAAS were used for analysis of the selected heavy metals. The data were analyzed by using one way analysis of variance and significant differences accepted at p ≤ 0.05. Tukey’s HSD test was used to separate means. The mean values of heavy metals concentrations (mgL-1) in dam water were Pb (1.109-2.815), Cu (0.013-0.061), Cr (0.197-0.561), Co (0.015- 2.849), Cd (0.061-1.327) and in the potential tributaries were Pb (0.468-3.218), Cu (0.057-0.258), Cr (0.232-0.526), Co (0.132-1.537) and Cd (0.258-1.178). The results revealed that there were significant differences in metal concentrations among the sites. This is due to the construction of the dam downstream of Jimma where untreated wastewaters generated by more than half million inhabitants were being directly discharged into main tributaries. Therefore, the concentrations of all metals (Pb, Cd, Cr and Co) in surface water exceed the WHO recommended limit for drinking water except Cu.

Keywords: Heavy metals; Gilgel Gibe I Hydroelectric Dam; Potential tributaries.