Genale â€“ Dawa river basin is one of the twelve river basins in Ethiopia with catchment arearnof 74,010 Km2 and has the third largest surface area (about 170,000km2) after Wabishebelernand Abay River basins. Genale-Dawa river basin has an estimated hydropower potential ofrn9,300 GWh/year and is expected to contribute about 5.8% of the total estimated potential ofrnthe country.rnOut of the nine large scale hydropower potential projects in the basin, this study deals withrnthe optimal operation of the cascade hydropower plants namely, GD-3, GD-5 and GD-6 thatrnare located on the main river channel of Genale River. The upper most hydropower plant,rnGD-3, is already under construction and the other two are under study.rnThe latest version of HEC-ResSim (Version 3.1) introduced by U.S. Army Corps ofrnEngineers in 2013 is used to simulate the multi-reservoir system network. The physical andrnoperational data are obtained from previous studies of these projects and used as input inrnreservoir network module of ResSim Model. Implicit (default) and Explicit (user defined)rnsystem storage balance has been used to get the maximum power and energy and plot thernoptimal guide curve so as to be used by the operator to attain the optimal overall energyrngenerated from the system. The explicit system storage balance has generated a better powerrnand energy for the reservoir system.rnThe study showed that, when the individual hydropower reservoirs deliver energy andrncapacity into a common power system, operating the projects as system has produce morernaverage energy or firm energy than the sum of individual projects operating independently.rnThe maximum overall average energy of the system found in this study is 4417.7GWh/yr.rnComparing with the feasibility study, the proposed model is capable to produce an extrarnamount of 487.7 GWh average electrical energy (a 12.4% increment) annually.rnThe optimal guide curve denotes that there is a tendency, in the optimal explicit systemrnstorage, to leave more water from GD-3 reservoir and prepare the storage to hold more waterrnduring high flood seasons. The water released from this reservoir can generate additionalrnpower at the downstream power plants. The reverse is true for the operation of therndownstream power plants. The pool level of GD-5 is almost in the flood zone except for thernfirst three months, January to March, where the pool level is in the active live storage zone. InrnGD-6 the reservoir pool level is in the flood zone throughout the year.