An optimal planning and preliminary-design (OPD) model was developed to maximize the annual net benefit of run-of-river (RoR) hydropower projects along a stream. The OPD model identifies optimal, non-overlapping RoR projects along the given stream. For each project, the OPD model determines intake location; penstock diameter and length; and type, number, and discharge of turbines. The OPD model applies genetic algorithm in two stages. The first stage optimizes design of individual potentially overlapping projects densely spaced at uniform intervals along the stream and the second stage determines the set of non-overlapping projects that maximizes the net hydropower benefit. Results of the OPD model compared favorably to results of previous models for a hypothetical case study. Further application of the OPD model to idealized case studies with different stream elevation profiles and with tributary flows indicated that the model gives reliable results. The OPD model was used to identify optimal RoR projects for the Guder River, Ethiopia. The OPD model identified 22 optimal RoR projects covering similar to 49 km of the 127-km-long Guder River. The total annual generated energy from the identified projects is similar to 540 GWh and the installed capacity for individual projects ranges between similar to 1 MW and 21 MW. (C) 2019 Elsevier Ltd. All rights reserved.