Water hammer is an unsteady hydraulic problem commonly found in closed conduits of hydropower plants, water distribution networks, and liquid pipeline systems. Because of either a malfunction of a system or inadequate operation conditions, a pipeline may collapse or burst erratically, resulting in substantial damages and human loss in some cases. Therefore, it is crucial that engineers design and/or analyze projects with reliable computing methods for all foreseeable operation situations. In this paper, transient flow situations in run-of-river plants are investigated for various operation conditions such as load rejection, load acceptance, and instant load rejection. A computer program using the method of characteristics is used in two case studies having Pelton and Francis turbines, respectively. On the basis of the field data of the plants, their computational models are set up, and resulting transient pressures are computed and compared with available measured pressures. Comparisons indicate that the mathematical model predicts the measured pressures closely for all simulations. © 2013 American Society of Civil Engineers.