A run-of-the-river hydroelectric generating plant is disclosed, in which river water is diverted downstream, used in the hydroelectric generation process, and sent back to the river. A high-mass, large diameter hydro rotor for hydroelectric power generation is disclosed. A large diameter circular horizontal water flow, the desired water flow regime, is created to float and rotate the high-mass hydro rotor, which is coupled to a turbine shaft. Extremely high torque and angular momentum is provided for conversion into extremely high energy output. The desired water flow regime can be augmented with different configurations of penstocks, intake channels, and discharge channels.