If you had enough vertical drop, and the right geography to create a string of dams, there would be no reason why you couldn't feed the output of one hydroelectric plant into the next one. The potential energy is determined by the vertical drop from the inlet to the turbine. Typically, you don't find the geography required to do that. Hydroelectric plants are constructed to maximize the potential energy by putting the turbines as low as possible and the dam as high as possible (while maintaining sufficient water supply to maintain the required flow).