Hydroelectric energy can be harnessed from large rivers. The great force of running water through the penstocks of a dam is capable of turning turbo-generator, which in turn, produces hydroelectric power. An example of such a Hydroelectric Power (HEP) station is the Aswan High Dam across River Nile, which supplies about 40% of Egypt's electric power supply. 

Aswan High Dam, River Nile, Egypt                                     How a Dam Works