Water flowing from Lake Mead is used to make electricity. Turbines and generators are used to change water from mechanical energy into electrical energy. Water in the Colorado River is used over and over again. Generators can be powered by many sources. A water turbine is a wheel with fins on it, somewhat like a windmill, except it uses water instead of air.
Magnets spinning inside wire coils in the generator make the electricity.
In this way the generator and turbine change mechanical energy into electrical energy. After having done its job of turning the water turbines to generate electricity, the water flows downstream. There it can be stored behind dams and used again. Hoover Dam was built to make Lake Mead, from water flowing down the Colorado River. There are three other dams on the main stem of the Colorado River.