Why does the Hoover Dam create hydro electric power?

Answer 1

If you spin a magnet inside a coil of wire, you will create an electric current. The stronger the magnet, or the larger the coil, or the faster the spin, the more electricity you will generate.

Hydro electric power is created the same way. The water behind the dam flows through sluice gates and turns large turbines (these are like propellers) which are attached to generators that produce the electrical current. The generators are essentially magnets inside a coil of wire, or a coil of wire turning inside a magnet - either way will work.