Hydroelectric power refers to the generation of electricity through the harnessing of the kinetic power of falling water. Conventional hydroelectric plants are created by damming powerful rivers and then controlling the water as it flows past turbines, converting potential energy into electrical energy with a generator. Many large hydroelectric plants use pumped-storage technology to respond to fluctuating electricity needs:
To take advantage of hydroelectric power's low costs and reliability, some large companies, such as Google, Microsoft, and Yahoo are locating their data centers near hydroelectric power facilities in the Northwest. All of these companies sited their facilities near hydroelectric power projects because their data centers consume large amounts of electricity. 

The simplest type of hydroelectric power station is called run-of-the-river; which indicates that there is no reservoir and a minimal (or nonexistent) dam. As water runs downstream, it moves through a turbine that feeds a generator, creating electricity. Run-of-the-river power must be used immediately, as these stations have no storage or control capacity. This type of station, which is often used for small or micro hydro projects, has significantly fewer environmental downsides than large dam projects because it relies on the natural flow of the river.

The differences among different types and sizes of hydroelectric power plants are more than just semantic; many states have different requirements for projects to be eligible for Renewable Portfolio Standards (RPS). For instance, California only accepts existing facilities that produce less than 30 MW, while Wisconsin recently (and controversially) allowed both small and large hydropower to count toward fulfillment of RPS.