Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source. The volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water. Swiftly flowing water in a big river, such as the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, such as Niagara Falls in New York, also has substantial energy in its flow.

At both Niagara Falls and the Columbia River, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity. In a run-of-the-river system, the force of the current applies pressure on a turbine. In a storage system, water accumulates in reservoirs created by dams and is released as needed to generate electricity.

In 1950 hydropower produced 360 billion kilowatts of power. In 2018 it produced 100 billion kilowatts of power.

Why the decrease? The onslaught of other alternative energies, the growth of so called clean coal (by the way there is no such thing), the increase in cheaper oil because of fracking and U.S increase in drilling. None of the above mentioned things are good for the environment. If hydro electric dams are built right and manageged correctly they could along with wind, solar and geothermal produce 75% of our energy needs.