The world's carbon and air pollution would be much, much worse without hydro power. Indeed, zero-carbon, zero-air pollution hydro power provides 15% of the world's electricity!
By contrast, hydro provides about 8% of America's power. Using water to make 8% of America's power and 15% of the global electricity avoids burning large amounts of coal, gas, and oil or nuclear power. Yet, in the energy world, there are always trade-offs.
Large hydro projects in many parts of the world, including America, have caused substantial damage to river systems, fish migration, and to communities living nearby. Indeed, China has forcibly moved millions of people from their homes in order to build massive dam systems to generate hydro power.
Around the world, hydro added 30,000 megawatts in 2013, an impressive amount that is equal to about 15 nuclear plants. Hydro power is the oldest form of power, but it is still very much part of our modern world.