Currently, there are about 40,000 large dams (dams higher than 15 feet [5 meters]) in the world, with 19,000 of those being in China and about 5,500 in the United States. By erecting dams that block effectively 15 percent of the fresh water flow on the planet’s surface, people have changed the Earth’s water cycle significantly. Water cycle changes can, in turn, change such things as temperature and cloud formation. Naturally, the bigger the dam, and the reservoir created behind it, the greater the effect. China’s Three Gorges Dam, located on the Yangtze River, is a huge dam that creates 20 times the kilowatts as America’s famous Hoover Dam. The reservoir created by Three Gorges contains over 5 trillion gallons (19 trillion liters) of water and is over 400 square miles (1,036 square kilometers) in surface area. Scientists have observed that the climate around the dam has cooled and that rainfall has also changed.