The Three Gorges Dam is located in Hubei Province, central China. It spans the longest river in Asia, the Yangzi River. The Three Gorges Dam uses water from three canyons, Qutang Gorge, Wu Gorge, and Xiling Gorge, to turn turbines to generate electricity.
According to IFLScience, the information about the Three Gorges Dam affecting the Earth's rotation seems to have originated from a 2005 NASA post about how the catastrophic 2004 Indian Ocean earthquake and tsunami affected the Earth's rotation.
The article explains how changing the distribution of mass on Earth can have a very small effect on the planet's moment of inertia. This is the same phenomenon that explains how an ice skater can increase her rotational speed by pulling her arms close to her body.
Similarly, the Earth’s motion can be affected after an earthquake due to the shifting of tectonic plates. This happened in 2004 after the Indian Ocean earthquake, according to NASA scientists. The earthquake shook the planet’s seismic structure, changing the distribution of mass and reducing the length of a day by 2.68 microseconds.
Theoretically, large displacements of water on Earth could have similar effects. In a 2005 post, Dr. Benjamin Fong Chao, a geophysicist at NASA's Goddard Space Flight Center, explained that the massive Three Gorges Dam could hold 40 cubic kilometers (10 trillion gallons) of water.
According to his calculations, the displacement of water in China's hydroelectric dam could increase the length of a day on Earth by 0.06 microseconds and move the Earth's poles by about 2cm. This number is not large compared to the impact of earthquakes, but it is quite significant when considering that the Three Gorges Dam is a man-made structure, IFLScience points out.
In fact, humans are also affecting the Earth’s rotation in other ways. Climate change is causing a redistribution of Earth’s mass. As temperatures rise, the Earth’s polar ice caps melt and tropical sea levels rise, causing more mass to be concentrated at the equator than at the poles. As a result, the Earth’s rotation slows and the days become slightly longer.
These fluctuations are not noticeable in everyday life on Earth, but can confuse ultra-precise timekeeping devices like atomic clocks. The resulting problem has led some scientists to argue that the world will need to account for a negative leap second (a second subtracted to synchronize with the Earth's rotation, meaning a minute will have only 59 seconds) in the next decade.
