When the Three Gorges Dam was built, 39 trillion kilograms of water from the Yangtze River built up behind it to 175 meters above sea level. This altered the Earth’s moment of inertia changed ever so slightly, causing the rotation to move more slowly. This is the same principle behind why figure skaters tuck in their arms to spin faster.
NASA has calculated that the dam only slows the rotation by 0.06 microseconds, which is six hundredths of a millionth of a second. Our planet’s rotation speed actually fluctuates fairly often, as it can be influenced by earthquakes, the moon, and the climate change-induced movement of the North Pole.