When this massive Chinese dam was built known as the Three Gorges Dam that blocks the Yangtze River, it caused over 39 trillion kilograms of water to be stocked up above sea level. The weight of this water being 175 meters from sea level has caused the Earth’s movement to slow down. It works the same way that a figure skater will tuck their arms in to ensure their rotations speed up, well here the Earth is essentially keeping its hands raised.
NASA has calculated that the dam has altered the Earth’s moment of inertia ever so slightly. Which means that the rotation has slowed down by 0.06 microseconds. What you may be surprised to learn is that Earth’s rotation speed is far from stable, it changes often. The rotations are influenced by earthquakes, the moon, and even climate change. For example, the melting ice caps of the North and South pole can also change the speed of the Earth’s rotations.
Three Gorges Dam is a hydroelectricity gravity dam, which is the world’s largest power station and can generate over 100 terawatts of electricity per hour. As well as producing electricity it is also intended to improve the Yangtze River’s shipping capacity by helping the river stay above certain levels. China sees this dam as one of their greatest economical and social successes, however, many argue that the massive scale flooding it has caused will prove dastardly in the future by the form of landslides and ecological damage. It has also caused the displacement of millions of people from their cultural and historical homes.
So what is happening in the case of this massive Chinese dam? The dam has caused so much water mass to be displaced that it has caused the rotations to slow down. It has to do with the moment of inertia, this is the phenomenon that describes that the longer the distance of a mass to its axis, the slower it will spin. So China has been able to increase the earth’s mass so much via the body of water build-up, that it has slowed down the rotations. Much like how an Olympic diver will tuck their body to somersault faster.