When we talk about feats of engineering, very few megastructures on Earth rival the sheer scale of China's Three Gorges Dam. It's located in Hubei province, spanning the mighty Yangtze River, and it's the world's largest hydroelectric project. Stretching more than 2 kilometers, it's more than 180 meters tall, and it's capable of holding back an astonishing reservoir of water that extends hundreds of kilometers upstream. This particular dam hit the major headlines not only because of its immense size, but also due to the profound effect it has on the environment.
Since the construction of this dam in 1994, millions of residents around the project site have had to be relocated. The impact the Three Gorges Dam has on its environment is well known and documented; however, there's also one lesser-known side effect. The sheer mass of water contained by the Three Gorges Dam subtly interacts with the planet's dynamics. This dam has a small yet powerful influence on the Earth's rotation — slowing it by a whopping 0.06 microseconds. Let's explore how this is possible.
The Three Gorges Dam's rotation problem
China's famous Three Gorges Dam has a tiny, but measurable influence on Earth's rotation. When the reservoir is full, it holds about 39 billion cubic meters (about 10 trillion gallons) of water. By shifting such an immense mass across the surface of the planet, the dam alters Earth's distribution of weight.
This matters because of the principle of conservation of angular momentum. This is the same rule in physics that explains why figure skaters spin faster with their arms pulled to their bodies, and slow down when they extend them. In the case of planet Earth, moving water away from the equator towards higher elevations spreads mass slightly further from the planet's axis of rotation. That added distance causes the Earth's spin to ease, although very subtly.
Scientists have calculated that when the Three Gorges Dam is at its full capacity, it slows the Earth's rotation just enough to lengthen the day by 0.06 microseconds. This millionth part of a second is far too small for humans to notice, but we can still detect it and measure it. It was Dr. Benjamin Fong Chao of NASA's Goddard Space Flight Center who figured out these calculations. It's amazing to think that some of humanity's largest engineering projects don't just change the environment locally, they're capable of affecting planetary mechanics as well.
The impact of the Three Gorges Dam
The Three Gorges Dam indeed slows the Earth's rotation, but its influence is minuscule when compared to the natural forces that are constantly at play. For example, massive earthquakes can redistribute rock deep inside the Earth's crust, nudging our planet's spin by similar or even larger margins. Seasonal melting and refreezing of the polar ice, along with the shifting ocean tides, continuously shuffle Earth's mass and affect the planet's rotation thousands of times more than the dam. Even the gradual tug of the Moon slows our planet more noticeably, although over a much longer timescale.
The Moon lengthens our day by about two milliseconds per century.
So, what does this all mean for humanity? In practical terms, nothing any of us would ever notice. A day stretched by a few millionths of a second won't give us more sleep time. It has zero impact on our clocks, calendars, and daily routines. Timekeeping systems are so precise that they account for natural fluctuations in the Earth's rotation. That's why we have such adjustments like leap seconds.
However, for science and technology, precision is everything. Satellites circling our planet and probes traveling to distant worlds rely on knowing the Earth's exact rotation and spin rate. Even tiny shifts, if unaccounted for, can nudge the spacecraft off course. That's why scientists measure the impact of melting ice, earthquakes, and even human-made megastructures, such as the Three Gorges Dam, on the Earth's rotation. The Three Gorges Dam also offers a humbling reminder that our world is not a perfect clock, but a dynamic, shifting body. The more we reshape it and influence its ecosystems and climate, the more its rhythm drifts.