China’s Three Gorges Dam — so named for the three chasms it encompasses — makes up the world’s largest hydroelectric dam. And the reservoir connected to the dam is capable of holding such a high volume of water that it is rumored to slow and change the rotation of the Earth.
The rumor has been in circulation for more than a decade, and turns out, it’s true. The rumor was fact-checked by Business Insider (BI) in June 2010 and has been reiterated over the years in variations of social media posts and a number of publications that made similar claims. In general, the claim asserted that once the reservoir of the dam was filled, the weight of the mass would be enough to slightly shift the rotation of the Earth. According to BI:
That water will weigh more than 39 trillion kilograms (42 billion tons). A shift in a mass of that size will impact the rotation of the Earth due to a phenomena known as “the moment of inertia”, which is the inertia of a rigid rotating body with respect to its rotation. The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. The longer the distance of a mass to its axis of rotation, the slower it will spin. You may not know it, but you see examples of this in everyday life. For example, a figure skater attempting to spin faster will draw her arms tight to her body, and thereby reduce her moment of inertia. Similarly, a diver attempting to somersault faster will bring his body into a tucked position.
Raising 39 trillion kilograms of water 175 meters above sea level will increase the Earth’s moment of inertia, and thus slow its rotation. However, the impact will be extremely small. NASA scientists calculated the shift of such a mass will increase the length of day by only 0.06 microseconds, and make the Earth only very slightly more round in the middle and more flat on the top. It will also shift the pole position by about two centimeters (0.8 inch). Note that a shift in any object’s mass on the Earth relative to its axis of rotation will change its moment of inertia, although most shifts are too small to be measured (but they can be calculated).
Named for the nearby gorges — Qutangxia, Wuxia, and Xilingxia — plans for the Three Gorges Dam were finalized in 2021, which explained why the claim once again made the rounds in May of that year. The dimensions and logistics noted in the above post are accurate. The dam was built along the Yangtze River and has a generating capacity of 22,500 megawatts – almost four times as much as the Grand Coulee Dam on the Columbia River in Washington. Measuring nearly 600 feet tall and running almost 1.5 miles long, the dam creates the Three Gorges Reservoir, which has a surface area of 400 square miles and extends upstream from the dam 370 miles, according to the U.S. Geological Service.
Snopes contacted NASA, which confirmed that the claim had originated in a Jet Propulsion Lab report published in June 2005. Scientists compared the effects of the dam to the Dec. 26, 2004, Indonesian earthquake, which prompted a tsunami that killed nearly 230,000 people.
Through a process known as the “moment of inertia,” the quake was found to have decreased the length of the day by 2.68 microseconds and Earth’s oblateness (flattening on the top and bulging at the equator) decreased by about one part in 10 billion. But to understand how this works, we need to explain the physical properties.
Shifts in mass like those resulting from earthquakes or reservoirs affect the rotation of the earth because of what is known as the moment of inertia, or rotational inertia. In the case of the dam, the moment of Earth’s inertia depends on its mass (water) and the distribution of that mass relative to the axis of rotation (i.e., the relocation of the water from other areas to the reservoir), according to the Khan Academy. The Earth’s axis is an imaginary pole that runs through the center of Earth from “top” to “bottom,” noted NASA. Earth spins around this pole and makes one full rotation each day complete with a day and a night. But as mass moves on the planet, this shift can slightly alter the rotation, and thus the length of days, on Earth.
Understanding the moment of inertia is also understandable when looking at a spinning top – an evenly distributed top will be able to better spin, but when mass changes, the rotation and spinning of an object also changes.
The phenomenon is not abnormal — a shift in any object’s mass on earth relative to the axis of rotation will change a moment of inertia, though most are too small to be measured. Earth’s rotation can be changed based on any of its dynamic processes, from winds and atmospheric pressures to earthquakes and glaciation — any time a large mass moves from one location on the planet to another. Rotational shifts were observed during the major earthquakes in Chile in 2010 and in Japan in 2011, both of which increased the Earth’s spin and hence decreased the length of the day.
In the 2005 NASA report, scientists argued raising enough water above sea level to fill the Three Gorges Reservoir would also increase Earth’s moment of inertia and thus slow its rotation — a small shift of about .06 microseconds per day, making the planet slightly more round in the middle and flat on top.
“If filled, the gorge would hold 40 cubic kilometers (10 trillion gallons) of water. That shift of mass would increase the length of day by only 0.06 microseconds and make the Earth only very slightly more round in the middle and flat on the top. It would shift the pole position by about two centimeters (0.8 inch),” write the scientists.
The change in inertia would also shift the position of the poles by about .8 inch – again, a process that is not that foreign. While the Earth’s poles reverse about every 200,000 to 300,000 years, the earth’s pivoted axis causes the north and south poles to shift slightly and often. Notably, from 1999 to 2005, Earth’s magnetic north pole went from shifting at most about 9 miles a year to as much as 37 miles in a year, according to a study published in the journal Nature Geoscience, and is expected to continue a trajectory toward Siberia.
So, while it may seem alarming to some that the construction of a dam and its subsequent reservoir has the capability to shift the Earth’s axis and alter the length of days, the concept is a rather normal element of life on Earth.