How a Giant Dam Slowed the Earth’s Rotation (A Little)

Did the Three Gorges Dam Really Slow Earth’s Rotation?

Yes—but only in the narrow scientific sense that moving a large amount of mass farther from Earth’s rotational axis slightly increases the planet’s moment of inertia. The estimated change is so small that it has no practical effect on people, calendars, navigation or ordinary timekeeping.

The Engineering Scale Behind the Calculation

The Three Gorges Dam crosses the Yangtze River in China’s Hubei Province. Its reservoir stretches for roughly 600 kilometers upstream and covers approximately 1,080 square kilometers when near its maximum operating level. The hydroelectric station reached an installed generating capacity of 22,500 megawatts after its final main generating units entered service in 2012.

Those dimensions are enormous by engineering standards, but one popular description needs correcting: the reservoir is not “nearly the size of Lake Superior.” Lake Superior’s surface area is more than 80,000 square kilometers, making it roughly 75 times larger than the Three Gorges Reservoir by surface area.

Why Moving Water Can Change a Planet’s Spin

Earth rotates according to the same fundamental mechanics that govern any spinning object. Its rotational speed depends partly on how its mass is distributed relative to its axis.

A familiar comparison is a spinning figure skater. When the skater pulls their arms inward, their mass moves closer to the axis and they rotate faster. Extending the arms moves mass outward and slows the rotation. The skater’s overall mass has not changed; only its distribution has.

A reservoir produces a remotely similar effect on a planetary scale. Water that once flowed downstream and remained closer to its natural river level is stored at a higher elevation across a broad region. That redistribution increases Earth’s moment of inertia by an exceedingly small amount, producing a correspondingly tiny reduction in rotational speed.

What the 0.06-Microsecond Figure Actually Means

A microsecond is one-millionth of a second. The NASA estimate of 0.06 microseconds is therefore equal to 60 nanoseconds, or 60 billionths of a second.

The figure refers to the estimated length of one complete Earth rotation under the modeled mass distribution. It does not mean the dam changed the length of the year. A year is determined by Earth’s orbit around the Sun, while a day is associated with the planet’s rotation.

It also does not mean that scientists can isolate the dam’s signal simply by looking at an ordinary clock. Earth’s rotational speed naturally varies because the atmosphere, oceans, groundwater, ice, mantle and core continually exchange and redistribute mass and angular momentum. Many of these natural variations are much larger than the calculated Three Gorges effect.

Common Claims: What Is True and What Is Not?

How Scientists Measure Earth’s Rotation

Ordinary GPS receivers are not the main instruments used to determine the precise duration of Earth’s rotation. Instead, international scientific networks combine several highly accurate space-geodetic techniques.

• Very Long Baseline Interferometry: Radio telescopes observe extremely distant quasars and measure tiny differences in the arrival time of their radio signals. This technique provides the most direct high-precision measurement of UT1, a time standard based on Earth’s actual rotational position.
• Global Navigation Satellite Systems: Networks using GPS and other satellite constellations help track changes in station positions, polar motion and Earth’s reference frame.
• Satellite Laser Ranging: Observatories fire laser pulses toward satellites fitted with reflectors and calculate distance from the pulses’ round-trip travel time.
• DORIS measurements: Ground-based radio beacons and receivers aboard satellites help determine precise satellite orbits and movements within Earth’s reference system.

The International Earth Rotation and Reference Systems Service combines observations from these and related systems to publish Earth Orientation Parameters. These describe polar motion, rotational time and other small changes required for astronomy, satellite operations, surveying and precise navigation.

What Else Changes Earth’s Rotational Speed?

The Three Gorges calculation is striking because it connects a human-built structure to planetary mechanics. However, the reservoir is only one tiny contributor among many processes that constantly change Earth’s mass distribution.

• The Moon’s tidal pull gradually slows Earth’s rotation over very long timescales, contributing to a long-term increase in the average length of a day.
• Atmospheric winds and pressure systems exchange angular momentum with the solid Earth, causing short-term fluctuations.
• Ocean currents and tides move enormous quantities of water around the planet and influence both rotational speed and polar motion.
• Earthquakes abruptly redistribute rock within the crust and mantle, sometimes producing calculable changes in rotation.
• Melting glaciers and ice sheets transfer water from high-latitude land into the oceans, altering Earth’s mass distribution and its rotational axis.
• Movements within the mantle and liquid core contribute to subtle variations that scientists continue to study.

Earth’s day is therefore not perfectly fixed. It becomes fractionally longer or shorter as planetary systems exchange energy and mass. Modern civil time remains stable because timekeeping institutions account for the difference between uniform atomic time and Earth’s irregular rotation.

The Dam’s Meaningful Impacts Are Here on Earth

The rotational calculation is scientifically interesting, but it should not distract from the dam’s much more significant environmental, economic and social consequences. These effects can be observed along the Yangtze River and within communities surrounding the reservoir.

Dams interrupt a river’s natural continuity. Sediment that would ordinarily move downstream may settle in the slower reservoir waters, while altered seasonal flows can affect floodplains, wetlands, fish spawning and riverbank erosion. Managing these effects requires long-term monitoring, environmental-flow releases, sediment planning and continued protection of vulnerable habitats.

The project has also involved an enormous human cost. Entire communities were moved as the reservoir filled, and numerous historic landscapes and archaeological locations were submerged or relocated. These consequences cannot be captured by electricity statistics alone.

Did the Reservoir Change the Climate?

A reservoir 600 kilometers long creates a large new water surface, so it is reasonable to investigate possible effects on nearby air temperature, humidity and rainfall. Water heats and cools more slowly than land, which can moderate temperatures immediately beside a reservoir.

However, local influence should not be confused with global climate control. An assessment described by the World Meteorological Organization and the China Meteorological Administration concluded that the reservoir’s detectable climatic effects were limited and primarily local. It did not support claims that the dam was responsible for extreme weather throughout the Yangtze Basin or around the world.

Regional droughts and floods are shaped by much larger forces, including monsoon circulation, Pacific and Indian Ocean temperatures, atmospheric pressure systems, snow cover and long-term climate change. Assigning such events to a single reservoir without strong evidence oversimplifies an exceptionally complex climate system.