Reservoir size and its impact on Earth's rotation

The Three Gorges Dam, located on the Yangtze River in Hubei province, has a reservoir capable of holding 39.3 billion cubic meters of water. This volume is enough to fill about 15.7 million Olympic swimming pools and is slightly more than 40% of Lake Geneva’s total volume.

Physics behind the Earth's rotational change

Researchers calculated in 2005 that the displacement of the reservoir’s massive volume to higher altitudes extended the length of the day by 0.06 microsecond. This effect is based on physics calculations rather than direct measurement and applies when the reservoir is filled to its maximum level of 175 meters.

Angular momentum and Earth’s rotation

The principle of the ice skater, where angular momentum is conserved, explains how the Earth's rotation is affected by mass redistribution. When mass moves away from the axis of rotation, the planet's moment of inertia increases, slowing its rotation. The 2004 Indian Ocean earthquake had a much greater effect, shortening the day by about 2.7 microseconds.

Magnitude of the dam’s effect

The effect of the Three Gorges Dam on the length of the day is extremely small. It would take about 45,600 years to accumulate a second of shift. Light travels only 18 meters in 0.06 microseconds, which is the length of an articulated bus.

Human impact on Earth’s rotation

The Three Gorges Dam is not the only geophysical trace of human activity. Between 1993 and 2010, the extraction of about 2,150 gigatons of groundwater shifted the Earth's rotation axis 80 centimeters eastward. This pumping represents about 55 times the mass retained at the Three Gorges.

Scale and engineering of the dam

The reservoir of the Three Gorges Dam stretches over 600 km, approximately the distance by air between Paris and Toulouse. Its surface area reaches 1,084 km², about ten times that of Paris within the city limits, and its drainage basin covers 1,000,000 km², nearly double the area of metropolitan France.

Human displacement and environmental impact

Between 1.13 and 1.4 million people were displaced due to the dam's construction, with the lower estimate exceeding the population of Marseille. By June 2003, 15 towns and 116 villages had been submerged by the rising waters, with 436 km² of land already disappeared.

Dam’s role during recent floods

In 2020, China experienced its strongest floods in more than three decades, and the reservoir approached its maximum capacity, reaching its highest level since 2003. Authorities claimed the dam limited damage and human losses, while its detractors viewed it as proof of its limitations as a flood protection tool.