China Built the World's Largest Dam — Now Engineers Are Tracking Its Deformation
October 29, 2026 · 21 min
The Three Gorges Dam holds back nearly 40 billion cubic meters of Yangtze River water behind a 181-meter wall that took about 28 million cubic meters of concrete, roughly ten times what went into the Hoover Dam. Engineers now track its deformation constantly, and that phrase has fueled years of alarming headlines.
This video argues that the deformation is the least interesting part. A gravity dam this size is designed to bend by a few millimeters under load. The real story is everything the dam set in motion around it: silt piling up in the reservoir, thousands of small earthquakes, a sediment-starved river eating its own banks, and mountainsides sliding into the water. None of it means the dam is about to fail. All of it means the dam can never be left alone.
Pouring a mountain
Concrete heats up as it cures. In a block this large, the outside cools while the core stays hot and expanding, which can tear the mass apart from inside. The builders embedded millions of meters of cooling pipe in the dam and circulated chilled river water through them during the pour. The structure was also modeled on scaled physical rigs and supercomputers so its geometry could pass violent flood peaks through the spillways without scouring the bedrock below.
Why the dam is supposed to move
A perfectly rigid wall would shatter under hydrostatic pressure. The dam is built to flex downstream when the reservoir rises and rebound when it drops. Seasonal heating, winter cold and a permanent temperature gradient between warm surface water and cold deep water add thermal stress on top. Concrete handles compression well but tension poorly, so the face can open hairline micro-cracks over time. In the early 2000s inspectors found dozens of surface fissures, some up to two meters long. A panel of senior engineers judged them thermal and shallow, filling was paused, the cracks were sealed with epoxy and resin, and the plant came online on schedule.
The silt problem
The Yangtze carries a huge sediment load from the Tibetan plateau. Behind the dam the water slows and drops it. The design answer is “storing clear water and releasing muddy water”: hold clean water in the dry season, open the bottom sluice gates during floods to flush the mud through. In the early years hundreds of millions of tons of sediment entered the reservoir annually, and only part of it could be flushed. Lost capacity means lost flood control.
Earthquakes and a viral image
Trapping that much water in a steep gorge presses on the crust and forces water into fault fractures, lubricating them. After filling began in 2003, local networks recorded thousands of micro-earthquakes, mostly below magnitude 3.0. The dam itself is rated for a magnitude 7.0 event. In the 2020 floods a stitched satellite image appeared to show the dam bulging. Operators released clean imagery, explained the mapping artifact, and confirmed the structure had shifted a few millimeters under peak load, exactly as designed.
The cascade and hungry water
To slow silting, China built a ladder of mega-dams upstream on the Jinsha River: Xiluodu, Xiangjiaba, Wudongde and Baihetan. They trap sediment and smooth flood peaks before water reaches Three Gorges. But the clear water leaving the dam still carries its energy and scours the channel instead. Stretches of riverbed have deepened by several meters, levees are being undercut, and the Yangtze Delta beneath Shanghai has stopped growing and begun to erode.
A permanent maintenance clock
Thousands of plumb lines, piezometers, strain gauges and GPS monitors feed supercomputer models of the dam’s health. Operators balance power generation, silt flushing, flood storage and downstream erosion every season. On the reservoir banks the bill has already come due: the annual 30-meter drawdown pumps water in and out of fractured slopes, triggering landslides that have forced further relocations and billions in slope reinforcement, on top of the million-plus people moved to build the reservoir.
Key takeaways
- Millimeter-scale flexing is a design feature of a gravity dam, not a failure sign.
- The early surface cracks were thermal, shallow, sealed, and have not been shown to compromise the structure.
- Silt, induced seismicity, downstream scouring and bank landslides are the real long-term costs.
- The upstream cascade solved one problem and shifted the erosion downstream to the delta.
- The dam’s safety now rests on uninterrupted monitoring and maintenance for as long as it stands.
More from the channel: @ChinaUnderStress