The largest dam in the world is the recently finished Three Gorges Dam on the Yangtze River, China. It’s more than 600 kilometers long and holds about 40 cubic kilometers of water.But it also stops an average of 151 million tons of sediment per year from making its way downstream, new research in Geophysical Research Letters shows. The river makes up for some of that in extra erosion downstream. But by the time the Yangtze washes out into the East China Sea, its sediment load is still lighter by 85 million tons per year.
With ocean waves eating away at it faster than new silt arrives, the Yangtze river delta has begun to shrink. For the record, that’s what happened to Louisiana’s barrier islands after we tamed the Mississippi.
Dams are tricky. On the face of it, they seem like a green miracle: They store water and generate clean power. But – as we’ve learned after damming nearly every American river – they block salmon runs, kill rare mussel species and give invasive plants a toehold by eliminating the annual scouring of spring floods. Worse, the massively expensive projects quickly depreciate: they begin to shrink from the bottom up as soon as they start to fill, as silt piles up behind the dam. At the same time, the precious water they’re hoarding drifts endlessly away into the sky.
At present rates, the authors calculate, the Three Gorges Dam will be full of sediment in about 150 years. Not to worry, though. The Chinese are building four more dams upstream, and they’ll keep much of that sediment from reaching Three Gorges.