H012
Does the tallest dam hold the most water?
The world's tallest dam rises 1,001 ft (305 m) out of a Chinese gorge. A dam in Africa less than half as tall holds about 23 times more water.
Height is only one of three dimensions. How much water a dam holds depends mostly on the valley behind it: how wide it is and how far back the water spreads. Jinping-I plugs a narrow canyon, so its reservoir stays small. Kariba blocks a broad river valley, and its lake covers about 2,100 sq mi (5,400 km²), larger than Rhode Island. Switzerland's Grande Dixence is almost as tall as Jinping-I but holds less than 1% as much water as Kariba.
One measurement rarely captures the size of something that has three. Before judging how big something is by its most impressive number, ask which dimensions that number leaves out.
Try it somewhere else
For the same price, you can buy one 18-inch pizza or two 12-inch pizzas.
Which gets you more pizza, and why doesn't adding up the inches give the answer?
See the numbers
| Dam | Height | Water held when full |
|---|
How much concrete does it take to build a dam?
More than the Great Pyramid. Hoover Dam holds 3.25 million cubic yards (2.5 million m³) of concrete, enough to pave a highway 16 ft (4.9 m) wide and 8 in (20 cm) thick from San Francisco to New York City. Switzerland's Grande Dixence holds about 7.8 million cubic yards (6 million m³), and Jinping-I about 9.7 million cubic yards (7.4 million m³), almost three times the volume of the Great Pyramid of Giza, about 3.4 million cubic yards (2.6 million m³).
All that concrete gets hot. Concrete gives off heat as it hardens. Engineers calculated that if Hoover Dam had been poured as one solid block, it would have taken 125 years to cool, and it would have cracked as it shrank. Instead, it was built as a grid of separate columns, raised 5 ft (1.5 m) at a time, with more than 582 mi (937 km) of 1-inch steel pipe running through them. River water and then ice-cold water from an on-site refrigeration plant flowed through the pipes until the dam was cool.
Why does a wide valley beat a tall wall?
Raising a dam adds water only as wide as the valley is at that height. In a narrow gorge, each extra foot of height adds a thin sliver. In a wide, gently sloping valley, the water spreads for miles in every direction, so even a modest dam floods a huge area. Lake Kariba is only about 95 ft (29 m) deep on average, but it stretches 170 mi (280 km) along the Zambezi River.
That's also why the tallest dams are usually in mountains: a narrow canyon is where a very tall wall is practical to build, and it's also the place where a tall wall holds the least water.
Related exhibits
Sources: Jinping-I height (305 m), reservoir capacity (7.76 km³), and structural volume (7.4 million m³); Kariba Dam height (128 m), Lake Kariba volume (180 km³), area (5,400 km²), length (280 km), and average depth (29 m), the world's largest reservoir by volume; Grande Dixence height (285 m), reservoir (about 0.4 km³), and concrete (about 6 million m³); Great Pyramid volume (2.6 million m³): Wikipedia articles on each. Hoover Dam height (726.4 ft), concrete (3.25 million cubic yards), highway comparison, 125-year cooling estimate, 5-ft lifts, and 582 mi of cooling pipe: U.S. Bureau of Reclamation. Lake Mead capacity (26.1 million acre-feet): Congressional Research Service (R45546). Acre-feet and cubic yards converted from metric where needed.