The Glen Canyon Dam on the Colorado River

What is the purpose of a dam? It can vary. Dams are barriers constructed to hold back water and raise its level, creating a reservoir. Depending on the type of dam and size of the reservoir, dams are commonly used to generate hydro power, create water diversion for agriculture, provide water supply, create lakes for recreation, or offer a means of flood control. Only 3% of dams in the US are hydropower facilities overall providing about 7% of our country’s electricity.

According to the Army Corps of Engineers, there are over 84,000 dams which have been built on our country’s three-thousand plus rivers.

1920 – 1970 was the grand age of dam building in the United States. We completed 10,000 new dams between 1920 and 1950 and 40,000 between 1950 and 1980.

While hydro power provides carbon-free energy, there are environmental issues with dams as they interfere with fish and wildlife, interrupt natural sediment and nutrient flow and interfere with periodic flooding. The trend in the U.S. over the past few decades has been to remove dams rather than to create more.  According the Wired magazine, 1,040 dams were removed in the U.S. between 1976 and 2015. Here’s a great article about dam removal from PBS/NOVA: Dam Removals

While the U.S. has been removing dams, the rest of the world is building more. In 2014 there were about 3,700 dams worldwide planned or under construction. There were zero dams constructed in the US in 2014.

“By the year 2000, the amount of water that was stored behind the giant dams on earth was between three and six times more than existed in all the world’s rivers—a redistribution of the planet’s supply of freshwater significant enough, according to the water expert Peter Gleick, to account for ‘a small but measurable change in the wobble of the earth as it spins.'” – from the book The Emerald Mile.

It’s tough to have a list of the biggest dams because there are different ways to measure them – by height, length, total volume of the dam, the size of the reservoir created, the amount of hydropower, etc.

By height, the U.S.’s tallest dam, the Oroville dam at 770 feet is merely the 21st tallest dam in the world. The tallest is Jinping-I dam in China at 1,001 feet which was completed in 2013. Here’s a pic:

The huge Jinping-I dam

Here’s a picture of the Oroville Dam in California, it’s made of earthfill: