# How do they build dams?

**URL:** <https://boards.straightdope.com/t/how-do-they-build-dams/456183>\
**Category:** Factual Questions\
**Created:** [July 11, 2008, 9:49pm UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183 "2008-07-11T21:49:50Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\
**Post date:** [July 11, 2008, 9:49pm UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/1 "2008-07-11T21:49:50Z")

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Well, a dam blocks water, so what blocks the water while they build the dam? A big turtle? Well, what’s behind _that_ turtle? 😉

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**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\
**Post date:** [July 11, 2008, 9:58pm UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/2 "2008-07-11T21:58:35Z")

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A [cofferdam](http://en.wikipedia.org/wiki/Cofferdam) blocks the water to a diversion route while the main dam is being built. ([cite](http://en.wikipedia.org/wiki/Hoover_Dam#Groundworks))

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [July 11, 2008, 10:13pm UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/3 "2008-07-11T22:13:29Z")

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After that, it’s [del]turtles[/del] coffer dams all the way down.

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**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\
**Post date:** [July 11, 2008, 10:56pm UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/4 "2008-07-11T22:56:41Z")

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[A picture saves 1000 words, or so.](http://upload.wikimedia.org/wikipedia/en/a/a6/Hoover-dam-contour-map.jpg)

That’s a plan of Hoover Dam. The faintly visible tunnels were dug first. Once they were open and carrying water, temporary cofferdams were built, leaving a dry section of riverbed while the river was being diverted through the tunnels.

Once the dam was built, the diversion tunnels were closed off, and Lake Mead was formed. The tunnels are still used as penstocks now.

[More about Hoover Dam and its construction is here.](http://en.wikipedia.org/wiki/Hoover_Dam#Groundworks)

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**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\
**Post date:** [July 12, 2008, 12:02am UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/5 "2008-07-12T00:02:50Z")

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Good dam question…good dam answers.

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**Author:** ![Moriarty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/moriarty/32/49_2.png) [@Moriarty](https://boards.straightdope.com/u/Moriarty)\
**Post date:** [July 12, 2008, 2:02am UTC](https://boards.straightdope.com/t/how-do-they-build-dams/456183/6 "2008-07-12T02:02:47Z")

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[QUOTE=Philster]  
Good dam question…good dam answers.  
[/QUOTE]

Thank you for that. I needed a good laugh.  
😃