How Are Dams Built with the Water Still There?

You have seen how a dam holds a massive body of water on one side of it. Have you ever thought about how engineers build one with all that water still there?

Following three straight-forward answers should give you a quick understanding of how it’s done.


They dont. They divert the water then build the dam. Then they divert the water back to where the dam is!
Youtube channel “Practical Engineering” released a really good video describing underwater construction techniques, that covers dam diversions and lots of other topics:


They typically build a “cofferdam” to temporarily divert water around the dam site. Then they build the dam, and once that’s built, they remove or destroy the coffer dam.

How Are Dams Built with the Water Still There?

The coffer dam does not need to be particularly big or strong to divert a river. The actual dam needs to be big and strong to hold back the reservoir that’s created by the dam

Fun dam fact: Glen Canyon Dam in UArizona is one of the largest in the US, and is under threat by all the sediment that the dam has trapped from moving downstream. The millions of tons of sediment are pushing on the dam and may eventually cause its collapse. So just dredge the sediment right? The problem: Some of it is highly radioactive from all of the uranium mining that took place in Utah and Colorado in the middle of the 20th century


How Are Dams Built with the Water Still There?

The Hoover Dam documentaries are pretty cool, in that case they actually made a tunnel beside the proposed dam and diverted the river (the Colorado) through it during construction, I think they still use the tunnel as overflow. This was from memory (what’s left of it) so I could be wrong.
Spend 8 minutes watching this


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