Well, a dam blocks water, so what blocks the water while they build the dam? A big turtle? Well, what’s behind that turtle? 
After that, it’s [del]turtles[/del] coffer dams all the way down.
A picture saves 1000 words, or so.
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.
Good dam question…good dam answers.
Thank you for that. I needed a good laugh.
