Eli5 – How do you build a dam?

Question

The recent dam collapse had me thinking how on earth do you build a dam? Huge river with loads of water – how do you even start without the water washing everything away? How do the workers manage to cope in such conditions?

In: 21

Answer 1

Often they divert the river. For example when the Hoover dam was build the first step was to create an artifical tunnel through the rock to divert the Colorado River. That left the dam sight dry allowing construction. When the dam was completed they sealed up the diversion and the resivoir behind the dam filled.