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

Usually places that have dams were just rivers, not lakes. So you let the river continue flowing under or around the dam until it’s done, then you finally close it off and fill up your new lake.