How do they build dams on rivers if there’s nothing stopping the water from flowing while they are building?

Question

How do they build dams on rivers if there’s nothing stopping the water from flowing while they are building?

In: 29

Answer 1

Cofferdams are structures built to keep water out of a working area.
They also will drill tunnels that may be used as spillways(overflow) infrastructure first and then divert the river through there while working.

The Hoover dam built the tunnels first to divert the river, then cofferdams to keep the area clear, then used some of the diversion tunnels for emergency spillways.

https://youtu.be/URC125wpMS4 from an engineering YouTube channel for a quick 10min *dive* into the topic.