NurseVooDooRN: It is designed with the arch upstream so that the pressure of the water strengthens the dam as the water presses it against the foundations and abutements.
Edit to add: PBS does a better job than I
“The Hoover Dam is a curved gravity dam. Lake Mead pushes against the dam, creating compressive forces that travel along the great curved wall. The canyon walls push back, counteracting these forces. This action squeezes the concrete in the arch together, making the dam very rigid.”
mmmmmmBacon12345: Cement/concrete is really strong against things that try to squish it and crush it, but really weak against things that try to pull it apart, we put rebar in it to help but it’s still wayyyy stronger against squishing
The curve of the dam means that as the water pushes on it it will try to become straight, but becoming straight would require it get wider but there are cliffs that push back against that. This puts the load on the dam in the way it is strongest since pushing in the middle creates a squishing force on the dam materials
If you have a straight dam then the water behind would be pushing the middle outward, this will try to stretch the dam forward and it is very weak to that so it won’t be able to hold as much water back
ElfMage83: Arches and domes are much better at spreading force than straight lines are. The arched shape of a dam lets the force of the water run along the dam and into the supporting structure.