system
Given the following post, respond as if you're commenting on Reddit:
human
Do the forces against a dam change as the reservoir behind it freezes, and if so how?

And how do civil engineers plan for this?
gpt
It's pretty unlikely that the reservoir will freeze. It will freeze *over,* which means it freezes on top, but if you drill down (usually at most a couple feet or so) you will find liquid water. It would have to be shockingly cold for a long time to completely freeze an average dam reservoir.

As for any pressure changes that might happen - well, it's not really pressure *changes* you have to worry about. Dams are already under a hell of a lot of pressure, simply because as you go deeper, the pressure rises, and it's all pushing against the dam. Dams are designed to withstand that pressure. What dams have to worry about is increased flowrate into the reservoir from things like flooding, spring runoff, etc. For that, dams have spillways which are basically giant drains - the extra water flow will go over the spillway and continue on down the river without increasing the pressure on the dam. 

Some spillways are controlled, which means the dam operator can open and close giant gates (called Tainter gates) to regulate how much water flows through the spillway - this allows them to drain off the excess water in a controlled manner, which among other things minimizes damage from the spillwater hitting the (usually concrete) footing of the dam.