system
Given the following post, respond as if you're commenting on Reddit:
human
Does the Hoover Dam need the water from Lake Mead to stay structurally sound?

I’ve been reading articles about how low the water level is in Lake Mead and it got me thinking about two things.

1. Does the concrete that was used to build the dam rely at all on being continuously wet (i.e. is the concrete prone to crumbling prematurely when it’s dry for extended periods of time)?

2. Since the dam was built to hold back tremendous pressure from the lake, now that the lake is so low is there concern that the dam could collapse into the lake because the water isn’t there to balance things out?

The thing was built during the Great Depression and wasn’t sure, at the time, if they ever factored the lake ever drying up in their engineering plans.
gpt
A few items of note. First, the concrete does not rely on being continuously wet. Once cured, water has little effect on concrete itself. Water, and especially repeated wet-dry cycles, can affect the reinforcement within concrete, but concrete itself is largely unaffected by water. However, Hoover Dam is largely built without reinforcement. It does have some small amount of reinforcement in it, but only a few areas such as the spillway actually rely on reinforcement for structural purposes.

Next, also consider that the dam itself is incredibly thick. Think about how a basement or cave remains at a constant temperature through the year. It takes a long time for changes to air temperature to work their way through the soil. And the same is true for moisture penetrating through concrete. It does happen, but it takes a long time and tends to average things out. So if low moisture levels could affect the concrete of the dam chemically, only decades-long swings in lake level would be relevant. 

Finally, do not worry about the dam collapsing if there is no water behind it. The dam was built not just to generate electricity, but to manage flooding on the Colorado. The designers planned that the water levels would wildly swing up and down. 

Structurally, a dam like Hoover is designed in such a way that the pressure from the water is resisted by the dam's raw weight. Lowering the water level simply reduces the component counteracting weight. At no point will any forces completely reverse direction. You could drain the entire lake and it would only reduce the loading on the dam.

Or, if you want to think of it a different way, we've already tested the dam in this condition! Think about how it was built. The river was first diverted around the dam site, and the dam was constructed on a dry riverbed. So when it was built, there was absolutely no water butting up against it. Furthermore, concrete only gains strength as it cures, so the concrete was slightly weaker when they started filling the lake than it is today.