It'd be interesting to see what would happen upon the collapse of Powell-Mead Civilization. And with the water flowing over the Hoover that'd make for a spectacular waterfall, until the water ate away at the concrete. I think this would be pretty quick. Look at how fast the water was eroding the Oroville Spillway
All that'd be left in the end would be a pair of giant concrete walls (assuming the waterfall didn't eat away at their foundations) with the colorado flowing between them. A sight to behold
But what about my idea of rain replenishing the groundwater? My idea is that there was a hedonistic Hydrological Empire known as Vegas. It rules the area for a while before the collapse of the great Hoova Dam destroys it. The replenished groundwater is enough for a pathetic shadow of the great empire to survive in the ruins of the great City of Vegas. A great monument to the founder of the Flagg* Dynasty becomes as bitterly ironic as Ozymandias. "I am Randall, King of Kings! Look upon my works, ye mighty, and despair!"