Downstream damage
Though the Hoover bursting would make for quite a disaster, it wouldn't be on the scale of Lake Missoula
The glacial lake, at its maximum height and extent, contained more than 500 cubic miles of water. When Glacial Lake Missoula burst through the ice dam and exploded downstream, it did so at a rate 10 times the combined flow of all the rivers of the world. This towering mass of water and ice literally shook the ground as it thundered towards the Pacific Ocean, stripping away thick soils and cutting deep canyons in the underlying bedrock. With flood waters roaring across the landscape at speeds approaching 65 miles per hour, the lake would have drained in as little as 48 hours.
So the scars of a Hoover flood might not be as prominent as Grand Coulee and evirons.
But I agree, a lot of man's works would disappear in a blink, but significant traces would linger for millions of years in geologically stable areas.
As a side note, I think some mountain valleys in the west paradoxically escaped glaciation, as did the Queen Charlotte Islands (Haida Gwai).