In Godzilla #9 (Apr 1978) Doug Moench wrote a plot where Hoover Dam threatened to break, flooding Las Vegas.
Radioactive breath kaiju rampaging through Nevada? Sure! Who cannot get behind that?
Hoover Dam flooding Las Vegas?
Nope – no way.
First off, Las Vegas is not downstream of Hoover Dam. Second, Las Vegas is about 2,000 feet above sea level. The lake behind Hoover Dam has a high-water mark about 1,229 feet above sea level. Water does not go uphill when a dam breaks.
Makes for a great visual though!