It's a Turbine Runner!
Here's what the explanatory sign days:
"HOOVER DAM TURBINE RUNNER
"This cast iron turbine runner (water wheel) which is 14.2 feet in diameter and weighs over 34 tons, powered the N-7 generator at Hoover Dam Power Plant from 1944 to 1982. The function of a turbine runner is to convert the force of filling water, delivered through the penstock pipes, to rotating energy which is then utilized by the generator in producing hydroelectric energy.
"This turbine runner was presented to the Boulder City Museum and Historical Association on April 30, 1983, by the Bureau of Reclamation, U.S. Department of the Interior."
Thanks go to Mark Holloway of Beatty, Nevada, for photographing the turbine runner in Boulder City, Nevada, and coming up with the bogus mutiple choice answers.