That wasn't winter rain/snow related fillup, I bet you. There are a bunch of other dams behind Hoover (my link mentioned a dozen) and my guess is that they let the water down when they were building 1-2, and when they finished, the main (Hoover) dam was closed up, and that filled up Lake Mead. There is simply no other explanation why so much water would have gone in the 60s from the lake, or why was it refilled that fast unless it was done by humans.
Did I say 100+? Make it almost 300 miles:
" the Southern Nevada Water Authority from applying for water rights in several basins in Nevada and the Snake Valley in Utah as part of a plan to send water to Las Vegas through a 285-mile long
pipeline."