Lake Mead was at low levels in both 1955 and 1965 just today at 1058 ft.
In 1955 it took only a few years to refill to capacity. In 1965 it took 20 years to get back to capacity.
Last time I checked the usage was lower and the sky was not falling at that time either.
Usage is an obvious problem. Snowfall is always variable. El Nino and La Nina push moisture have a big impact on moisture and the fronts that move across the Western US.
PDF (note that the current level is about 1058)
https://www.usbr.gov/lc/region/g4000/hourly/hourly.html