I've live along side the Sierra/Cascades almost all my adult life ... it rains mostly on the westside, not on the eastside ... what little water does come down on the eastside follows the Owens Valley directly to LA, or into Mono Lake or Pyramid Lake (both of which have no outlet) ...
This is called the "rain shadow effect" ... as the moist air is lifted up over the Sierra ridge, water is precipitated out, as the air drops back down the other side, humidity crashes to near nothing ... yes, there are other mountains "downstream", but the air has already been bled out and is too dry to precipitate anymore water ... what rain the Desert Southwest does get is usually `monsoonal` from the Gulf of Mexico ... rather than `zonal` from the Pacific Ocean ...
I'm sorry ... Lake Powell and Lake Meade have plenty of water for a Phoenix population of 100,000 ... in 1950 ... there was NEVER enough water there to support the current 2 MILLION people in Phoenix ... math is hard for liberals? ... it's impossible for politicians ...
ETA:
Rain shadow - Wikipedia