Since we are already in a 20 year plus drought in the Western states it is almost inevitable that Lake Powell comes close to completely drying up

The consequences for places like Phoenix, Las Vegas, San Diego and Los Angeles and various other places in Utah, Arizona, California and Nevada and Mexico are nothing short of a disaster of epic proportions. For example, we have a large aquifer under Palm Springs which is slowly being depleted at which time all the waterfalls in developments around Palm Springs might have to end. The same might be true of Las Vegas and Phoenix. However, places like Los Angeles and even San Diego and San Francisco might still be okay as long as water is still falling on the Sierras and further north because of the canals that California has dug from Northern California all the way to a way to bring through water weight all the water over the coastal range near the Grapevine in California to Los Angeles. Likewise, Hetch Hetchy dam which dammed up an area equally as beautiful as Yosemite Valley earlier in the 20th Century supplies enough water for now to the San Francisco Area (as long as snow and rain continues to fall in large enough quantities in the Sierras and further north in the Cascades starting around mt. Shasta North.

So, because of the Canal running from northern to southern California and because of the Hetch Hetchy Dam in the Sierras San Francisco and Los Angeles might be okay and if Los Angeles shares water with San Diego, then San Diego might be okay even if the Colorado River completely dries up with little or no water left at all in the coming years. 

But, places like phoenix and Las Vegas and even Palm Springs might not fare so well as others dependent completely upon the Colorado River aquifer.

Another thing to realize is that the Sacramento River begins near Mt. Shasta but flows all the way to the San Francisco Bay and out to sea and is the 2nd largest river on the west Coast of the U.S. 2nd only to the Columbia River that borders Washington and Oregon near Portland, Oregon.

So, it's possible that San Diego and Los Angeles and San Francisco will still be okay water wise even if the Colorado River completely dries up. But, this might also turn Phoenix and Las Vegas into basically Ghost towns unless something water wise is figured out for them.