You clearly haven’t seen the hundreds of miles of canals that supply water to SoCal already.
The Western USA has a big problem with water, such that water rights from the Colorado are fought over, tooth and nail. That one river is SO important to the western desert states.
If there is a dry winter in the Sierra’s and western Rockies, then the entire desert SW knows water is going to be an even bigger problem.
Phoenix, Las Vegas and the entire LA Basin are deserts, yet those cities (and their satellite “suburbs”) have grown exponentially over the last 25, 50, 100 yrs. In fact, you wouldn’t believe pictures of those cities from 100 yrs ago!
Now, I’m not saying the job is as simple as building a pipeline from Canada, but stranger things have happened…and if you look at the oil pipelines that have been built (and fought over…Keystone XL for instance), it is not out of the realm of possibility. However, as with everything these days, the cost would be eye watering!