Last estimate I saw for a Columbia River to Los Angeles pipeline was $150G. Expensive, but not out of reach for the US government. And there's enough water there to take care of the southwest for a long time.

The nasty things will happen across national borders. Like, when things in California get tight, will Mexico continue to receive any of the Colorado? Or worse, what about the Upper Ganges Aquifer, already shrinking and used by both India and Pakistan.