Doesn’t northern California have a lot of snowmelt? I get that California is geographically large, but at least Southern California getting water from Northern California wouldn’t involve all the cross-state bickering that other states do for water access (i.e. Tennessee and Georgia disputing over water).
Much of the water used in Los Angeles already comes from the Owens Valley, 200 miles to the north.
The interstate fights over Colorado River water have been epic. It is the primary source for Southern California, taking more water than Arizona and Colorado combined. If California would decide to take care of their own problems for a change it would have a very salutary effect on the whole West.