Donald Trump ran as a champion of the forgotten man, and few have been forgotten more by the political class than California’s parched farmers. On Friday the President made good on a campaign promise to deliver more water to more people.

California has an arid climate in the best of times. Yet tens of billions of gallons of water each year are wasted because of restrictions on pumping in the Sacramento-San Joaquin River Delta that are intended to protect fish including smelt and chinook salmon.

One...