In California, it takes about 1.1 gallons of water to grow an almond; 1.28 gallons to flush a toilet; and 34 gallons to produce an ounce of marijuana. But how many gallons are needed to save a three-inch delta smelt, the cause célèbre of environmentalists and bête noire of parched farmers?

To protect smelt from water pumps, government regulators have flushed 1.4 trillion gallons of water into the San Francisco Bay since 2008. That would have been enough to sustain 6.4 million Californians for six years. Yet a survey of young...