When California officials got serious about building two giant tunnels to divert freshwater out of the Sacramento-San Joaquin Delta, it didn’t take critics long to propose alternatives. One of the first was a grassroots scheme that, at first, seemed radical and counterintuitive: Let winter floods retake vast parts of the San Joaquin Valley – the very farmland that needs those Delta water diversions. The floods would recharge depleted groundwater that could then be used to irrigate the farms, preventing the need for Delta water exports.