Only a close look at the Middle River revealed anything amiss in this part of the Sacramento-San Joaquin Delta. Instead of flowing north toward San Francisco Bay, as nature intended, the Middle was headed south. On the other side of Bacon Island, the Old River was doing the same thing.

The backward flow of these two obscure channels is at the core of a proposal to build California’s biggest water project in decades: a $15-billion diversion and tunnel system in the delta, the ecologically failing hub of the state’s waterworks.