CLARKSBURG, Calif.  ­— Atop a dirt levee his great-grandfather built in the 1800s to hold back California’s mightiest river, Northern California farmer Russell van Loben Sels looks out over the site of a new water project, one that would be the state’s most ambitious in a half-century.

Promoted by Gov. Jerry Brown, the $15.7 billion project would run giant twin pipes, each four stories high, underground for 35 miles and eventually pull thousands of gallons of water a second from the stretch along the Sacramento River where van Loben Sels farms to cities and farms to the south.

In what all agree will be the decisive year for the project, Brown’s plan — which is facing obstacles to environmental approval in the Sacramento-San Joaquin River Delta and mounting uncertainty over the financing — is splitting farmers and political leaders.

In the delta, a land of tree-lined river banks, pear orchards and Gold Rush-era Victorian homes, signs saying, “Stop the Tunnels,” hang on farm gates and shop walls. People fear the tunnels would let the state take too much water from the delta.

“I do resent the fact they look at the delta as being sort of expendable to protect their farms,” van Loben Sels said, driving a narrow river road that would be replaced by widened highways and massive water intakes. “It’s just the destruction of the delta.”