The state has reached a key milestone in its $15.7 billion proposal to build tunnel bypasses around the Sacramento-San Joaquin River Delta as an environmental group is threatening to sue to stop it.The twin tunnels, both 40 feet in diameter and 35 miles long, would be California’s most ambitious water project since the 1950s and 1960s. The project would enable Sacramento River water to be sent directly to southern and central California farms and cities, bypassing the Delta.