Until a few weeks ago, the McCormack-Williamson Tract in the California Delta was an island of low-lying farmland, more than two square miles protected from the surrounding rivers and sloughs by earthen levees. Today the tract is an immense lake, up to 15 feet deep, with fish prowling the water and ducks skimming the surface. The adjacent Mokelumne River, swollen by the intense storms that have drenched the state this winter, caused a levee to break, allowing the water to rush in.