Every few winters during heavy rainstorms, the Sacramento River overflows its banks and floods a remote sprawl of rice fields and grassy scrub called the Yolo Bypass. These events benefit juvenile salmon by creating a temporary area of refuge and abundant food, especially of small invertebrates that proliferate in the shallow floodwaters. Before dams and levees tamed the river early last century, such floods were an annual occurrence in California’s Central Valley, where Chinook salmon and waterfowl historically teemed but have since dwindled in numbers. To help revive ailing fish populations, environmentalists want to see the levee system that separates the Yolo Bypass from the river modified with a large notch so that the area—some 17,000 acres —will once again flood every year. “Allowing the Yolo Bypass to function more often as the natural floodplain that it is will help increase the number of salmon in California,” says John McManus, president of the Golden State...