A five year survey released by the California Department of Water Resources reveals half of the levees that guard California cities from a major flood don’t meet modern standards, and if a levee were to break in the wrong place, it could cut off the drinking water supply to the Bay Area for months or even years. Much of the state’s 13,000 miles of levees were built in the 1800s to protect farmland in the Sacramento and San Joaquin valleys from potential flood.