After Katrina, California voters approved nearly $5 billion in bonds to shore up the state's aging flood protection system. With more than half the money spent, officials say they need up to $12 billion more to finish.In Sacramento, a 42-mile-long levee along the Sacramento and American rivers protects the Natomas area, a flood-prone basin transformed from cropland into a community of 100,000 with an airport, nearly three dozen schools and two interstates. If the levee were to fail, "it would be beyond catastrophic," said Angelique Ashby, the city's vice mayor. The levee's unacceptable rating led federal emergency officials to impose strict building limits that have prevented homeowners from making repairs and delayed construction of schools and a police station, she said. About half the $810 million needed for upgrades has been raised from state and local sources, and much work has been done. The city is seeking federal financing of the rest. In 2009, a congressional advisory panel recommended that Congress invest in levees, create national levee programs and enact policies to increase awareness about the risks of flooding. But Congress has yet to adopt the group's report. In the meantime, experts are warning that aging and weak flood-control systems will likely face stiffer tests as climate change makes severe storms more common in the coming years. "This is going to be a national problem and it just hasn't dawned on people how big it's going to be," said Jeffrey Mount, a levee management specialist and founder of the Center for Watershed Sciences at the University of California, Davis. "We're in a never-ending cycle of flood and rebuild." ___ John Flesher reported from Brookport, Ill., and Traverse City, Mich., and Cain Burdeau reported from New Orleans. AP reporter Troy Thibodeaux contributed from New Orleans and AP video journalist Haven Daley contributed from Sacramento.