There's a reason they call it the "Mighty" Mississippi: As heavy rains pound the river's basin and drive water levels above their record heights, the mayor of Memphis has warned city residents to prepare for a possible "large-scale disaster." Officials have begun building sandbag buffers around the city and raised levees where water levels have climbed to dangerous heights. Thousands have evacuated the city, leaving their homes behind to live in tents and campers at higher grounds. One woman showed CBS News correspondent Mark Strassman photos of her submerged home north of Memphis. "What's next? We've never had anything like this. We've lost crops to the water. We've never lost a house," she said. Things haven't been worse since the Mississippi flood in 1927, the most destructive in U.S. history, after which engineers built a system of levees and reservoirs that protect some four million people today. But that system is certainly being tested. The river is expected to crest next Wednesday at 48 feet.