This NASA Image of the Day does a really good job of helping to visualize how a large-scale flood control system works. To prevent the rising Mississippi from flooding downstream cities like Baton Rouge and New Orleans, authorities opened some of the bays in the Morganza spillway, allowing water from the Mississippi to flow out into a levee-defined floodplain.
Five days after the U.S. Army Corps of Engineers opened a flood control structure, or spillway, onto the Morganza Floodway, water had spread 15-20 miles (24-32 kilometers) southward across the Louisiana landscape.
The leading edge of the flood water was about 3 miles (4.5 kilometers) below Krotz Springs, between U.S. Route 190 and Interstate 10 (off the bottom of the image), according to Ricky Boyett, a spokesman for the Army Corps. Arrival at the Atchafalaya River is imminent, though the southward flow of water in the floodway has been slower than projections. The region had been suffering through a significant drought, so the ground and side waterways have been able to absorb more water than originally anticipated.
The false-color images combine infrared, red, and green wavelengths to help distinguish between water and land. Clear water is blue, and sediment-laden water is a dull blue-gray. Vegetation is red; the brighter the red, the more robust the vegetation. Gray patches away from the center of the floodway are likely farm fields that have recently been burned or cleared.
The same levees and spillway you see here also prevent the Mississippi from changing course. Without them, the River would probably take a more direct course to the Gulf of Mexico, likely through the same basin region that is the floodplain now.
For the people who live in the floodplain, well, life is weird and depressing right now. CNN has a really interesting story about a town where residents were told, days in advance, "the depth of water from right here will be 15 feet," and had to pack up their homes and lives—some taking everything, including the kitchen sink.
Thanks to Tim Heffernan for the CNN story!