Mississippi River FloodingThis gallery is part of a special National Geographic News series on global water issues.People watch as floodwaters from the Mississippi River gush through newly opened gates in the Bonnet Carre Spillway in Norco, Louisiana, on Monday. The spillway, built in response to the Great Flood of 1927, diverts water from the Mississippi into Lake Pontchartrain. At Memphis (map), Tennessee, the swollen Mississippi river crested Tuesday at nearly 48 feet (14.6 meters)—not quite a foot below the record flood level. The Mississippi River originates at Lake Itasca in Minnesota (map) and flows about 2,300 miles (3,701 kilometers) to the Gulf of Mexico. The current flood is expected to reach Natchez, Mississippi, by May 21, and arrive in the Gulf around the end of this month. Even now residents in Arkansas, Mississippi, and Louisiana are piling up sandbags and building earthen barriers as the surge of floodwater continues its journey downriver.The Mississippi and its dozens of tributaries, which form the world's third largest drainage basin, have been flooding for centuries. Spanish explorers wrote about a huge flood that widened the Mississippi River to about 80 miles (128 kilometers) at one point in 1543. Under normal conditions, the river's widest point is about a mile (1.6 kilometers) across near Alton, Illinois.Jeff Masters, meteorological director for the website Weather Underground, noted that the Mississippi is, in a sense, two rivers—the upper and lower Mississippi. The lower Mississippi begins at the confluence of the Ohio River at the borders of Illinois, Missouri, and Kentucky (map). Heavy rains and snowmelt in the Ohio River Valley can add to Mississippi River flooding as the waters meet and continue southward.This year's floods were caused by intense rainfall and the melting of heavy winter snows in Minnesota and the Dakotas, Masters said. But the 2011 flooding is only the latest in a series of major floods that have soaked the country's midsection since the early 1900s.(Read more about how we can mitigate Mississippi River flooding in National Geographic Freshwater Fellow Sandra Postel's most recent blog post: "Mississippi Floods Can Be Restrained With Natural Defenses.")—Willie Drye

Photograph by Patrick Semansky, AP