Weather forecasters say the Mississippi River, the United States' largest water system, is expected to crest near record levels in the southern state of Tennessee.
Serious flooding already has been reported along the river. The National Weather Service said the Mississippi is expected to crest on Tuesday at about 14.6 meters in Tennessee's largest city, Memphis.
Authorities there have ordered residents to leave more than 1,300 homes, and thousands of other homes also may be affected by flooding.
Downstream in the state of Louisiana, authorities are expected to open the Organza spillway in an effort to lower river levels in the cities of Baton Rouge and New Orleans.
Authorities warn that even if the spillway is opened, water levels could reach between 1.5 to 7.6 meters. If that happens, the floodwaters may inundate farmland and several nearby communities.
Some information for this report was provided by AP and Reuters.