Factbox: Five facts about Mississippi River flooding

Thu May 5, 2011 4:42pm EDT

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(Reuters) - An unusually large snow melt and wet spring has caused a chain reaction of flooding from Canada, the Dakotas and Minnesota in the north through the Midwest, and experts said it would cascade all the way to the Gulf of Mexico. Here are five facts about the Mississippi River:

MIGHTY MISSISSIPPI: The Mississippi begins at Lake Itasca in Minnesota, curls south for more than 2,300 miles through the United States and empties into the Gulf of Mexico. The river and its tributaries touch all or parts of 31 states. The basin forms a funnel that stretches from Montana and southern Canada in the west to New York state in the east that runs down to a spout in Louisiana.

FLOODWAYS: The U.S. ordered the Birds Point-New Madrid Floodway opened for only the second time to relieve pressure on the town of Cairo, Illinois. The first time was in 1937. A two-mile gap was blown to allow water to escape to the 130,000-plus acre floodway in Missouri and gaps were opened at the south end to let water flow back.

Two floodways in Louisiana that have permanent gates, the Bonnet Carre and Morganza spillways, also may be opened. If they are, it would be the first time three floodways have been used in the same year. The Bonnet Carre Spillway has been opened nine previous times.

HISTORIC FLOODS:

1927 GREAT FLOOD: Heavy rains for a year led to massive flooding with the Mississippi reaching 80 miles wide at some points. The death toll was estimated at anywhere from 250 up to 1,000, with more than 600,000 left homeless and levee breaches inundating 26,000 square miles. Floodways were adopted as a response.

1937: Massive flooding from Cairo to Arkansas City forced the use of the Birds Point-New Madrid Floodway for the first time. The record crests set in 1937 at Cairo, New Madrid, Missouri, and two other locations already have been eclipsed this year. Bonnet Carre Spillway also used for the first time.

1973: The largest volume of water to flow down the Mississippi since 1927 caused 28 deaths and displaced 45,300 people. The Bonnet Carre Spillway was opened for a record 75 days and the Morganza Spillway was used for the first time.

1993: Record flooding on the Upper Mississippi and tributaries led to about 50 deaths. More than 1,000 levees were topped or failed and thousands of people went homeless.

2008: Heavy rain forced the opening of the Bonnet Carre Spillway for the first time in 11 years. Flooding delayed rebuilding of levees damaged by Hurricane Katrina in 2005.

LOCKS AND DAMS AND SECURITY: The U.S. Army Corps of Engineers controls 27 lock and dams on the Upper Mississippi from Minneapolis to St. Louis to maintain a 9 foot navigation channel. The Corps was established as a separate branch of the Army within the U.S. military in 1802, in part to build coastal fortifications and survey roads and canals. It has evolved into the lead federal flood control agency.

WILDLIFE: At least 260 species of fish, a quarter of all in North America, mussels, migratory birds and others use the river system.

SOURCES: Charles Camillo, historian for the Mississippi River Commission and the Mississippi River and Tributaries Project; U.S. National Park Service; U.S. Army Corps of Engineers; National Weather Service; and the U.S. Geological Survey.

(Compiled by David Bailey; Editing by Greg McCune)

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