Mississippi River Basin and the Midwest of the United States

The Mississippi River is the largest river basin in the United States, and the third-largest river basin in the world. It is the site of frequent, sometimes devastating floods. All of the 11 major tributaries of the Mississippi have also experienced major floods, including events that have at least quadrupled the normal river discharge in 1993, 1973, 1927, 1909, 1903, 1892, and 1883. Three of the major rivers (Mississippi, Missouri, and Illinois) meet in St. Louis,

Satellite image of the St. Louis, Missouri, area that shows the Mississippi, Missouri, and Illinois Rivers at normal flow stages (Image taken August 14, 1991; NASA images created by Jesse Allen, Earth Observatory, using data provided courtesy of Landsat Project Science Office)

which has seen some of the worst flooding along the entire system.

Floods along the Mississippi in the 1700s and 1800s prompted the formation of the Mississippi River Commission, which oversaw the construction of high levees along much of the length of the river from New Orleans to Iowa. By 1926 more than 1,800 miles (2,896.8 km) of levees had been constructed, many of them higher than 20 feet (6.1 m). The levees imparted a false sense of security against the floodwaters of the mighty Mississippi, and they restricted the channel, causing floods to rise more quickly and forcing the water to flow faster.

Many weeks of rain in late fall 1926 followed by high winter snowmelts in the upper Mississippi River basin caused the river to rise to alarming heights by spring 1927. Worried residents all along the Mississippi strengthened and heightened the levees and dikes along the river, hoping to avert disaster. The crest of water was moving through the upper Midwest and had reached central Mississippi, and the rains continued. In April levees began collapsing along the river, sending torrents of water over thousands of acres of farmland, destroying homes, killing livestock, and leaving 50,000 people homeless. One of the worst-hit areas was Washington County, Mississippi, where an intense late April storm dumped an incredible 15 inches (457.2 cm) of rain in 18 hours, causing additional levees along the river to collapse. One of the most notable was the collapse of the Mounds Landing levee, which caused a 10-foot-deep lobe of water to cover the Washington County town of Greenville on April 22. The river reached 50 miles in width and had flooded approximately 1 million acres, washing away an estimated 2,200 buildings in Washington County alone. Many perished trying to keep the levees from collapsing and were washed away in the deluge. The floodwaters remained high for more than two months, and people were forced to leave the area (if they could afford to) or to live in refugee camps on the levees, which were crowded and unsanitary. An estimated 1,000 people perished in the floods of 1927, some from the initial deluge, more from famine and disease in the months following the initial inundation by the floodwaters. More than 1 million people were displaced from their homes, and a total of 27,000 square miles (43,450 km2, or 16.6 million acres) were flooded. Crop losses amounted to $102.6 million, and 162,000 homes were inundated.

Another wet year along the Mississippi was 1972, with most tributaries and reservoirs filling by the end of summer. The rains continued through winter 1972-73, and the snowpack thickened over the northern part of the Mississippi basin. The combined snowmelts and continued rains caused the river to reach flood levels at St. Louis in early March, before the snow had even finished melting. Heavy rain continued throughout the Mississippi basin, and the

Satellite image of the St. Louis, Missouri, area that shows the Mississippi, Missouri, and Illinois Rivers at stages at the height of the 1993 flood (Image taken August 19, 1993; NASA images created by Jesse Allen, Earth Observatory, using data provided courtesy of Landsat Project Science Office)

The Blackfoot River in Montana, showing natural meanders, oxbow lakes, and floodplain (James Steinberg/ Photo Researchers, Inc.)

river continued to rise through April and May, spilling into fields and low-lying areas. The Mississippi was so high that it rose to more than 50 feet above its average levels for much of the lower river basin, and these heights caused many of the smaller tributaries to back up until they too reached this height. The floodwaters rose to levels not seen for 200 years. At Baton Rouge, Louisiana, the river nearly broke through its banks and established a new course to the Gulf of mexico, which would have left New orleans without a river.

The floodwaters began peaking in late April, causing 30,000 to be evacuated in st. Louis by April 28, and close to 70,000 in the region. The river remained at record heights throughout the lower drainage basin through late June. Damage estimates exceeded $750 million (1973 dollars).

In late summer 1993 the mississippi and its tributaries in the upper basin rose to levels not seen in more than 130 years. The discharge at st. Louis was measured at more than 1 million cubic feet (28,320 m3) per second. The weather situation that led to these floods was remarkably similar to that of the floods of 1927 and 1973, only worse. High winter snowmelts were followed by heavy summer rainfalls caused by a low-pressure trough that stalled over the midwest, because it was blocked by a stationary high-pressure ridge that formed over the East Coast of the united states. The low-pressure system drew moist air from the Gulf of mexico that met the cold air from the eastern high-pressure ridge, initiating heavy rains for much of the summer. The rivers continued to rise until August, when they reached unprecedented flood heights. The discharge of the mississippi was the highest recorded, and the height of the water was even greater because all the levees that had been built restricted the water from spreading laterally and caused it to rise more rapidly than it would have without the levees in place. more than two-thirds of all the levees in the upper mississippi River basin were breached, overtopped, or damaged by the floods of 1993. Forty-eight people died in the 1993 floods, and 50,000 homes were damaged or destroyed. Total damage costs are estimated at more than $20 billion.

The examples of the floods of 1927 and 1993 on the mississippi reveal the dangers of building extensive levee systems along rivers. Levees adversely affect the natural processes of the river and may actually make floods worse. Their first effect is to confine the river to a narrow channel, causing the water to rise faster than if it were able to spread across its floodplain. Additionally, since the water can no longer flow across the floodplain, it cannot seep into the ground as effectively, and a large amount of water that the ground would normally absorb must now flow through the confined river channel. The floods are therefore larger because of the levees. A third hazard of levees is associated with their failure. When a levee breaks, it does so with the force of hundreds or thousands of acres of elevated river water pushing it from behind. The force of the water that broke through the Mounds Landing Levee in the 1927 flood is estimated to be equivalent to the force of water flowing over Niagara Falls. If the levees were not in place, the water would have risen gradually and would have been much less catastrophic when it eventually entered the farmlands and towns along the Mississippi River basin.

The u.s. Army Corps of Engineers is mitigating another hazard and potential disaster where the Atch-afalaya branches off the mississippi. over geological time the Mississippi River has altered its course so that its mouth has migrated east and west by hundreds of miles. Each course of the river has produced its own delta, which subsides below sea level after the river migrates to another location. subsidence of the delta deposit occurs primarily because the river no longer replenishes the top of the delta, and the buried muds gradually compact as the weight of the overlying sediments expels water from the pore spaces. As the delta subsides to sea level, waves add to the erosion, keeping the delta surface below sea level. At the present time the lower mississippi River follows a long, circuitous course from where the Atchafalaya branches off from it, past New orleans, to its mouth near Venice. The mississippi is ready to switch its course back to its earlier position, following the Atchafalaya, which would offer it a shorter course to the sea, and would take less energy to transport sediment to the Gulf of mexico. If this were to occur, it would be devastating to the lower delta, which would quickly subside below sea level. The city of New orleans is currently below sea level and protected from the river, storms, and the Gulf of mexico only by high levees built around the city. To prevent this disaster the Army Corps has constructed an extensive system of diversions, levees, and dams at the mississippi/Atchafalaya junction to keep the mississippi in its channel.

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