Track 1) Flood plain regulation, hazard assessment, and ecosystem preservation

Mississippi , Missouri, Ohio, Meramec, and Illinois River Floods 

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 eleven major tributaries of the Mississippi River have also experienced major floods, including events that have at least quadrupled the normal river discharge in 2007, 1993, 1973, 1927, 1909, 1903, 1892, and 1883. Three of the major rivers (Mississippi, Missouri, and Illinois) meet in St. Louis, which has seen some of the worst flooding along the entire system. Climate change is starting to impact these river systems, with a predicted 20% increase in rainfall and a consequent 51% increase in river discharge over the next 30 years (Pan, 2005). These changes will severely impact the way people live in along the river basins. We must understand and plan for these changes. Our affiliated researchers in GIS and Atmospheric Sciences are able to soil moisture/water content with models of heavy rainfall to make forecasts of floods and hydrologic conditions.

Floods along the Mississippi River in the seventeen and eighteen hundreds prompted the formation of the Mississippi River Commission, which oversaw the construction of high levee’s along much of the length of the river from New Orleans to Iowa. By the year 1926 over 1,800 miles of levees had been constructed, many of them over twenty feet tall. The levees gave people a false sense of security against the flood waters of the mighty Mississippi, and the levees restricted the channel causing floods to rise more quickly, and forcing the water to flow faster.

Many weeks of rain in the late fall of 1926 followed by high winter snow melts in the upper Mississippi River Basin caused the river to rise to alarming heights by the Spring of 1927. Residents all along the Mississippi were worried, and were strengthening and heightening 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, 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 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, whose collapse caused a ten 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 one million acres, washing away an estimated 2,200 buildings in Washington County alone. Many people perished trying to keep the levees from collapsing and were washed away in the deluge. The flood waters 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 flood, and more from famine and disease in the months following the initial inundation by the flood waters.

1972 was another wet year along the Mississippi, with most tributaries and reservoirs being filled by the end of the summer. The rains continued through the winter of 1972/1973, and the snowpack thickened over the northern part of the Mississippi basin. The combined snow melts 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 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 river heights caused many of the smaller tributaries to back-up until they too were at this height. The flood waters rose to levels not seen for 200 years. At Baton Rouge, 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 flood waters began peaking in late April, causing 30,000 people to be evacuated in St. Louis by April 28th, and close to 70,000 people in the region. The river remained at record heights throughout the lower drainage basin through late June. Damage estimates exceeded 750 million dollars (1973 dollars).

In the late summer of 1993 the Mississippi River and its tributaries in the upper basin rose to levels not seen in more than one hundred and thirty years. The discharge at St. Louis was measured at more than one million cubic feet 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 snow melts 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 the water 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 dollars.

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. They first effect they have is to confine the river to a narrow channel, causing the water to rise faster than if it were able to spread across its flood plain. Additionally, since the water can no longer flow across the flood plain it can not seep into the ground as effectively, and a large amount of water that would normally be absorbed by the ground now must 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 came into the farmlands and towns along the Mississippi River basin.

With the history of flooding and the potentially worsening conditions with the continued construction of levees, it is surprising that legislation allows rapid development of many floodplain areas such as the confluence region of the Mississippi, Missouri, and Illinois Rivers in St. Louis. This causes loss of vital habitat and wetland areas, as well as putting the people and property in the flood plains at great risk. At present, it is estimated that the flood walls in downtown St. Louis are not strong enough to withstand a flood even of the height of the 1993 flood- yet we continue to build new levees upstream that reduce the storage capacity of the flood plain, will raise flood levels in St. Louis, increase the velocity of the river, and cause great damage if nothing is done. Costs of recovery from floods are borne by taxpayers. Part of our mission is to educate the public about river dynamics and hazards, and the need to preserve flood plain ecosystems as floodplains and natural water storage areas, not suburban sprawl. The SLU Center for Environmental Science is uniquely situated geographically, historically, and staffed to research these issues, inform the public about the significance and dangers of flood plain development, and change local, regional, and national policy on floodplain development. We have faculty with expertise in flood hazards, flood plain ecosystems, surface and ground water quality, and federal flood plain regulation.

Projected flood stages are based on current climate conditions, not a wetter climate phase that we are entering. The region will likely suffer more frequent and larger floods than in the past.