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Floodplains by Design

Mississippi River and Tributaries

When the Levees Break: Lessons Learned from the “Great Flood” of 1927


Bonnet Carré Floodway

Floodways along the Mississippi, like those at Bonnet Carré, are key components of the flood protection measures in the Missippi River.

2011 Flood

Birds Point New Madrid Floodway

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2011 Flood

A Glimpse of Louisiana

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In 1927, an epic flood struck the lower Mississippi River. In more than 100 places, floodwaters surged over or through levees, inundating an area the size of South Carolina and displacing 700,000 people for months. Officially, some 400 people died, though historians estimate the true death toll must have been in the thousands.

In 2011, a larger volume of floodwater flowed through the lower Mississippi River. This time, no levees failed, nobody died and no land flooded that was not intended to be flooded. That last phrase – intended to be flooded – holds the key to the dramatically different outcomes between the two floods. 

Before 1927, management of the Mississippi River was fragmented, with different counties, towns, and landowners overseeing various flood-protection schemes. They were united, however, in their faith that levees alone could protect them from floods. The 1927 flood exposed both the fallacy of this belief and the vulnerability of fragmented management.

In response to that disaster, the U.S. Army Corps of Engineers implemented a plan to manage the Mississippi as an integrated system. That system included standards for levee design, construction and location, including setting many of them back to give the river more room. It also featured eight locations where, during the largest floods, water can move onto portions of the historic floodplain, which reduces pressure on levees and which proved vitally important to the system’s successful performance during the 2011 flood. (Note: During the spring of 2011, levees on other rivers and outside of this Mississippi system did fail and lead to unintended flooding.) 

The failure in 1927 and the success in 2011 underscore the importance of managing river basins as whole systems. The story is not one of complete success, though. The response to the 1927 flood led to a system designed to optimize mainly two benefits – flood control and navigation. 

Today, society expects a great deal more from its rivers, and scientists have a much greater appreciation for how rivers and floodplains work. For example, we now know that floodplains are the most important habitat in a river system for supporting fish populations. Additionally, floodplain wetlands can remove excess nutrients or sediment from rivers, thereby improving water quality, which reduces costs at treatment facilities.

Given this, how would a modern system-scale approach to the Mississippi compare to what was produced in the 1930s? For example, the current system relies on floodplains as critical “relief valves” during the largest floods, but those floodplains are not generally managed to optimize benefits like fisheries and water quality. A system that sought greater balance across a broader range of benefits could use those same floodplains to support fish populations and to improve water quality. Increasing demands and potential synergies like these drove the development of a shared vision among a broader coalition of stakeholders through the formation of America’s Watershed Initiative.

Such lessons could prove pivotal for those who are working to shape the future of other great rivers, like Colombia’s Magdalena, which was also hard hit by floods in 2011 – floods that killed at least 192 people, left more than 69,000 homeless and caused some $5 billion in damages. The Magdalena is at a much earlier stage of development than the Mississippi, so decision makers now have the chance to plot the future of their “national river,” which is the principal source of inland fisheries and provides a source of water for 32.5 million people.

Learning from the challenges and problems presented by a system designed 85 years ago for the Mississippi and seeing the achievements made when multiple stakeholders are brought together to solve complex problems has given people in the Magdalena basin the opportunity to obtain greater balance across a broader range of benefits. It has been an opportunity to demonstrate to the rest of the world the value of such an approach. And, by exchanging knowledge with planners along China’s Yangtze River, decision makers at the Magdalena can implement the best practices in the sustainable site selection and construction of dams that will help reduce flood risks and provide a clean source of electric power to Colombians.

It is the exchanges of lessons and knowledge like these between the world’s great rivers that is core to the Great Rivers Partnership's mission.

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