More Than Climate, Engineering Worsening Flooding Along Mississippi

TUSCALOOSA, Ala. — Through discovering ancient floods along the Mississippi River, a group of scientists, including a University of Alabama professor, found human-led engineering, not climate, is the largest influence on worsening floods.

Published today in the journal Nature, the findings add to the ongoing debate on whether flood risk is greater now because of climate variability or modifications to the river.

Seemingly large floods during this century and the 20^th century were put into context when researchers used sediment cores and tree rings to extend flood records 500 years into the past, much older than modern monitoring that began at the tail end of the 19^th century.

“There was not really clear evidence one way or the other just because the instrumental record is so short,” said Dr. Matthew D. Therrell, UA professor of geography and co-author on the paper in Nature. “The short instrumental period makes it difficult to understand the true range of natural variability within the system.”

The team found the magnitude of the 100-year flood is 20 percent greater now than 500 years ago, with river engineering accounting for three-fourths of the increase. Human alterations to the Mississippi River, with assistance from climate variability, have exacerbated current flood risk to unprecedented levels, according to the paper.

“The engineering is having this big effect, more so even than climate,” Therrell said. “The amount of flooding is worse because of the engineering along the river.”

Led by Dr. Samuel E. Munoz, a former post-doctoral researcher at Woods Hole Oceanographic Institution in Massachusetts and now at Northeastern University, the team looked at recorded streamflow data collected by the United States Army Corps of Engineers and U.S. Geological Survey, cores of sediment, and tree rings to look at the long-term record of flooding for the Mississippi.

Therrell contributed data he collected from tree rings along the river, funded through an ongoing project through the National Science Foundation. Tree rings, which record the annual growth cycle, can tell a lot about environmental change within the system, and can pinpoint when floods occur.

Although they do not do as well as sediment cores at revealing the magnitude of a flood event, the combination of sediment cores and tree ring gave both frequency and magnitude of flooding within the system. This novel method for combining sediment cores and tree rings to understand flooding is one of the first of its kind,  Therrell said.

“Anytime you have two completely different proxies that are saying the same thing, you can be more much more confident than if just one is saying it,” he said.

Other institutions with contributing authors on the paper include Southern Illinois University, Coastal Carolina University and Texas A&M University.