By David Jacobson, Temblor
Creating a large channelized box
The Mississippi River flows for over 2,300 miles from Northern Minnesota to the Gulf of Mexico. Because it flows through one of the most fertile agricultural regions in the country, various types of flood defenses have been put in place to protect cities and fields vital to the country’s economy. While levees have been credited with protecting areas within the floodplain, other modifications to the river, such as artificial channelization, have actually increased the region’s flood hazard, according to a new study published today in Nature. This increase in flood hazard due to artificial channelization has meant that additional investments have had to be made in flood defenses.
Since the 19th century, federal money has been used to protect against the threat of flooding along the Mississippi River. This began with the building of levees, but has now expanded to include spillways as well as an artificially shortened and straightened main channel. Such channelization has turned once meandering rivers into arrow straight sections. While this has helped navigation along the river, it has also changed the relationship between the river’s discharge (volume of flowing water) and its stage (a river’s water level relative to a defined point). This means that over the last 150 years as land use and river engineering intensified, both the frequency and magnitude of flooding also increased. In fact, the authors of this article suggest that over the last five centuries, 100-year floods (a flood with a 1% chance of occurring annually) have increased by approximately 20%.
It’s us, not climate change
While climatic controls such as El Nino frequency and the Atlantic Multidecadal Oscillation have also played a role in increasing flood frequency, the authors estimate that only 25% of the total flood frequency increase can be attributed to climate. The figure below shows how the observed trend in the largest flood of the century significantly deviates from the trend where only climatic controls are considered. This deviation becomes most significant during the 20th century when river engineering was at its peak.
So what do we do?
What all of this illustrates is that the tools we are using to protect ourselves from floods, are also making us more vulnerable to them. Moving forward, this means that as defenses continue to be built at elevating costs, it will come at the expense of communities and industries in the lower Mississippi delta and floodplain. So, do we continue to build up defenses knowing full well that we are in fact making the overall situation worse? Or, do we stop and let mother nature take its course?
Samuel E. Munoz, Liviu Giosan, Matthew D. Therrell, Jonathan W. F. Remo, Zhixiong Shen, Richard M. Sullivan, Charlotte Wiman, Michelle O’Donnell & Jeffrey P. Donnelly, Climatic control of Mississippi River flood hazard amplified by river engineering, 5 april 2018 | VOL 556 | NATURE