Earth sciences: Flood control lands Mississippi in deep water

Management of the Mississippi River could be making floods worse and more frequent, with the current overflow risk at a level unprecedented in the last half-century, reports a paper published in this week’s Nature.

The 2011 flooding of the Mississippi led to US$3.2 billion in agricultural losses and infrastructure damage - this notwithstanding 150 years of river engineering in the form of channel straightening, man-made embankments and overflows. Whether these measures have the desired effects is challenging to assess, however, because stream flow records dating back to 1897 cannot capture the river’s natural variability before human interference began to take place.

Samuel Munoz and colleagues assembled a longer flooding history - reaching back to the early sixteenth century - by looking at both trees and sediment cores taken from nearby lakes. When inundated by floodwaters, oak trees grow abnormally, preserving a timeline of floods in their growth rings. When the Mississippi overflows, meanwhile, it dumps carried sediments into lakes on the floodplain, producing a detailed register of each deluge. The authors compared these with historical climate records.

They found that the Mississippi’s flood magnitude has increased by about 20 per cent over the last 500 years, with the highest levels in the last century. Although the river does slowly rise and fall with North Atlantic climate fluctuations, the authors found that this can only account for a quarter of the recent increase in flood intensity. The remaining 75 per cent of the heightened flood hazard, they suggest, comes from human changes to the Mississippi and its basin.

doi: 10.1038/nature26145

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