Climatic control of Mississippi River flood hazard amplified by river engineering

A suite of river discharge, tree-ring, sedimentary and climate data shows that the Mississippi’s flood magnitude has risen by about twenty per cent over the past half-century, largely owing to engineering works. Engineering magnifies Mississippi's rising rivers Instrumental records of river discharge do not go far enough back in time to place recent flood activity in a longer-term context, making it difficult to understand how climate variability and human activity might have affected flooding. Now, Samuel Munoz and colleagues reconstruct the past flood frequency of the Mississippi River from a compilation of river-discharge, tree-ring, sedimentary and climate data. The results show that the magnitude of the 100-year flood has gone up by about 20 per cent over the past 500 years. Climate cycles account for most of the variability in flooding on multidecadal timescales, but engineering works account for about three-quarters of the long-term increase. Over the past century, many of the world’s major rivers have been modified for the purposes of flood mitigation, power generation and commercial navigation 1 . Engineering modifications to the Mississippi River system have altered the river’s sediment levels and channel morphology 2 , but the influence of these modifications on flood hazard is debated 3 , 4 , 5 . Detecting and attributing changes in river discharge is challenging because instrumental streamflow records are often too short to evaluate the range of natural hydrological variability before the establishment of flood mitigation infrastructure. Here we show that multi-decadal trends of flood hazard on the lower Mississippi River are strongly modulated by dynamical modes of climate variability, particularly the El Niño–Southern Oscillation and the Atlantic Multidecadal Oscillation, but that the artificial channelization (confinement to a straightened channel) has greatly amplified flood magnitudes over the past century. Our results, based on a multi-proxy reconstruction of flood frequency and magnitude spanning the past 500 years, reveal that the magnitude of the 100-year flood (a flood with a 1 per cent chance of being exceeded in any year) has increased by 20 per cent over those five centuries, with about 75 per cent of this increase attributed to river engineering. We conclude that the interaction of human alterations to the Mississippi River system with dynamical modes of climate variability has elevated the current flood hazard to levels that are