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In April 2011, the Ohio River began flooding farmland and cities from Pennsylvania to Illinois that were not protected by levees. The US Army Corps of Engineers (USACE) had realized as early as March that the torrential rains and heavy snow melt across the Upper Midwest were setting up the Mississippi River Basin for an epic flood year (USACE 2011). By late April, lakes and reservoirs along the Wabash and Ohio rivers were filled to capacity and cities without levees, such as Metropolis, Illinois, had lower sections covered by floodwaters. Cairo, Illinois, and many of the cities on the lower Mississippi River were protected by levees.
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Olson et al. (2012) studied this question.
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