In the spring and summer of 2011, concurrent floods developed in the two primary subbasins of the Mississippi River: the Ohio River and the upper Mississippi River (defined here as the part of the Mississippi River above the confluence with the Ohio River). As the separate flood waves traveled toward their confluence in April 2011, the U.S. Geological Survey (USGS) was alerted to the possibility that the New Madrid Floodway (hereafter referred to as "the floodway"), shown in figure 1, might be activated for the first time since 1937 (Mississippi River Commission, 2012).