Floods and Fluvial Activity during the Medieval Climate Episode

Widespread warming during the Medieval climate episode between about 1200 and 600 years ago in the U.S. significantly influenced fluvial processes through related impacts on runoff and sediment delivery. Floodplain sedimentation associated with overbank floods on the upper Mississippi River (UMR) indicates that floods on average were of relatively small magnitude during the relatively warm Medieval episode, but the stratigraphic record also clearly shows an anomalous high frequency of large magnitude UMR floods punctuates this episode, especially between about 1000 and 600 years ago. The anomalous high variance in UMR flood magnitudes between 1000 and 600 years ago is similar to the anomalous high variance of UMR floods that have accompanied global warming since 1950 when the frequency of floods exceeding one standard deviation above the long-term historic mean flood is nearly twice the normal expected occurrence. The causal mechanism for the anomalous high frequencies of large floods during Medieval and recent times apparently relates to the potential for occasional widespread heavy rains and/or heavy snowfalls associated with Gulf derived air masses of high water vapor content being favored in the region during warmer times. Furthermore, many of the largest UMR floods involve early spring rains on the melting winter snowpack. Medieval flood records for other large U.S. rivers are inadequate for detailed comparisons with the UMR record, but geologic evidence of channel incision thought to be related to increased frequencies of large floods has been postulated by others for many sites on small drainage systems in the Great Plains and the Southwest. Regional comparisons among drainages of the Upper Mississippi Valley, Great Plains, Uinta Mountain West, and Southwest support the idea that Medieval timescale climate changes have resulted in widespread, broadly synchronous changes in fluvial activity, although directions and magnitudes of such changes vary regionally.