LARGE UPPER MISSISSIPPI RIVER FLOODS AND CLIMATE CHANGE OVER THE PAST 7000 YEARS

Sedimentological properties of UMR (upper Mississippi River) Holocene alluvium indicate large floods during the last 7000 years have occurred in a very episodic fashion that is strongly linked to relatively modest climate changes. There has been a tendency for floods to be smaller and more variable during prolonged warm episodes compared to floods being larger and less variable during prolonged cool episodes. Thus, a general tendency for moderate to relatively large floods between about 7000 and 5500 years ago, followed by an episode of smaller floods between about 5500 and 3300 years ago, then returning to generally larger floods after about 3000 years ago is broadly coincident with modest shifts in local effective climate conditions from cool/moist to warm/dry and back to cool/moist during the same times. However, on shorter timescales, there also has been a strong tendency for increased occurrences of very large floods during the beginning phases of times characterized by rapid climate change. Particularly noteworthy are occurrences of large floods about 4700, 2500-2200, 1800-1500, 1280, 1000-750, and 550-400 calendar years B.P., all times that independent climatic proxies indicate as periods of change. The anomalous high frequency of large floods on the UMR since about 1950 have occurred during a period of rapid global warming, and the association appears similar to anomalous frequencies of large floods evident in the geologic record of past periods when global warming was also apparent. The strong sensitivity to climate change supports a view that planning and policy decisions related to anticipated future flooding conditions should reflect scenarios of potential future climates rather than assuming randomness in climatic and hydrologic time series as is commonly done at present. Research was supported by the National Science Foundation (ATM-0112614).