SENSITIVITY OF LARGE UPPER MISSISSIPPI RIVER FLOODS TO CLIMATE CHANGE

Long, century-scale, instrument records for the upper Mississippi River (UMR) support the idea that extreme floods tend to be more sensitive to climatic change than are high-frequency low-magnitude floods, and a similar relationship is indicated in the geologic record of floods during approximately the last 7000 years. The high sensitivity between large floods and climate change naturally raises concerns about possible related hydrologic consequences of global warming. Model results and instrument records both support the idea that global warming magnifies hydrologic variability and enhances the hydrologic cycle of the UMR basin.

A relatively high-resolution sedimentary record of UMR floods is present along the river reach on Wisconsin’s western border because both the channel and floodplain have been progressively aggrading throughout the Holocene. Centimeter scale vertical analyses of sediment properties indicate that large floods on the UMR have commonly accompanied the beginning of warm and dry climate episodes in the region, but long-term persistence of warming and drought eventually result in smaller floods of high short-term variability. 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 climate conditions from cool/moist to warm/dry/ and back to cool/moist during the same times. Short-term occurrences of large floods were common about 4700, 2500-2200, 1800-1500, 1280, 1000-750, and 550-400 calendar years B.P., all times that approximate rapid warming and drought in the upper Midwest identified by others. The recent high frequency of large floods on the UMR since the early 1990s may be a modern analogue because these floods have accompanied major hemispheric warming during the same period.

Research was supported by the National Science Foundation (ATM-0112614).