LAURENTIAN GREAT-LAKES HYDROLOGY AND LAKE LEVELS UNDER THE TRANSPOSED1993 MISSISSIPPI RIVER FLOOD CLIMATE

The Laurentian Great Lakes are North America's largest water resource, and include six large water bodies (Lakes Superior Michigan, Huron, E rie, Ontario, and Georgian Bay), Lake St. Clair, and their connecting channels. Because of the relatively small historical variability in sy stem lake levels, there is a need for realistic climate scenarios to d evelop and test sensitivity and resilience of the system to extreme hi gh lake levels. This is particularly important during the present high lake level regime that has been in place since the late 1960s. In thi s analysis, we use the unique climate conditions which resulted in the 1993 Mississippi River flooding as an analog to test the sensitivity of Great Lakes hydrology and water levels to a rare but actual climate event. The climate over the Upper Mississippi River basin was computa tionally shifted corresponding to a conceptual shift of the Great Lake s basin 10 degrees west and 2 degrees south. We applied a system of hy drological models to the daily meteorological time series and determin ed daily runoff, lake evaporation, and net basin water supplies. The a ccumulated net basin supplies from May through October 1993 for the 19 93 Mississippi River flooding scenario ranged from a 1% decrease for L ake Superior to a large increase for Lake Erie. Water levels for each lake were determined from a hydrologic routing model of the system. La kes Michigan, Huron, and Erie were most affected. The simulated rise i n Lakes Michigan and Huron water levels far exceeded the historical re corded rise with both lakes either approaching or setting record high levels. This scenario demonstrates that an independent anomalous event , beginning with normal lake levels, could result in record high water levels within a 6- to 9-month period. This has not been demonstrated in the historical record or by other simulation studies.