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Annual bankfull flows have long been thought to be the primary drivers of fluvial geomorphology. However, sedimentation in the upper Connecticut River basin after Hurricane Irene, 2011, suggests that low frequency, high magnitude events may dictate drainage evolution in upland watersheds. During Hurricane Irene, the Deerfield River, an upland tributary to the Connecticut River, produced as much as 40% of the Connecticut’s total sediment yield, even though the Deerfield catchment composes only 5% of the Connecticut’s watershed. Previous studies have indicated that Hurricane Irene deposits are unusually fine-grained, inorganic, and enriched in potassium, a signature that resembles the fine-grained matrix of Pleistocene glacial tills blanketing the hillslopes of the watershed. A series of six cores were sampled from Sherman Reservoir on the Deerfield River to search for a Hurricane Irene deposit and investigate the Deerfield’s exceptional sediment yield. The cores were analyzed for loss-on-ignition, mercury concentration, grain size distribution, potassium-zirconium ratio, bulk density, and radionuclide concentration. The cores revealed no distinct Hurricane Irene depositional layer, even though the catchment produced the equivalent of 10 to 40 years of sediment flux as a result of the hurricane.
Therefore, the Deerfield’s sediment contribution to the Connecticut River during Hurricane Irene must have come primarily from the lower 65% of the watershed below Sherman Reservoir. It is also possible that Hurricane Irene sediment bypassed Sherman Reservoir or is preserved in the reservoir in locations where we did not core or displays a signature different from the fine-grained one that we looked for. These hypotheses imply that the interactions between precipitation, erosion, and sedimentation in upland watersheds are heterogeneous and governed by the glacial legacy of the landscape. Further study should explore the differences in the Deerfield catchment above and below Sherman Reservoir that might account for the striking disparity in sediment yield. Additional research in the region can aid in the development of soil conservation and erosion prevention programs in upland watersheds. Identification of patterns of erosion during and after extreme weather events will contribute to infrastructure protection and flood management.
Andrews, Lucy, "Extreme precipitation and erosion in upland watersheds: a case study from Sherman Reservoir, Massachusetts" (2014). Geology Honors Projects. 21.
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