THE RELATIVE ROLE OF HIGH MAGNITUDE, LOW FREQUENCY EVENTS IN INFILLING OFF-RIVER WATERBODIES: INSIGHTS FROM TROPICAL STORM IRENE AND THE 2012 SPRING FRESHET

Tropical Storm Irene’s most extreme rainfall resulted in record-breaking sediment loads from upland tributaries to the Connecticut River. However, was the event exceptional with respect to resultant deposition downstream? Off-river waterbodies to the Lower Connecticut River, such as cut-off meanders and blocked valley lakes, are a particularly important floodplain environment, which have been shown to serve as a focal point for the trapping of sediment and associated contaminants. This study evaluates the relative role of extreme events like Tropical Storm Irene in infilling these off-river environments. To meet this objective we compare the magnitude and composition of resultant sedimentation from Irene to that observed following the 2011 and 2012 spring freshets. Tropical Storm Irene deposits were identified as compositionally distinct, grey layers that were relatively inorganic compared to sediments deposited by the annual Spring Freshet. Sediment within the Irene deposit was enriched in elemental potassium and depleted in zircon, a finding consistent with glacigenic lacustrine and till sediments. Decreased mercury levels in the Irene deposits suggest that this event served to cap highly contaminated, industrial era sediment with a layer relatively clean, fine-grained silt and clay. Resampling of these waterbodies in Fall 2012 revealed preservation of the 1.5-6 cm thick Irene deposit at all but one site, and was covered with 1.5-3 cm of more recent sediments deposited following the event. Sediment contributions from rare events, like Tropical Storm Irene, were found to be less influential than the annual Spring Freshet in the long-term infilling of waterbodies along the Lower Connecticut River. However, sediments from Irene are compositionally unique and serve to highlight the importance of this event in removing glacially derived fines from the river’s upland catchments.