RESULTANT SEDIMENTATION FROM TROPICAL STORM IRENE IN THE LOWER CONNECTICUT RIVER

Here we present results from a rapid response study that documents resultant sedimentation from Tropical Storm Irene along the Lower Connecticut River. The study is focused to coves and other off-river waterbodies adjacent to the main channel of the lower river, where trapping is typically enhanced towards the mouth due to the flood-dominated asymmetry in fine-grained sediment flux via tidal channels connected to the main river. Irene was the event of record in terms of freshwater discharge for many of the tributaries to the river’s upper watershed. Record high sediment concentrations in excess of 1000 mg/L persisted towards the mouth of the river, although peak flows in the lower river were more moderate (having a return period of roughly 7 years). Moderate discharge in the lower river resulted in tides persisting towards the mouth, which in combination with record sediment loads presented near-ideal conditions for fine-grained sediment trapping at backwater sites along the lowest portion of the river’s tidal reach. On average, the overall thickness of the Irene deposit was roughly twice the thickness of sediments deposited during the spring freshet of 2011, with sedimentation rates increasing towards the mouth of the river. Irene sediments are primarily composed of inorganic clays and silts likely derived from the upper watershed’s glaciated landscape. These sediments are compositionally distinct from the more organic rich sediments collected during the spring freshet of 2011. Mercury levels within the Irene deposit are also significantly lower than sediments collected during the spring freshet, as well as older underlying material deposited during the industrial era. Results therefore suggest that the event did not serve to remobilize legacy contaminants stored in the system, but rather to cap contaminated sediments with a relatively clean, impermeable layer of clays and silts.