THE EFFECT OF FLOOD CONTROL DAMS ON DOWNSTREAM SEDIMENTATION FROM LARGE DISCHARGE EVENTS. A STUDY OF TROPICAL STORM IRENE AND THE CONNECTICUT RIVER

In late August, 2011, Tropical Storm Irene passed directly over the Connecticut River watershed causing several low order streams to swell beyond historical discharge maxima. Resultant discharge on the lower mainstem Connecticut River only amounted to a one in seven year-sized event. Variation in precipitation volume and intensity can only partially explain the observed disparity in hydrograph response between upper and lower watershed. Furthermore, floodplain sediment deposits in the lower watershed are anomalously fine and enriched in glacial clays similar to those found in Illinoisan till and lake varves. Thus, both the hydrologic and sedimentological response to Irene are distinct from historical floods in the watershed.

Here we investigate the role of flood control dams as they affected Irene’s Connecticut River hydrograph as well as the sediment it transported. The prevalence of these structures since the mid twentieth century has dramatically altered how large discharge events contribute to the floodplain sediment record. We will illustrate how sediment entrainment and deposition during the Irene event differed from those of past events and evaluate how the system might have responded in the absence of these flood control reservoirs.