14-2 Reconstructing a Millennium of Merrimack River Flooding from Oxbow Lake Sediments

Session: Lake Sedimentary Records of Past Climate and Environment

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Abstract:

Recent catastrophic floods in New England combined with projections of increased precipitation underscore the need to better predict flood risk extent. Understanding trends in flood frequency and magnitude typically relies on instrumental streamflow records, however, these datasets are often too short to capture the full range of variability in extreme events. Extending streamflow records with paleoflood data offers a valuable way to increase the sample size of extreme events, yet few paleoflood studies exist in New England. This study integrates sedimentary archives, hydraulic modeling, and earth system model output to reconstruct long-term changes in river discharge and flood frequency. We use sediment cores from Horseshoe Pond in Concord, NH, an oxbow of the Merrimack River, to reconstruct floods over the past millennium. We develop a composite flood score integrating bulk density, organic content, and grain size to quantify flood variability through time. Flooding and velocity in the modern and paleochannels are simulated with a hydraulic model (HEC-RAS), allowing us to better constrain paleo-discharge estimates. To place the sediment-based reconstructions in a broader climate context, we then compare our results with moisture reconstructions from tree rings and annual peak discharge estimates derived from the Community Earth System Model-Last Millennium Ensemble (CESM-LME) over the same period. Our paleoflood reconstructions extend the instrumental record, providing a more comprehensive understanding of hydrologic variability in New England and improving our ability to assess future flood risk.

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