ANTHROPOGENIC IMPACTS TO SEDIMENTATION IN THE MERRIMACK RIVER ESTUARY: INSIGHTS FROM FIELD, HISTORICAL, AND MODELING APPROACHES

Land clearing, river impoundments, and other human modifications to drainage basins and within estuarine systems can drive coastal change at local to regional scales. Measuring the impact of these anthropogenic changes on the coastal landscape requires integration of methods from multiple disciplines of coastal science. To demonstrate one such approach, we present a late-Holocene record of changing sediment sources and accumulation rates as recorded in a suite of sediment cores collected across a tidal flat in the Merrimack River estuary (Massachusetts, USA). We pair sedimentology, geochronology, bulk- and stable-isotope organic geochemistry, and hydrodynamic simulations with historical data to evaluate human and natural impacts on coastal sediment fluxes. During the 17^th to 19^th centuries, accumulation rates increased by an order of magnitude in the central tidal flat, likely in response to enhanced delivery of terrestrial sediment resulting from upland deforestation. By integrating historical sources with field measurements of sediment accumulation and a point-modeling approach, we demonstrate that coincident anthropogenic land clearing and dam building, upland sediment storage, and estuarine hydrodynamics together result in a subtle and spatially variable overall increase in accumulation (0.56 – 2.6 mm/yr) within the estuary. This study provides insight into the response of formerly glaciated fluvial-coastal systems to human modifications, and underscores the role of estuarine environmental conditions in modifying upland signals of land-use change.