Human and natural impacts on sedimentation in the Merrimack River Estuary, Massachusetts, USA

Abstract:

Estuarine sedimentation responds to myriad, often interconnected, human and natural drivers, such as sea-level rise, upstream land-use change, local urbanization, and changing hydrodynamics. Measuring the impact of humans on estuaries requires the 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 geochemistry, and hydrodynamic simulations with historical data to evaluate human and natural impacts on estuarine sedimentation. During the 17th to 19th 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. The overall increase in accumulation within the estuary (0.56 – 2.6 mm/yr) is subtle and spatially variable as a result of coincident anthropogenic land clearing and dam building, upland sediment storage, and estuarine hydrodynamics. By integrating historical sources with field measurements of sediment accumulation and hydrodynamic modelling, this study provides insight into the response of formerly glaciated estuarine systems to human modifications, and underscores the role of humans in modifying coastal sedimentation, and that of estuarine environmental conditions in modifying upland signals of land-use change.