ESTUARINE RECORDS OF LONG-TERM HUMAN IMPACTS ON FLUVIAL SEDIMENT DISCHARGE FROM A LARGE NEW ENGLAND RIVER

Coarse- and fine-grained sediment discharge to the coast can aid in the stability of barrier islands and backbarrier marshes, respectively, as they adapt to sea-level rise and increased storminess. This study presents geologic and historical records to ascertain the anthropogenic impacts from damming and deforestation on sediment discharge from the Merrimack River to the coast in northern Massachusetts. A kilometer-long transect of five piston cores collected across an estuarine tidal flat reveals a high degree of spatial heterogeneity in deposits. A ca. 400-year-old, 40–50-cm thick peat deposit (organic content: 11–13 %; bulk density: 0.5–0.9 g/cc) is buried approximately 70 cm below the sediment surface in the most river-proximal cores. The two most shore-proximal cores contain 20–40 cm thick fining-upward sequences (from poorly-sorted muddy medium-to-coarse sand to muddy fine sand) interpreted as tidal-channel fill. Together, these deposits highlight a dynamic record of channel migration and marsh formation, accretion, and loss. Radiocarbon and short-lived radioisotope dating, combined with geochemical records of industrial contaminants, indicate very low accretion rates at this site during the past 500 years, ranging from 0.5 to 1.9 mm/yr. Peak rates coincided with the period following the industrial revolution (1850–1955) but have since remained high (1.8 mm/yr, 1955–2015). These data demonstrate that this environment has undergone minimal sedimentary reworking and that sediment accretion has long been outpaced by rates of historic relative sea-level rise (1–2.8 mm/yr). Historical records demonstrate that peak deforestation coincided with dam-building on larger New England rivers, likely buffering the sediment pulse from enhanced soil erosion. While colonial deforestation may have facilitated rapid marsh expansion in this system along small coastal rivers, these geologic and historical records indicate that sediment discharge from larger rivers may have undergone only a relatively modest increase as a result of human activities.