39-5 Characteristics and styles of legacy sediment deposition associated with historic mill dam sites in Connecticut USA

Session: Functions of River Corridors, Floodplains and Wetlands

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Abstract:

Southern New England preserves a remarkable history of land use-land cover change characterized by ~250 years of deforestation and agricultural expansion in the late 17th to mid-20th centuries, followed by a dramatic reduction in agriculture and forest regrowth.  This transformation on hillslopes coincided with the construction, use and eventual abandonment of mill dams along stream and river corridors, which were built to fuel industries such as the production of textiles, grain, and lumber.  Today, the legacy of historic land use and mill dams can be observed in landforms left behind (e.g., stone walls, dams) and as well as in soils modified during land use practices and sediment deposited in floodplains, wetlands, and directly upstream of abandoned mill dams.  In this study, we examine the characteristics and styles of legacy sediment deposition associated with historic mill dam sites along river corridors in Connecticut.  We present a geospatial summary of dams and reservoir history within the 508 km² upper Thames Watershed in eastern Connecticut from 1811 to present, and highlight a number of case studies in the region where we have completed detailed sediment coring investigations to better understand sedimentation behind historic mill dam sites.  Sites range widely in terms of reservoir filling (complete to partial); sediment thicknesses and sedimentation rates (0.1-2 cm/yr); and metals behavior (highly contaminated with Hg, Pb, Zn, Cu, etc. vs. low values simply associated with global atmospheric deposition).  They also exhibit different complicating aspects, such sites with multiple dams and evidence of multiple pulses of sedimentation in the lower reservoir due to a breach of the upper dam, as well as sites that had previously been abandoned or breached that have recently returned to having a reservoir due to beavers occupying the site and building dams.  Overall, these examples highlight the multitude of factors that come into play when studying sedimentation behind historic mill dams in river corridors of Connecticut and emphasize that despite having similar land use history and density of mills as the more widely studied mid-Atlantic region, the northeast US generally has less legacy sediment and styles of preservation are much more variable.

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