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Abstract

This one-day field trip highlights recent research into the late Holocene geomorphic evolution and land use history of Piedmont stream valleys near Raleigh, North Carolina. European settlers began building water-powered milldams in the eastern United States in the 1600s, and dam construction continued until the early twentieth century. At the same time, regional-scale land clearing associated with agriculture and development increased upland erosion rates 50–400 times above long-term geologic rates. Much of the eroded sediment was subsequently aggraded on floodplains and impounded behind milldams. This trapped "legacy" sediment, commonly mistaken for natural floodplain deposition, has gone largely unrecognized until recently. This study focuses upon 1st to 4th order streams in W.B. Umstead State Park that drain into the Neuse River basin. There are seven water-powered milldam locations within the park and adjacent areas. Geomorphic mapping demonstrates that upland soil erosion and valley bottom sediment aggradation was substantial following European-American land acquisition and their conversion of large amounts of forest land for agricultural purposes.

We observe three distinct sedimentary units in stream bank exposures that are corroborated by 14C dating. Pre-European sediments range from ca. 4400–250 yr B.P. and consist of quartz-rich axial stream gravels and off-channel organic rich clays. Two legacy sediment units are differentiable; pre and post-dam, and range in age from ca. 300–100 yr B.P. The pre-dam sediments consist primarily of fluvial sands, and are interpreted as channel aggradation in response to soil erosion from upland land clearing prior to dam construction. Post-dam sediments are distinguished by finer grain size and sedimentology consistent with slackwater deposition, including sandy "event" layers, interpreted to be the result of large floods into the former mill ponds. Stream bank magnetic susceptibility (MS) measurements exhibit large and consistent increases at and above the pre-European-legacy sediment contact, suggesting that MS is a suitable proxy for legacy sediment identification along North Carolina Piedmont streams.

Estimates of aggraded legacy sediment from two stream reaches in Umstead State Park indicate that the volume of eroded upland soils is approximately balanced by valley bottom sediment aggradation, and that area-averaged depth of upland soil loss was equivalent to 3–15 cm across this part of the Piedmont. We evaluate the current impact of legacy sediment erosion on stream water quality by capturing the total suspended sediment load (TSS) during discharge events using ISCO samplers at 5 sites on Reedy and Richland Creek. We document a TSS increase as water passes through reaches containing milldam deposits. This suggests that modern stream water impairment in the Piedmont may result where milldams were constructed and legacy sediments impounded. The field trip concludes by examining an active beaver (Castor canadensis) pond–wetland meadow complex above the historic Yates Mill pond. Beavers may prove to be valuable assets in the restoration of Piedmont stream systems still suffering from centuries of poor land and soil management.

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