THE LASTING IMPACTS OF POST-COLONIAL AGRICULTURE AND WATER-POWERED MILLDAMS ON CURRENT STREAM WATER QUALITY

European settlers began building water-powered milldams in the eastern US in the 1600s, and dam construction continued until the early 20th 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 (Trimble, 1974). Much of the eroded sediment was subsequently aggraded on floodplains and impounded behind milldams. This trapped “legacy” sediment (LS), commonly mistaken for natural floodplain deposition, has gone largely unrecognized until recently (Walter and Merritts, 2008). This study focuses upon 1^st to 4^th order streams in Umstead State Park (Wake Co, NC) that drain into the Neuse River basin. There are 10 water-powered milldam locations within the park. Geomorphic mapping demonstrates that LS accumulation was extensive along the park’s valley bottoms.

We observe three distinct sedimentary units in stream bank exposures that are corroborated by ¹⁴C dating. Pre-legacy sediments range from ca. 4400 to 250 a^-1 BP and consist of quartz-rich axial stream gravels and off-channel organic rich clays. Two LS units are differentiable; pre and post dam, and range in age from ca. 300 to 100 a^-1 BP. The pre-dam LS consist primarily of fluvial sands with an increase in non-quartz lithics, and are interpreted as channel aggradation in response to soil erosion from upland land clearing prior to dam construction. Post-dam LS are distinguished by finer grain size and sedimentology consistent with slackwater deposition. Stream bank magnetic susceptibility (MS) measurements exhibit large and consistent increases at and above the PL-LS contact, suggesting that MS is a suitable proxy for LS identification along NC Piedmont streams.

We evaluate the current impact of LS 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 LS. Site specific stage-discharge calibrations allow us to predict the reach-specific contribution of LS-dominated bank erosion to the local TSS flux. Our study suggests that modern stream water impairment in the Piedmont may result where milldams were constructed and legacy sediments impounded.