THE LASTING IMPACT OF HUMAN-INDUCED SOIL EROSION ON PIEDMONT STREAM MORPHOLOGY AND WATER QUALITY NEAR RALEIGH, NORTH CAROLINA

European settlers began building milldams in the eastern U.S. in the 1600s, and this continued until the early 20th century. Geomorphic mapping in Wake and Durham Counties, NC, demonstrates that upland soil erosion and valley bottom sediment aggradation on floodplains and behind milldams was substantial following the conversion of forest land for agricultural purposes. We observe three distinct sedimentary units in stream bank exposures that are supported by ¹⁴C dating and magnetic susceptibility (MS). Pre-European sediments range from ca. 8700 to 250 a^-1 BP consisting of quartz-rich channel sands, gravels, and off-channel organic-rich clays. Two legacy sediment units are differentiable; pre and post-dam, and range in age from ca. 300 to 100 a^-1 BP. Pre-dam sediments are primarily fluvial sands, recording net channel aggradation in response to soil erosion from upland land clearing prior to dam construction. Post-dam sediments are finer grained and consistent with slackwater deposition, often including sandy “event” layers that we interpret as floods into former millponds. Stream bank MS measurements exhibit consistent increases at and above the pre-European – legacy sediment unconformity.

Mapping indicates that the volume of eroded upland soils is approximately balanced by valley bottom aggradation, and the thickness of soil loss was equivalent to 10 ± 5 cm spread evenly across the landscape. We evaluate the current impact of legacy sediment erosion on stream water quality by capturing the total suspended sediment load (TSS) during discharge events, and document a TSS increase as water passes through millpond reaches. In addition, we utilize sediment fingerprinting to investigate TSS sources in tributary streams to Falls Lake (Neuse R.), the drinking water source for ~500,000 people. We measured 55 elemental and 2 radionuclide concentrations from both stream TSS and composite source sediment samples (stream banks, forests, pastures, construction sites, and roads). Results reveal that stream bank erosion of legacy sediment is the largest contributor to the TSS load in three out of four basins, and is significant in the fourth. Construction sites and timber harvesting are also significant source areas. Stream bank erosion of legacy sediments is a factor in the present-day impairment of NC Piedmont streams.