7-27 Eroding Legacies: How Past Land-Use and Erosion is Impacting Water Quality Today Within a Southern Appalachian Mountains Headwater Basin in Western North Carolina

Session: Undergraduate Research, Part I (Posters)

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Abstract:

During the late 1800’s and early 1900’s, European settlement in the Southern Appalachians brought increases in population and major shifts in land use practices, including clear cutting for logging and agriculture.  Widespread erosion of slopes led to deposits of anthropogenic ‘legacy’ sediments in valleys that continue to influence streams today. Headwaters regions are critical to flow generation, water quality, and ecosystems, yet their impact is poorly studied. In this study, we investigate whether legacy sediments in the 0.4 km² Gribble Gap watershed (Cullowhee, NC) continue to impact stream channel stability and water quality. 

Preliminary work with sediment augers showed that legacy sediments are present in all parts of the valley floor. Seven sites were assessed in detail using an integrated geomorphic and sedimentologic approach; two of the sites were in a relatively undisturbed area and served as a control. Methods included collection of channel cross-sections data, bank stability assessments, soil texture and color analysis, bed-material size analyses, XRF geochemistry, tree-core dating, and turbidity measurements during baseflow and stormflow conditions. These data allowed identification of legacy sediments, evaluation of channel traits, and quantification of turbidity responses to rain events. 

Legacy sediments, 0.5 – 2 m thick (having a measured approximate sedimentation rate of 0.55 cm/yr from ~1940 – 2000), are generally sandy to silty in texture. In this region, channels are incised with steep, unstable banks composed of legacy sediments, have finer bed material, and pronounced turbidity increases during storms, with peak values exceeding 50 NTU. Tree-core and channel cross-section data indicate continued channel incision (>1 cm/y in recent decades) in the most disturbed reaches. In contrast, control sites with minimal modern disturbance showed greater channel stability and substantially lower turbidity responses despite the presence of legacy deposits. Overall, the streams were observed to be highly sensitive and have undergone multiple aggradation degradation cycles. These results demonstrate that legacy sediments remain a significant driver of stream instability and water quality degradation in Appalachian headwaters. Limiting modern impacts and allowing recovery in headwater systems may reduce the long-term influence of historic land-use legacies on downstream watersheds.

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