Woody debris, channel morphology, and sediment storage along headwater streams of Great Smoky Mountains National Park, Tennessee-North Carolina

摘要

Few studies have investigated channel morphology and sediment storage along headwater streams in the southern Appalachians. This dissertation examines the distribution and geomorphic effects of coarse woody debris (CWD) along second-order streams in Great Smoky Mountains National Park (TN-NC, USA). Along study reaches located in old growth, logged, and debris-slide-affected areas, I categorized 119 CWD channel obstructions based on their size and geomorphic function. I quantified channel morphology and sediment storage associated with these obstructions.;Along debris-slide-affected reaches, I found that CWD typically accumulated into large, complex channel obstructions. In areas unaffected by recent debris slides, CWD obstructions were typically formed by single trees anchored to channel banks, suggesting that transport of CWD is rare along these channels. According to dendrochronological evidence, CWD obstructions along debris-slide-affected reaches have shorter residence times than obstructions on other study reaches, suggesting that debris slides periodically flush CWD from channels. Results show that in-stream CWD loading tends to be higher in old growth watersheds than in watersheds that were logged in the 1920s. However, high CWD loading occurs along logged area streams where steep valley walls border channels and where riparian forests escaped clear-cutting.;Along three study reaches, CWD-influenced aggradation has led to channel abandonment. This finding complicates the interpretation of valley floor landforms because aggraded portions of abandoned channels are higher in elevation, yet younger, than surrounding floodplain surfaces. Results presented in this dissertation indicate that CWD obstructions promote in-channel sediment storage and encourage erosion of channel banks. Overall, however, CWD obstructions stored between three and six times more sediment than was lost due to CWD-influenced bank erosion. CWD-influenced sediment storage was dominated by two large log jams formed by debris slides, which stored 1,200 m3 and 2,800 m 3 of sand and gravel, respectively. The eventual disintegration of these log jams should lead to significant increases in sediment yield. Results from this study indicate that the effects of CWD should be considered in the interpretation of valley floor morphology and in the quantification of sediment storage in forested watersheds.