EP33A-1041
Braided River Response to Eight Decades of Human Disturbance, Toklat River, Denali National Park and Preserve, AK
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Mariah Ellis Richards, Colorado State University, Geosciences, Fort Collins, CO, United States; Denali National Park, Physical Sciences, Denali National Park, AK, United States, Sara L Rathburn, Colorado State University, Fort Collins, CO, United States, Derek B Booth, University of California Santa Barbara, Santa Barbara, CA, United States, Denny Capps, Denali National Park, Denali National Park, AK, United States and Ellen Wohl, Colorado State University, Department of Geosciences, Fort Collins, CO, United States
Abstract:
The spatially complex and stochastic nature of braided rivers complicates quantifying natural rates of sediment transport and limits our understanding of braided river response to human disturbance. The Toklat River in Denali National Park and Preserve, a 135-km-long braided tributary of the Kantishna River draining the north-facing slopes of the Alaska Range, exemplifies these challenges. Eight decades of localized channel confinement due to installation of a causeway in the 1930’s and over three decades of gravel extraction since the 1980’s have occurred on the Toklat River where it crosses the Denali Park Road. Research associated with these disturbances has developed a unique multi-scalar and temporally diverse dataset that records the responses of the river over a 10-km reach. We have evaluated trends in short-term sediment storage through lidar differencing and analyzed long-term planform change through morphologic metrics such as braiding index, active braidplain width, and percentage of occupied floodplain derived from aerial photographs. Two reference reaches along comparable adjacent braided rivers, with varying levels of confinement and no gravel extraction, illuminate the relative influence of confinement and gravel extraction on sediment flux and planform changes. Preliminary comparisons of lidar-derived DEMs show volumetric decrease in sediment within the active braidplain within the gravel extraction area and downstream of the causeway, suggesting enhanced degradation due to both extraction and confinement. Areas of braidplain degradation also show increasing variance in surface elevations and decreasing braidplain width and braiding index with time. These data support the usage of morphologic metrics to identify long-term degradation trends in large, braided systems where detailed elevation data are unavailable. Despite the pronounced local expression of sediment discontinuity, watershed topography and zones of long-term sediment storage in this system appear to spatially constrain morphologic responses triggered by human disturbances. These findings suggest a spectrum of river resilience relevant to Park management of the Toklat River in particular, and valuable to improving contextual assessments of disturbed braided river systems in general.
