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Braided river response to eight decades of human disturbance, Denali National Park and Preserve, AK




Richards, Mariah, author
Rathburn, Sara, advisor
Booth, Derek, committee member
Nelson, Peter, committee member
Wohl, Ellen, committee member

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The spatial complexity and stochastic nature of braided rivers complicate our ability to quantify natural rates of sediment transport and limit our understanding of braided river response to human disturbance. The Toklat River in Denali National Park and Preserve, a 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 adjacent to the Denali Park Road. A unique, multi-scalar and temporally diverse dataset records the responses of the river over a 10-km reach. I evaluated trends in short-term sediment storage through LiDAR differencing and analyzed long-term planform change using braiding index, braiding beltwidth and topographic ruggedness 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 these human disturbances on channel and planform change. Comparisons of 2009 and 2011 LiDAR-derived DEMs showed a statistically significant volumetric loss of -30,300 ± 27,600 m3 over 4 km of active braidplain within the study reach. Braidplain sediment loss adjacent to the channel-confining Denali Park Road bridge crossing was comparable to that removed biennially through gravel extraction downstream (17,100 m3). Upstream of both the gravel extraction site and the bridge crossing, the braiding beltwidth decreased by 400 m and the braiding index lowered from eight to one between 1988 and 2011. The reference reaches did not display such noticeable morphologic adjustments, implying upstream migration of gravel extraction and confinement impacts, which can significantly alter flow character, leading to increased localized stream power, degradation and infrastructure damage. These results are relevant to assessing the variety and spatial extent of human disturbance on braided river systems in general.


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