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Master of Science (MS)
Grossman, Eric E.
Evaluation of historical meandering since 1937 shows that the active floodplain of the middle reach Skagit River between Rockport and Sedro-Woolley, Washington, has periodically been a significant source of sediment to the lower Skagit River and delta. In response to recent findings that the Sauk-Suiattle on average only supplies approximately 30% of the sediment load observed at Mount Vernon, I examined the geomorphic change and potential sediment production of the middle reach to test whether it is a significant source to the lower river. I tested the hypothesis that the 20-mile (32-km) middle reach has been a net source of sediment to the delta, at least since the mid-2000s that comprehensive sediment load monitoring has been conducted at Mount Vernon. A new tool constructed with ArcGIS model builder that integrates analyses of lateral meander migration from aerial photogrammetry with digital elevation data (e.g. lidar) automates the calculation of sediment volume produced by bank erosion versus stored in bars, islands, and side channels through time. While the results reveal changes in net sediment production through time, they show that for the period 2006 through 2015, recruitment of floodplain sediment from the middle reach to the active channel produced ~27% of the annual sediment mean load measured at Mount Vernon. The sediment source was dominated by lateral incision at rates of 3-8 m/yr in several areas of high-relief (3-15 m) banks characterized by unconsolidated, friable glacial outwash or lahar deposits. Decadal-scale sediment production from the floodplain to the active channel associated with highest meander rates and storage within the reach correlate with periods of frequent moderate floods, whereas periods of net sediment export to the lower river correlate with highest peak flows that presumably erode and flush sediment from the system. These results help quantify recent channel dynamics, rates of change, and sources of sediment that influence sediment transport and recent sediment aggradation patterns downstream, that are in turn important to flood risk and salmon habitat. The results and model also help inform decision makers how these sources of sediment and their impacts may change with projected increases in the magnitude and frequency of floods and sediment production expected with climate change across the Pacific Northwest. The methods and model developed in this study can be readily applied to other systems with historical time-series of mapped floodplain and channel changes to evaluate sediment budgets and impacts to habitats associated with meandering behavior.
Western Washington University
Skagit River (B.C. and Wash.)
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Rothleutner, Amelia Deuell, "Sediment Budget of the Middle Reach Skagit River, Washington 1937-2015 Reveals Decadal Variations in Sediment Export and Storage" (2017). WWU Graduate School Collection. 624.