| Summary: | 2018 Summer. Includes bibliographical references. River channels and floodplains transport, transform, deposit, and store organic carbon (OC) as active participants in the carbon cycle. Two of the largest stocks of OC in floodplains include soil and downed large wood (LW). This dissertation investigates floodplain OC stocks in LW and soil, and the geomorphic controls on soil OC stocks in the central Yukon River Basin in the Yukon Flats region of interior Alaska. The Yukon Flats region contains discontinuous permafrost, has a semiarid boreal climate, and has experienced little human modification. Almost all studies of floodplain OC have occurred in the temperate regions, despite permafrost regions storing large amounts of OC in the subsurface due to cold and wet conditions. In addition, relatively little is known about the geomorphic processes that control soil OC distribution on the landscape, particularly over large regions. Wood has been removed for navigation and infrastructure protection in many river corridors, and thus knowledge of natural wood loads, particularly on floodplains, is limited. I first present floodplain downed large wood measurements for the Yukon Flats region, and compare those measurements to downed wood loads in unaltered floodplains in two additional biomes, the subtropical lowlands and the semiarid temperate mountains. Average volumes of downed LW are 42 m3ha-1, 50 m3ha-1, and 116 m3ha-1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. I find patterns in LW loads reflect climatic controls, such as decay rate and primary productivity, as well as increases in floodplain downed wood loads with recent disturbances such as fire. Next, I assess the geomorphic controls on floodplain soil OC concentrations along the Yukon River and four of its tributaries using a large dataset of floodplain soil samples, finding that river basin characteristics and geomorphic unit characteristics likely influence the spatial distribution of soil OC on the landscape. Average ...
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