Lininger, Katherine Blom, authorWohl, Ellen, advisorCovino, Tim, committee memberLeisz, Stephen, committee memberRathburn, Sara, committee member2018-09-102019-09-062018https://hdl.handle.net/10217/191287River 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 OC concentration within floodplain soil is 2.8% (median = 2.2%). Most floodplain soil OC likely comes from riparian vegetation, which is influenced by channel migration rates and the development of geomorphic units within the floodplain. Greater variability in OC concentrations among geomorphic units compared to among river basins indicates that a bottom-up approach to estimating OC on the landscape (scaling up from small-scale landscape units) may be necessary. Finally, I estimate the soil OC stock in the floodplains of the Yukon Flats and find that my estimate results in approximately an 80% increase in OC stock when compared to a previously published database. The residence time of floodplain sediment is constrained using radiocarbon dates taken from cutbanks, and indicates that OC may be stored in floodplains for over 7000 years before being eroded by the channel. This dissertation provides much needed information on the geomorphic controls on floodplain OC storage in permafrost regions, which are undergoing relatively rapid warming due to anthropogenic climate change. In addition, it highlights the importance of accounting for floodplains as unique landscape units and mediators of OC fluxes, water, and nutrients.born digitaldoctoral dissertationsengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.Text