THE INFLUENCE OF CHANNEL MIGRATION RATE AND GRAIN SIZE ON DIFFERENCES IN FLOODPLAIN ORGANIC CARBON STORAGE BETWEEN TWO RIVERS IN INTERIOR ALASKA

Research on the terrestrial carbon cycle has not adequately addressed floodplain organic carbon (OC) storage at timescales of 10¹-10³ years. Floodplains provide a temporary storage area for sediment and associated OC, but many questions remain regarding differences in floodplain sediment OC storage across diverse river systems as a result of geomorphic influences. Rates of channel migration and floodplain erosion and the dominant grain size of the floodplain likely influence large-scale spatial patterns of floodplain OC storage and the residence time of floodplain sediment and OC. We present results of floodplain OC storage in sediment within the active layer of the floodplain along two rivers in interior Alaska: the Dall River (meandering single-thread) and Preacher Creek (multithread with braided channel characteristics). We expected the Dall River floodplain to have higher OC storage per unit area than Preacher Creek in part due to differences in rates of floodplain erosion and grain size. Floodplain OC storage was determined from sediment samples collected along transects at 5 sites along the Dall River (drainage areas 780 - 2720 km²), and 4 sites along Preacher Creek (drainage areas 1230 - 2780 km²). We estimated channel migration rates using aerial photos from the 1970s/1980s and 2011. We also used tree cores to estimate rates of channel migration, and we determined soil texture class of sediment as a measurement of dominant grain size. Average values of floodplain sediment OC along the Dall River and Preacher Creek are 243 and 176 Mg C ha^-1, respectively, and the Preacher Creek dominant soil texture class is coarser than that of the Dall River. Segment-averaged rates of channel migration obtained via aerial photo analyses range from 0.08 to 0.4 m yr^-1 for the Dall River and 1.9 to 2.6 m yr^-1 for Preacher Creek. Rates of channel migration obtained through dendrochronology are slower and more uncertain than those found via aerial photo analysis. The smaller average value of OC storage per area along Preacher Creek is likely linked to the faster rate of floodplain turnover caused by channel migration and the differences in grain size. This work helps to constrain the large-scale spatial distribution of carbon storage within the landscape in interior Alaska, a region undergoing rapid climate change and permafrost thaw.