HOW MIGHT LOGJAMS REDUCE FLOODPLAIN FINE SEDIMENT AGGRADATION AND ORGANIC CARBON STORAGE IN MOUNTAIN STREAMS?

Large in-stream wood and logjams have been linked to retention of sediment and the storage of organic carbon along river corridors. We find that unconfined valley segments along mountain streams of the Colorado Front Range store more carbon per area than more confined valley segments, but increased storage is also linked to lower-gradient streams and those at higher elevation. Among the unconfined valley segments, stream reaches with only a single channel of flow across the valley bottom store more carbon per area than more complex channel reaches with numerous channels of flow across the valley bottom, induced by positive feedbacks associated with logjam occurrence. Storage of organic carbon in these multithread channel segments appears to be limited by two factors related to the abundance of logjams and in-stream wood: (1) a reduced capacity to store floodplain fine sediment and (2) breakdown of organic carbon in multithread hotspots with increased channel complexity. When also considering elevation, drainage area, stream gradient, and valley width among the unconfined valley segments, those with more abundant logjams retain shallower fine sediment and lower organic carbon content within floodplain sediment. Examination of fluorescence signatures of surface and soil water shows that complex multithread segments likely facilitate the transformation and mineralization of organic carbon. We provide a conceptual model that describes how logjams and channel complexity are linked to observed changes in fluorescence signatures and measured depths of fine sediment.