Logjams and Channel Morphology Influence Sediment Storage, Transformation of Organic Matter, and Carbon Storage Within Mountain Stream Corridors

Abstract

AbstractThe flow of organic matter (OM) along rivers and retention within floodplains contributes significantly to terrestrial carbon storage and ecosystem function. The storage and cycling of OM largely depend upon hydrogeomorphic characteristics of streams and valleys, including channel geometry and the connectivity of water across and within the floodplain. To examine the role of river morphology on carbon dynamics in mountain streams, we (a) quantify organic carbon (OC) storage in fine sediment, litter, and wood along 24 forested gravel‐bed stream reaches in the Rocky Mountains of CO, USA, (b) examine morphological factors that regulate sediment and OC storage (e.g., channel width, slope, logjams), and (c) utilize fluorescence spectroscopy to examine how the composition of fluorescent dissolved OM in surface water and floodplain fine sediment are influenced by channel morphology. Multivariate regression of the study reaches, which have varying degrees of confinement, slope, and elevation, indicates that OC storage per area is higher in less confined valleys, in lower gradient stream reaches, and at higher elevations. Within unconfined valleys, limited storage of fine sediment and greater microbial transformation of OM in multithread channel reaches decreases OC storage per area (252 ± 39 Mg C ha−1) relative to single‐thread channel reaches (346 ± 177 Mg C ha−1). Positive feedbacks between channel morphology and persistent channel‐spanning logjams that divert flow into multiple channels may limit the aggradation of floodplain fine sediment. Although multithread stream reaches are less effective OC reservoirs, they are hotspots for OM decomposition and provide critical resources to downstream food webs.