My research shows that logjams may reduce otherwise high amounts of floodplain sediment storage in unconfined valleys, provide hotspots for the transformation of organic matter, and thus limit carbon storage. This research expanded upon the last portion of my dissertation, which examined the role of channel morphology and logjams on floodplain sedimentation and organic carbon storage, to include fluorescence analysis of dissolved organic matter in surface water and sediment through collaboration with Laurel Lynch and Tim Fegel. This complemented surveys of 24 study reaches, over 600 sediment samples, and used five years of logjam surveys in and around Rocky Mountain National Park. The preprint is available here, for the manuscript currently in review. Our results inform two conceptual models described below.
Figure 5. Conceptual figure of sediment dynamics in (A) confined, (B) multithread, and (C) unconfined systems and observed dissolved organic matter dynamics in (D) single thread-multithread-single thread transitional stream segments. (A) Shear stress (τ) is a function of the specific weight of water (γ), stream gradient (S), and flow depth (h), which changes with discharge (Q). In multithread channels (B) where Qt = Q1 + Q2 + Q3 + … Qn for n number of channels, flow depth is not additive as is Qt so h, and thus τ, decrease substantially across the valley bottom. Increased transit time and hyporheic exchange in multithread channels facilitates extraction and mixing of terrestrial and microbial organic matter (OM) from floodplain sediment, increasing the transformation and mineralization of OM and resulting in higher OM diversity within multithread and downstream (D) systems. Unconfined single-thread channels have high in situ OM inputs from seasonal sedges and evidence of groundwater inputs that may cause higher observed OC concentrations (C).