Importance of Meander to Reach Scale Heterogeneity in Stream Chemistry Changes
Patricia M. Fox1, John N. Christensen1, Dipankar Dwivedi1, Bhavna Arora1, Cam Anderson2, Curtis Beutler1, Rosemary W. H. Carroll, Christian Dewey3, Wenming Dong1, Boris Faybishenko1, Michelle Newcomer1, Alexander W. Newman1, Kenneth H. Williams1, Peter Nico1* (firstname.lastname@example.org), Eoin Brodie1
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of Massachusetts, Amherst, MA; 3Oregon State University, Corvallis, OR
Understanding river corridor biogeochemical function and scaling up that understanding to watershed scale requires an understanding of the functional traits within the river corridor and how they interact to control river chemical outputs. Hypothesized important traits that have been investigated in the East River system are relief, bedrock composition, meander morphology, and the composition of intrameander sediments. The impact of these traits has been evaluated at the single meander (10s of m), short-reach (~1 km), and long-reach (10s of km) scale. At the long reach scale, Arora et al. (2020) identified bedrock composition as a key trait with higher solute concentrations in shale-dominated upper reach as compared to a granodiorite-dominated reach. River water constituents derived primarily from shale included calcium, dissolved inorganic carbon, dissolved organic carbon, magnesium, molybdenum, nitrate, and sulfate ions. However, the ultimate exports to the river were controlled by coupling with other traits in a chemical species-specific way. For example, Fox et al. (2022) showed that intrameander floodplain sediments mediated export of hillslope shale-derived sulfate to the river through sulfate redox cycling coupled with river hydrologic stage. The importance of river stage coupling with intrameander sediments was also shown in the case of nitrate. Dwivedi et al. (in preparation) showed that the rising limb has the highest denitrification potential, followed by the base flow and recession limb. Additionally, meanders with larger amplitude exhibited higher denitrification potential due to longer lateral flow paths through intrameander sediments. However, highlighting the challenge of scaling, Fox et al. (in preparation) showed that river chemistry changes due to lateral hyporheic exchange were concentrated at key locations along apparently geomorphologically similar river reaches. The locations of these disproportionate solute inputs stayed the same year over year, were independent of discharge magnitude, and were likely due to the largely stable texture structure of the meander sediments. However, discharge magnitude did impact the magnitude of solute inputs, with extreme discharge years, high or low, showing greater solute inputs than average years. Taken together, these studies highlight how the coupling of traits leads to emergent behavior observable at the river corridor scale and provide insights into how increased hydrologic variability will impact river water chemistry. They also demonstrate that detailed trait characterization (e.g., textural structure) is important for accurate scaling.