“…As mentioned before, these boundary conditions covary over space and time (Creed et al., 2015), constraining exchange dynamics, residence times, and substrate supply (e.g., Dwivedi et al., 2018; Gomez‐Velez et al., 2017; Song et al., 2020; Wu et al., 2021). Future work should consider the temporal dynamics and correlations of discharge and concentrations (e.g., Dwivedi et al., 2018; Gomez‐Velez et al., 2017), the influence of heterogeneity in sediments (e.g., Sawyer, 2015), and the potential implications of including our conceptualization into reach‐scale water quality models, where the effects of concentrations on reaction rates play a significant role (e.g., Schmadel et al., 2020).…”