Upgradient Storage Features Control Timing and Magnitude of Solute Export at Stream Outlets

Abstract Concentration‐discharge (C‐Q) relations are used to infer water and solute storage and transport but are often informed by coarse‐temporal data. Here, we use high‐frequency and long‐term C‐Q datasets from two adjacent, but geologically and geomorphically distinct, watersheds in Colorado to evaluate how vertical and lateral flowpaths collectively control outlet water quality. We calculate C‐Q slopes at monthly, annual, and long‐term time scales and evaluate hysteresis behavior for three major solute classes—geogenics, metals, and biogenics. Geogenic C‐Q behavior is the most consistent across temporal scales and reflects vertical flowpath shifts from snowmelt to groundwater. In contrast, metals and biogenics have greater variability, particularly during high‐flow periods, as driven by laterally distributed landscape features, such as mine adits, an iron fen, floodplains, wetlands, and beaver ponds. Lateral heterogeneity in solute stores exerts major control on downstream chemistry and should be considered when predicting water quality changes.