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Abstract
Riverine processes play important roles in global biogeochemical cycles. However, current modeling frameworks for scaling biogeochemical processes in rivers lack fundamental hydrologic and ecological controls on biogeochemical cycles. We used several approaches to improve our understanding of scaling riverine biogeochemical processes. First, we evaluated common modeling assumptions about river and catchment hydrogeomorphology and biogeochemistry, by scaling headwater stream denitrification measurements to eight small river networks. Using the model results, we identified additional factors important for understanding biogeochemical cycling, and illustrated strategies for improving river biogeochemistry simulation. River network model results revealed the importance of incorporating hydrologic linkages between the river channel, floodplain surface, and hyporheic zone as a basis for scaling biogeochemical cycles. Thus, we parameterized a detailed three-dimensional hydrogeomorphic model for the 16 km2 Nyack Floodplain on the Middle Fork Flathead River, Montana, and used hydrologic model results as a basis for 1) evaluating the influence of floodplain surface and hyporheic storage on hydrologic residence time (i.e., mean matrix traversal time; MTT), 2) analyzing observed patterns of hyporheic carbon quantity and quality, and 3) scaling an interdependent set of flow path dissolved oxygen and nitrate models to the whole Nyack Floodplain study area. Our hydrologic residence time results revealed the importance of floodplain surface and hyporheic storage for MTT, specifically that whole-floodplain MTT was strongly correlated with hyporheic exchange. Dissolved organic carbon (DOC) concentration decreased with MTT but DOC lability increased, suggesting that although the Nyack hyporheic zone is a net sink for DOC, recalcitrant DOC is replaced with more bioavailable DOC along hyporheic flow paths, increasing the lability of DOC transported downstream. Our biogeochemical model explained 67% and 27% of the variance in dissolved oxygen and nitrate measurements, respectively, that spanned the floodplain longitudinally, laterally, and vertically, and river discharge conditions and seasons. Paired with a realistic model of floodplain hydrogeomorphology, relatively simple biogeochemical models explained complex patterns of observed biogeochemical dynamics observed throughout the hyporheic zone. Thus, understanding the physical template that drives hydrologic flux and storage of biogeochemical constituents is fundamental for understanding biogeochemical cycles across large spatial and temporal scales.