From mountains to the ocean: quantifying connectivity along the river corridor

Rivers are the landscape's arteries; they convey water, solutes, energy, and living organisms from the hillslopes, floodplains, aquifers, and atmosphere to the oceans. As water moves along this complex circulatory system, it is continuously exchanged with the surrounding alluvial aquifer, termed hyporheic exchange, which strongly conditions and constrains the biogeochemical evolution of water at the local scale with basin-scale consequences. Over the last two decades, considerable efforts have focused on the use of detailed mathematical models to explore the hydrodynamics and biogeochemical effect of hyporheic exchange at the scale of individual channel morphologies. While these efforts are essential to gain mechanistic understanding, their computational demand makes them impractical for basin applications. In this talk, a parsimonious but physically based model of hyporheic flow for application in large river basins is presented: Networks with EXchange and Subsurface Storage (NEXSS). At the core of NEXSS are the up-scaling of detailed mathematical models and a characterization of the channel geometry, geomorphic features, and related hydraulic drivers based on scaling equations from the literature and readily accessible information such as river discharge, width, grain size, sinuosity, channel slope, and regional groundwater gradients. As a proof-of-concept, we use NEXSS to characterize the spatial and temporal variability of hyporheic exchange and denitrification potential along the Mississippi River basin. This modeling approach allows us to map the location of critical hot spots for biogeochemical transformation, their geomorphic drivers, and cumulative effect. Finally, we discuss new avenues to incorporate exchange with floodplains and ponded waters, which also play a key role in water quality along the river corridor. This new modeling approach is critical to transition from purely empirical continental models of water quality to hybrid approaches that incorporate fundamental physics and take advantage of available hydrogeomorphic data. In particular, hybrid models will be instrumental for predicting outcomes of river basin management practices under present and future socio-economic and climatic conditions.