National-extent Modeling of River Corridor Functions: What Level of Process Representation Improves Management Outcomes for Water Quality and Aquatic Ecosystems?

National-extent hydrologic models provide a consistent framework to understand water runoff and storage, streamflow generation and routing, water use and return flows, transport and fate of constituents, and health of aquatic ecosystems. A key challenge is upscaling river corridor processes to integrate with national-extent watershed and groundwater components. For more than 50 years, river corridor modeling focused mainly on empirical routing of hydrographs. Here we discuss advances in the framework for river corridor modeling that improve the physical basis of water quality and ecological flow modeling. Previously, most water-quality models specified reactivity as spatially uniform "whole-stream" retention coefficients based on poorly estimated travel times. Channel hydraulic geometry modeling, grain size estimation, and related information influencing reactivity is increasingly available and we describe a national synthesis of these underutilized data, applied to improve estimation of in-channel residence times and hydrologic exchange fluxes that bring main channel waters into contact with reactive sediment interfaces. We discuss how improved data sources are used with hybrid physically-based and statistically-based water quality models to arrive at spatially explicit estimates of nutrient uptake rates that distinguish the relative roles of modeled and GIS-mappable river corridor elements (e.g., streambed hyporheic zones, riparian bank areas, floodplains, small ponds, and lakes and reservoirs). We found that improving the physical basis of the river corridor in water-quality predictions allows better identification of dominant reaction zones where constituents are stored and transformed. Such improvements are a fundamental step towards prioritization of management strategies, such as channel bed management, riparian bank restoration, or floodplain reconnection, for improved river water quality and aquatic ecological functions. This work evolved from activities of the River Corridor Synthesis Working Group at the USGS J.W. Powell Center.