IMPACTS OF STREAM RESTORATION ON SURFACE WATER-GROUNDWATER INTERACTIONS, STREAMBED REDOX CONDITIONS, AND NUTRIENT DYNAMICS

Stream restoration is a billion dollar industry in the U.S., aimed at returning degraded streams to their pre-disturbance condition or correcting a specific, negative human impact. Surface water-groundwater (SW-GW) interactions are a critical component of stream ecosystem function, and there has been a call for increased emphasis on SW-GW interaction as an integral part of river restoration. Here, we explore how stream restoration by Natural Channel Design (NCD) impacts rates of SW-GW exchange, and associated redox conditions and nutrient dynamics in streambeds. These impacts are considered in the context of how they may impact nutrient export to inland or coastal receiving waters. We assess the impact of NCD by comparing restored streams to their reference sites, by monitoring changes at a restored reach from pre- to post- restoration, and by comparing features of several restored streams to identify universal characteristics of restored systems. Our findings suggest reference reaches have a mixture of discharging groundwater and stream water in the hyporheic zone, while restored reaches have predominantly stream water in the hyporheic zone; as a result, upwelling groundwater has a stronger influence on streambed geochemistry at reference sites. The most rapid rates of SW-GW exchange were found immediately upstream of cross-vanes (a common feature in NCD), with downwelling rates > 3 m/day - an order of magnitude higher than rates observed at reference or pre-restoration reaches. The rapid rates of SW-GW exchange immediately adjacent to cross-vanes result in short hyporheic residence times and limited biogeochemical transformations of dissolved solutes. Distinct patterns of biochemical cycling in the hyporheic zone at restored sites are driven by moderate rates of SW-GW exchange flux around secondary bedforms, such as pools and riffles. Despite clear patterns of nutrient transformations in the hyporheic zone at restored sites, overall rates of stream water exchange with the hyporheic zone appear relatively small (e.g. ~0.4% of stream discharge within a 45-55 m reach). For this reason, the primary ecological benefit of stream restoration appears to be development of heterogeneous patches of varying water exchange flux and redox conditions, rather than net changes in nutrient loads in streams.