Collaborative Research: Changes in hyporheic exchange and nitrous oxide generation due to streambed alteration by macro-roughness elements

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Boulders and large cobbles are ubiquitous roughness elements in mountain rivers and their presence provides vital ecological and hydromorphological functions. They are routinely used in restoration projects to increase habitat quality for many aquatic species because they impact on stream morphology by inducing erosional and depositional areas, whose size and extent depend on the boulder/cobble density, sediment inputs, and flow conditions during sediment mobilizing flows. However, little is known about the role of roughness element induced-streambed changes in regulating hyporheic exchange, which is the process moving stream water into and out of stream bed sediments to form the subterraneous hyporheic zone. The hyporheic zone promotes important biochemical transformations, which affect stream and pore water chemistries with implications at both local and global scales because it is a primary driver of bioactivity in streams and a key source of the greenhouse gas nitrous oxide, N2O. Sustainable management of water resources including river restoration (a $1B/year industry in the US) would benefit from knowledge of hyporheic processes to predict the effects of regulated flows, climate, and land use on ecosystems, nutrient cycles, and solute transport at the local and watershed scales. The scientific goal for this project is to understand, quantify, and model the effects of boulder and cobble density and relative sediment supply during forming flows on hyporheic exchange, chemistry, and N2O production during low flow conditions. This will be addressed with flume and field experiments, supported by analytical and numerical modeling, and tested with hypotheses that boulder or cobble-induced changes in streambed morphology (1) have a primary role in regulating both the quantity and chemistry of hyporheic exchange at the reach scale, and (2) alter aerobic respiration, nitrification, and denitrification rates and increase the rates of apparent production and release of N2O.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.