Large-scale patterns of hyporheic exchange are predictable within some river systems, but our understanding of the factors driving hyporheic processes and the magnitude of hyporheic exchange needed to influence biophysical patterns at larger scales remains limited. We investigated the patterns, magnitude, and potential effects on biota of reach-scale hyporheic exchange in an alluvial river of the Pacific Northwest. The river was topographically similar to and in the same geographic region as other systems where large-scale hyporheic exchange and associated biological responses have been observed. We hypothesized that predictable reach-scale patterns of hyporheic exchange would occur in alluvial valley segments of the river and that hyporheic upwelling would be associated with reach-scale patterns of physical and biological characteristics. We used in-channel piezometers and synoptic stream flow measurements to quantify hyporheic exchange. We measured temperature, dissolved O2, pH, specific conductivity, chlorophyll a biomass, primary production, and benthic macroinvertebrates as indicators of physical and biological responses. Contrary to our expectations, we found no evidence, physical or biological, of reach-scale hyporheic exchange. Hyporheic connectivity in this river system probably is constrained by geologic and geomorphic characters as well as the legacy of human land use in the basin. Thus, our results illustrate the variability of hyporheic processes that can occur among alluvial river systems and may have implications for watershed management.
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