Surface water and streambed sediment interaction: The hyporheic exchange
Stream and pore waters continuously interact and mix within streambeds due to spatial and temporal variations in channel characteristics (e.g., spatiotemporal variations of streambed pressure, of volume of alluvial material surrounding a river, of streambed hydraulic conductivity, and sediment transport). This mixing is typically referred to as hyporheic exchange and the hyporheic zone defines the interfacial zone between rivers and their surround aquifers and riparian zones. It is an important ecotone and a place where many biogeochemical reactions occur such as nitrification and denitrification. The significance of hyporheic exchange in affecting surface and subsurface water quality and linking fluvial geomorphology, groundwater, and riverine habitat for aquatic and terrestrial organisms has been emerging in recent decades as an important component of conserving, managing, and restoring riverine ecosystems. In this chapter, we present the concepts, characteristics and environmental effects of hyporheic exchange, and we review the methods for measuring and predicting its characteristics, i.e. hyporheic flux and hyporheic residence time.