MACROPORES WITHIN THE HYPORHEIC ZONE: ABUNDANCE, DIMENSIONS, AND IMPORTANCE AT BASEFLOW

Small-scale exchange of water in streams between channel and subsurface (hyporheic exchange) is beneficial to water quality and stream ecology as it can buffer in-stream temperatures, increase the occurrence and rates of important biogeochemical reactions, attenuate pollutants, and create habitat for organisms. Macropores are physical void spaces within stream bed and bank sediment formed by mechanisms such as burrowing invertebrates, rotting of roots, and microerosion processes. Macropores can increase hyporheic exchange by acting as preferential flow paths. However, few hyporheic zone studies consider how macropores affect stream hydrology or solute transport and there are few data on fundamental characteristics of macropores such as abundance, dimensions, and hydrologic role. In this study we surveyed and quantified reach scale macropore abundance and dimensions (i.e., diameter, depth, height on bank) in five streams of varying land use and gradient near Blacksburg, VA. We also determined baseflow activation of macropores at the reach scale by combining macropore survey data with stage and stream discharge measurements in a HEC-RAS computer model to predict macropore saturation at baseflow. Results show that macropores are common geomorphic features, with average macropore frequencies ranging from 0.65 to 1.08 macropores per meter across the five streams. Results also suggest relationships between macropore frequency and stream geomorphology as well as sediment type. Macropore diameter, depth, and location on bank varied widely, suggesting that macropore hydrologic significance differs among macropores. Macropore activity (i.e., saturation) and significance at baseflow varied among streams, suggesting the importance of the macropore frequency and formation mechanism. This work quantifies the frequencies, dimensions, and baseflow activity of macropores within streams, providing insight into macropore impacts on hyporheic exchange and associated benefits.