Benthic responses to groundwater–surface water exchange in 2 alluvial rivers in northwestern Montana

Abstract

AbstractWe tested the hypotheses that groundwater–surface water exchange regimes affect spatial distribution of algal biomass and Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in main-channel riffle habitats of 2 northwestern Montana rivers flowing through alluvial flood plains. We used a stratified random design to sample riffles with contrasting groundwater–surface water exchange regimes, but with similar bedform, current velocity, and substrata grain size. We examined general patterns of exchange between river channel and hyporheic zone waters by measuring vertical hydraulic gradients (VHG) and hydraulic conductivities using mini-piezometers. Riffles near the upstream limit of each flood plain were characterized by strong, hyporheic recharge (−VHG, downwelling), whereas riffles throughout the lower half of each flood plain were characterized by weak, dispersed hyporheic discharge (+VHG, upwelling). There were no differences in mean seston concentrations between riffles on either flood plain wit...