Processes of surface-subsurface water exchange in a low energy sand-bed stream

The surface-subsurface water exchange at base-flow conditions was assessed in a natural sand-bed stream. The aim was to examine the relative contribution of sediment turnover and vertical hyporheic flow in a natural environment. Sediment turnover was determined using coloured sand columns implanted into the stream bed. Direction and velocity of hyporheic flow was recorded with a fluid tracer injected into the stream bed. Large amounts of wood caused a heterogeneous stream bed relief. Grain size distribution of sediments was homogenous with a porosity of 37%. The hydraulic conductivity ranged from 3 (.) 10(-6) m s(-1) to 1 (.) 10(-4) m s(-1) with a mean of 7 . 10(-5) m s(-1). Groundwater upwelling was a major process (0.39 1 m(-2) h(-1)) and stream water was passing through the hyporheic zone at a rate of 0.12 1 m(-2) h(-1). Since hyporheic velocity was high in comparison to flume studies, the transfer of stream water through bed sediments might have been induced by the wood in addition to local topography in the sand-bed. Sediment displacement at base-flow was found in 79% of the stream bed. Release and trapping of pore water by sediment turnover contributed greatly to the surface-subsurface water exchange. Pumping and sediment turnover were significant processes of the vertical connectivity in the natural sand-bed stream.