Groundwater-surface-water interactions in a braided river: a tracer-based assessment

Natural tracers (alkalinity and silica) were used to infer groundwater-surface-water exchanges in the main braided reach of the River Feshie, Cairngorms, Scotland. Stream-water samples were collected upstream and downstream of the braided section at fortnightly intervals throughout the 2001-2002 hydrological year and subsequently at finer resolution over two rainfall events. The braided reach was found to exert a significant downstream buffering effect on the alkalinity of these waters, particularly at moderate flows (4-8 m(3) s(-1)/congruent toQ(30-70)). Extensive hydrochemical surveys were undertaken to characterize the different source waters feeding the braids. Shallow groundwater flow systems at the edge of the braided floodplain, recharged by effluent streams and hillslope drainage, appeared to be of particular significance. Deeper groundwater was identified closer to the main channel, upwelling through the hyporheic zone. Both sources contributed to the significant groundwater-surface-water interactions that promote the buffering effect observed through the braided reach. Their impact was less significant at higher flows (> 15 m(3) s(-1)/>Q(10)) when acidic storm runoff from the peat-covered catchment headwaters dominated, as well as under baseflow conditions (<4 m(3) s(-1)/<Q(70)), when upstream alkalinity was already buffered owing to headwater groundwater sources assuming dominance. The significant temporally and spatially dynamic influence of these groundwater-surface-water interactions was therefore seen to have important implications for both catchment functioning and instream ecology. Copyright (C) 2004 John Wiley Sons, Ltd.