Influence of ground water on surface water conditions in a glacial flood plain of the Swiss Alps

Hydrological exchange processes in alluvial flood plains occur along several major pathways operating at different scales. Ground water–surface water interactions exert major control on structural and functional attributes of stream ecosystems. The role of ground water on environmental conditions in surface water habitats was investigated as part of a holistic ecological study of Val Roseg, a complex glacial flood plain in the Swiss Alps. Six channel types were identified based on surface hydrological connectivity with the main channel and water source: the main channel, side channels, intermittently-connected channels, mixed channels, groundwater channels, and tributaries. The groundwater channels have no upstream connection with surface channels since they are fed by alluvial or tributary aquifers. Compared with other channel types, groundwater channels are characterised by relatively high and constant temperatures, low turbidity, and high specific conductance, bicarbonate and nitrate concentrations. Different types of groundwater habitats were delineated, based on their silica concentrations and temporal patterns of specific conductance. Increases in the concentrations of major ions with declining discharge clearly show a shift from a glacier meltwater-dominated flood plain in summer to a groundwater-dominated system in autumn and winter. A conceptual model was developed to portray the different groundwater reservoirs and flux pathways thought to structure the habitat template of the Val Roseg hydrosystem. This model serves as a framework for future studies of the role of ground water in sustaining what appear to be diverse and abundant benthic and interstitial communities in this harsh environment.