Shifting dominance of subcatchment water sources and flow paths in a glacial floodplain, Vol Roseg, Switzerland

The influence of river-catchment hydrological interactions on discharge patterns, lotic ecosystem size, and surface water chemistry were investigated in a glacial floodplain of the Swiss Alps (Val Roseg). Discharge, the extent and form of the stream network, specific conductance, and concentrations of major ions were measured periodically at multiple sites across the floodplain from August 1996 to January 1998. These data are used to (1) describe the seasonal cycle of contraction and expansion of the floodplain channel network and (2) identify hydrochemical tracers for detecting different sources and pathways of water. The seasonal changes in discharge, spatiotemporal patterns of channel length, and water chemistry within the floodplain are linked to the shifting dominance of different hydrologic reservoirs and flow paths within the catchment. During the expansion phase in spring, snowmelt is the main source of water for the entire floodplain, although it circulates via subsuface pathways on the catchment slopes before entering the floodplain. Peak discharge and maximum expansion of the channel network in summer are associated with the melting of the glaciers. Englacial water enters the floodplain mainly via surface flow paths and is distributed across the floodplain via surface connectivity and shallow groundwater pathways. During the contraction phase in autumn, seepage of subglacial water from the main channel is the primary water source for the upper floodplain. In the lower floodplain, surface flow is sustained by upwelling of deep alluvial and hillslope groundwaters. The relative contribution of deep groundwaters increases during winter, a phase of low discharge and maximum contraction of the channel network.