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 pat terns, lotic ecosystem size, and surface water chemistry were investigated in a glacial floodplain of the Swiss Alps (Val Roseg). Discharge, the exten t and form of the stream network, specific conductance, and concentrations of major ions were measured periodically at multiple sites across the flood plain 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 sour ces and pathways of water. The seasonal changes in discharge, spatiotempora l patterns of channel length, and water chemistry within the floodplain are linked to the shifting dominance of different hydrologic reservoirs and fl ow paths within the catchment. During the expansion phase in spring, snowme lt is the main source of water for the entire floodplain, although it circu lates via subsuface pathways on the catchment slopes before entering the fl oodplain. Peak discharge and maximum expansion of the channel network in su mmer are associated with the melting of the glaciers. Englacial water enter s the floodplain mainly via surface flow paths and is distributed across th e floodplain via surface connectivity and shallow groundwater pathways. Dur ing the contraction phase in autumn, seepage of subglacial water from the m ain channel is the primary water source for the upper floodplain. In the lo wer floodplain, surface flow is sustained by upwelling of deep alluvial and hillslope groundwaters. The relative contribution of deep groundwaters inc reases during winter, a phase of low discharge and maximum contraction of t he channel network.