PHYSICOCHEMICAL CHARACTERIZATION OF CHANNEL TYPES IN A GLACIAL FLOODPLAIN ECOSYSTEM (VAL-ROSEG, SWITZERLAND)

Val Roseg in the Swiss Alps is a complex alluvial valley formed in gla cial outwash. The braided flood plain, 2.6 km long and 130-510 m wide, begins 1.2 km downstream of the glacier terminus and extends to a ''k nickpoint'' at 1990 m a.s.l. where water upwells before entering a con strained reach. A long-term study has been initiated to investigate ha bitat heterogeneity and how such heterogeneity (1) contributes to the biodiversity of benthos, groundwater fauna, and periphyton in a harsh environment and (2) influences ecosystem processes such as productivit y and decomposition dynamics. As a first step we have distinguished di fferent channel types based on the correspondence between hydrological connectivity and physico-chemical attributes. This functional charact erization will serve as a habitat template to structure future ecologi cal research in the Val Roseg flood plain. Six distinct channel types have been identified within the floodplain ecosystem: (i) Main channel , (ii) Side channels, (iii) Intermittently-connected channels, (iv) Mi xed channels, (v) Groundwater channels, and (vi) Tributaries. Distinct seasonal and daily runoff patterns, caused by ice melt, change the hy drological connectivity between individual channel types. Results clea rly demonstrate that the whole flood plain shifts from dominance by su rface water at high summer discharge to a groundwater-controlled syste m in winter. Temporal variability, rather than the means of environmen tal values, has been used to differentiate between individual floodpla in channel types. Groundwater channels exhibit the highest spatial but the lowest temporal variability. In contrast, intermittently-connecte d channels are characterized by a low spatial but an extraordinary tem poral variability. High spatio-temporal heterogeneity resulting from a diversity of channel types is believed to play a major role in mainta ining what appears to be remarkably high biodiversity in this glacial flood plain.