Physico-chemical heterogeneity in a glacial riverscape

Abstract

Spatio-temporal heterogeneity in physico-chemical conditions associated with the annual expansion/contraction cycle in a complex glacial flood plain of the Swiss Alps was investigated employing a landscape approach. The diverse and dynamic aquatic habitats of the flood plain were visualized as an aquatic mosaic or riverscape. Based on samples collected at ca. monthly intervals for 1.5 yr along 17 floodplain transects, the 3 components of riverscape heterogeneity, extent, composition, and configuration, were quantified using categorical maps and indices of landscape patterns for turbidity and specific conductance. Changes in the spatial heterogeneity of 13 other physico-chemical parameters were further analyzed by means of a within-dates principal component analysis. Riverscape heterogeneity (RH), quantified by applying several indices of landscape pattern to turbidity and specific conductance data, was minimum during groundwater-dominated base flow in winter. Despite an increase in surface connectivity in the channel network with rising discharge, RH rose in spring and summer as additional chemically-distinct water sources (i.e., snowmelt runoff and glacial ablation) contributed to surface flow within the flood plain. Most other physico-chemical variables measured during this study exhibited the same spatio-temporal heterogeneity as turbidity and specific conductance. Overall, the glacial flood plain shifted from a monotonous physico-chemical riverscape in winter to a complex mosaic in summer, this seasonal pattern being clearly driven by hydrological factors operating at the catchment scale rather than by autogenic processes within individual water bodies. Although RH exhibited a predictable annual pattern in response to the seasonal flow regime, we expect the channel network to undergo future modifications from stochastic factors associated with flood events and long-term changes reflecting movements of the glaciers.