Geomorphic implications of subglacial drainage configuration: rates of basal sediment evacuation controlled by seasonal drainage system evolution

Although basal sediment evacuation by subglacial meltwater is widely assumed to dominate the sediment budget of most temperate glaciers and ice caps, its potential influence for glacial geomorphic processes and landforms has largely been overlooked in the glacial literature. Data collected at Haut Glacier d'Arolla, Switzerland during the 1998 ablation season are used to test theoretical predictions that subglacial drainage configuration should exert a primary control on rates of basal sediment evacuation by subglacial meltwater. Seasonal evolution of the subglacial drainage system enables observation of sediment evacuation from both distributed and channelised drainage systems. Daily suspended sediment load exhibits an extreme, nonlinear increase with respect to discharge between winter and summer periods, which correspond to periods of predominantly distributed and channelised subglacial drainage, respectively. These results suggest that subglacial drainage configuration should significantly impact upon: (a) rates of subglacial erosion and debris production; (b) the nature of debris transport and the proportion of sediment in glacial versus fluvioglacial transport pathways; and (c) rates and styles of ice-marginal sedimentation. In contrast to their distributed counterparts, channelised drainage configurations are predicted to encourage subglacial erosion through the removal of basal sediment from the ice–bedrock interface and, with the exception of supraglacial transport, will discourage sediment transport by alternative glacial mechanisms. Direct sedimentation at the glacier margin may therefore be reduced, resulting in smaller moraines despite an overall increase in sediment production. An appreciation of the coupling between glacier hydrology and geomorphic processes suggests new frameworks for the understanding of rates and styles of glacial erosion, sediment transfer and deposition, and the complex geomorphology of ice-marginal environments.