Subglacial hydraulics of South Cascade Glacier

The subglacial hydraulics of South Cascade Glacier are examined using natural variations of water and solute flux discharged from the glacier, the travel time and dispersion of artificial tracers injected into the glacier and the magnitude and variation of water levels in boreholes drilled to the glacier bed. The observations are unique in scope and temporal coverage, and present the first comprehensive picture of the drainage system of any glacier. The glacier is divided into three drainage basins, each drained by a single stream. The two largest basins account for about 99% of the glacier area and are the subject of this dissertation.;In the main basin, an englacial conduit system and a distributed subglacial system drain to a single subglacial conduit that exits the glacier. The englacial conduit system is arborescent, such that smaller conduits feed into fewer larger conduits. Most of the surface meltwater drains directly into the conduit system and bypasses the subglacial flow system. The subglacial system is distributed and is characterized by high pressures, close to the ice overburden pressure. The spatial pattern of pressures indicate a localized low pressure hydrologic sink, representing drainage to the main basal conduit. Assuming Darcian flow in the subglacial system, the range of transmissivity is 10;The second stream drains a basin that is roughly half the size of the main basin. Unlike the main basin, most of the water is routed through a distributed subglacial flow system. Observations in ice tunnels and calculations, based on data derived from tracers, indicate that the water probably flows through interconnected bands of highly permeable debris. Model calculations indicate that flow through the debris layer can explain the observed variations of this stream.;These results challenge the assumption that subglacial hydraulics are uniform. Independent subglacial systems of different hydraulic characteristics can exist in adjacent drainage basins. Within each basin the hydraulics can be spatially nonuniform. Englacial and subglacial flow systems can be coupled in series, which transfers surface water directly to the bed, or coupled in parallel, which isolates the bed from direct water input. These findings have important consequences for predictive models of glacier movement--it may be equally important to understand how water reaches the subglacial system as it is define the hydraulics of the subglacial system.