FATE OF COARSE SEDIMENT IN THE SUBGLACIAL DRAINAGE SYSTEM OF THE MATANUSKA GLACIER, ALASKA

Melt-water streams draining temperate glaciers are known to transport considerable sediment via suspended and bed load. However, recent studies of sediment in meltwater discharging from vents along the margin of the Matanuska Glacier in south-central Alaska show bed load to be <1% of the total load despite the availability of coarse material at and near the glacier bed. A possible explanation for this is that much of the coarse material is being trapped within the subglacial drainage system due to an adverse slope of the glacier bed near the terminus. An estimate of the mass of coarse material potentially being trapped was calculated by determining the grain-size distribution of the debris-rich basal ice and subglacial sediment at the Matanuska and measuring the suspended sediment flux in meltwater streams draining the glacier for a period of five consecutive melt seasons. The results show that 1.14 x 10^8 to 8.72 x 10^8 kg of coarse material could be trapped at the glacier bed each year. If deposited within 1 km of the glacier terminus this would be equivalent to a layer 1.2 to 9.4 cm thick. How the coarse sediment might be distributed, however, would depend on the geometry of the adverse slope(s) and the type and pattern of the subglacial drainage system.