Example of the dependence of ice motion on subglacial drainage system evolution: Matanuska Glacier, Alaska, United States

The horizontal ice motion of the Matanuska Glacier, Alaska, was monitored along the western portion of the terminus during late May to the end of August of 1996 and 1997. The daily positions of five (1997) to six (1996) stations anchored into the ice, were measured using total station surveying equipment. The velocity curves for each station were stacked and smoothed to generate one velocity profile for each of the two study periods. The velocity curves for each summer show a similar seasonal trend of velocity increasing abruptly and significantly in early June, attaining a seasonal high in late June and early July, then generally decreasing through mid-August. Data from both field seasons show that velocity increases after mid-August through the end of the field season. In addition, short-term, small-scale fluctuations in the two records indicate the glacier responds to meteorological events, such as sunny periods and rainfall, as illustrated by abrupt changes in velocity. The velocity records are compared to input and output proxies in an attempt to develop a conceptual model for the evolution of the subglacial drainage system throughout the melt season. The current hypothesis is that Matanuska Glacier's subglacial drainage system consists of low, broad canals in subglacial sediment. The velocity records have a hysteretic relationship with the discharge of a stream that is sourced by glacial discharge vents. At the start of the melt season, there are large increases in ice velocity with very little change in stream discharge, indicating the subglacial drainage system has not fully developed to accommodate the increasing influx of meltwater generated by the onset of summer temperatures. It is hypothesized that meltwater influx increases through late June and early July, and subglacial water storage causes increased basal water pressure. Drainage system evolution and increased basal water pressure are suggested to control the horizontal ice velocity. Late melt season ice flow reduces drainage system capacity faster than the meltwater inputs decrease. Some drainage areas may cease discharging water because subglacial pathways become disconnected, which causes storage to again increase.