DYE TRACER EXPERIMENTS ON THE MATANUSKA GLACIER, AK

Theory holds that the subglacial drainage systems of many glaciers evolve through the course of the melt season. However, it is difficult to directly observe or measure the conditions of the inaccessible subglacial hydrologic systems of most glaciers. In an attempt to learn more about the drainage system of the Matanuska Glacier, we conducted multiple dye tracer experiments during July and August, 2000. Discharge is minimal during the colder months when it is sourced by ground water flow and geothermal melting; in the summer, discharge peaks at over 3000 cfs on warm, sunny days. Based on our measurements of subglacial discharge and glacier surface velocity throughout the melt season, we have already proposed a conceptual model of the evolution of the subglacial drainage system (Ensminger et al., 1999, GSA Special Paper 337). This study was designed to test and further refine that model. In a series of experiments, a fluorescent dye tracer was introduced into a superglacial stream flowing directly into a moulin. Automatic sampling systems collected samples every ten minutes for 4 hours at subglacial discharge vents located 300m to 400m downglacier. A flourimeter measured the dye concentration (in mg/L). Results from an experiment conducted in mid July exhibited multiple dye concentration maxima. Other experiments, conducted in late July and August, exhibited simpler dye concentration patterns, with only one maximum. Also, some of the moulin-to-vent connections were lost. This shift in dye curve characteristics matches the expected shift from the "mid-to-late season stage" to the "late-season stage" of our model. As the glacier changes to the "late-season stage", we hypothesize that freezing begins in many of the paths. Freezing closes some connections, causing an increase in storage and a reduction in the previously distributed character of the subglacial drainage system. These changes explain the observed simplification of the dye concentration curves. Our dye tracer experiments support the proposed model, although data only span two of the four proposed stages of development.