MEANDER MIGRATION IN SUPRAGLACIAL STREAMS, JUNEAU ICEFIELD, ALASKA

The rate and direction of migration of stream meanders remains poorly understood, largely because of the time scale needed to assess form-process adjustments in alluvial rivers. The difficulty in understanding controls on meander migration can be partially overcome by examining supraglacial streams as an analogue. Research was conducted on supraglacial streams in a firn field of the Vaughan Lewis-Gilkey Glacier Research Area on the Juneau Icefield, Alaska, as part of the Juneau Icefield Research Program. Supraglacial streams are carved into the surface of glaciers where meltwater can downcut faster than the rate of glacier surface ablation. Supraglacial stream meanders are best formed in firn in areas with few crevasses. Each day a longitudinal groove forms in supraglacial stream channels caused by diurnal fluctuations in stream discharge. In meandering reaches of supraglacial streams, the meanders migrate simultaneously with the formation of the daily grooves, leaving a three-dimensional record of daily meander position on the channel walls.

With respect to direction of meander migration, Extension dominates over translation in meanders with high sinuosity, low radius of curvature, and low peak daily discharge. In this type of meander, maximum shear stress occurs on the outer bank near the apex of the meander bend. Conversely, translation dominates over extension in meanders with low sinuosity, high radius of curvature, and high peak daily discharge. In this type of meander, maximum shear stress occurs on the outer bend downstream from the apex of the meander bend. With respect to rate of meander migration, extension and translation both increase as peak daily discharge increases. Meanders in our supraglacial streams migrate 8 to 77 centimeters per day, proportional to what might be expected in alluvial rivers over one or more decades.