SUPRAGLACIAL STREAM INCISION INTO DEBRIS-COVERED ICE, MATANUSKA GLACIER, AK

This paper presents new data on the lateral and vertical incision of streams that flow on debris-covered glacier ice at Matanuska Glacier, Alaska. Lateral and vertical incision processes often form meanders on the glacier's surface. We hypothesize that the development of meanders in glacier ice is analogous to the lateral migration of stream channels in alluvium, or else their incision vertically into the ice substrate is more analogous to the vertical incision of bedrock stream channels. To test these hypotheses two streams developed on debris-covered ice were monitored over a six week period from June 1st through July 12th during the 2003 melt season. Meteorological, morphological, hydrological and sedimentological data were collected and anaylzed.

The total ablation recorded at both study sites was 1.7 m. The average daytime temperature recorded by a meteorological station located on the glacier’s moraine was 12.3°C (53.6°F) and the average night time temperature was 9°C (48.2°F). Average daily ablation was directly influenced by daytime temperatures. The daily ablation rate is hypothesized to influence the evolution of the streams’ morphology. Both streams are characterized by meanders having steep overhangs on their cutbank sides, average meander wavelengths of 11.5 m, and average meander amplitudes of 8.6 m. The average recorded surface velocities of the streams are 0.682 m/sec and 0.348 m/sec. Both streams incised vertically ~0.7 m during the study period, but the faster-flowing stream incised laterally twice as much (1.98 m) as the slower stream (0.93 m) and its cutbank had a low-angle overhang (27°) compared to that of the slower stream (45°). The faster-flowing stream was also characterized by suspended-sediment concentrations (0.73g/L ) and bedload volumes (4687.7 cm3) at least twice as great as the slower-flowing stream (0.36g/L and 10.5cm3, respectively). The ice channel of the faster-flowing stream also had more sediment settled on its floor (0.533g/cm2) than did the slower-flowing stream (0.356g/cm2). The fact that the stream with the greater stream capacity incised predominately laterally rather than vertically suggests that this stream was near its local base level. Thus, supraglacial streams developed on debris-covered ice that are near local base level behave like stream channels in alluvium.