CHANNEL DEVELOPMENT OF SUPRAGLACIAL STREAMS IN CLEAN ICE: IMPLICATIONS FOR UNDERSTANDING BEDROCK-CHANNEL FORMATION

Field observations and measurements of supraglacial streams at the Matanuska Glacier hold implications for the study of bedrock-bound stream-channel development. Glacial ice is analogous to bedrock in that it is a cohesive, resistant substrate; bedrock-channel studies to date have neglected stream formation in this setting. In this study, forms common to bedrock-bound channels were observed and photo-documented in several ice-walled streams, including step-pool reaches, small slot gorges, sculpted wall forms, and ripple-marks. Cross-sectional profiles were measured almost daily for three weeks at 2-m intervals along a 20-m reach of a meandering stream for a total of over 100 profiles, along with several profiles measured at 15-m intervals along the entire length of the stream. Profiles exhibited asymmetrical parabolic channel shapes, typically with overhanging ledges (~40% of measured profiles) sloping at angles up to 30º from the horizontal. The average width:depth ratio of 5.2 for these profiles reflects the incisional nature of the streams, which is typical of bedrock channels.

Erosion in bedrock channels is commonly attributed to bed quarrying/hydraulic wedging, abrasion, and cavitation. However, most erosion in ice-walled channels is accomplished by frictional melting at the water-ice wall interface, with some localized work being done by hydraulic wedging (observed during a catastrophic event). The low water velocities (1.04 m/s) and sediment loads (0.123 g/L) measured in these streams negate cavitation or abrasion as being major contributors to channel erosion during the short time scale over which these channels develop. It is notable, then, that such similar forms can be observed in both ice and bedrock, such as basalt or limestone, when these forms are the result of distinctly different erosive forces (thermal vs. mechanical). This observation implies that there may be a more fundamental, common control governing the development of bedrock stream systems.