INVESTIGATION OF ENGLACIAL AND SUBGLACIAL DRAINAGE NETWORK EVOLUTION AND ITS EFFECT ON SEASONAL DISCHARGE AND ABLATION PATTERNS AT EXIT GLACIER, ALASKA

Concerns of climate change and rapidly retreating glaciers have placed great importance on glacial monitoring. One of the biggest challenges of this discipline is understanding the englacial and subglacial environments. Surface meltwater that enters englacial and subglacial networks can affect ice flow and calving rates, and can sometimes lead to surges or outburst floods. Because these systems often freeze during the winter and re-form each summer, examining their seasonal evolution is crucial for assessing the impact that internal drainage may have on the overall behavior of a glacier each year.

Exit Glacier is a 5-km long temperate mountain glacier located in Kenai Fjords National Park in Alaska. The glacier is currently in retreat, with the position of the terminus approximately 300 meters upstream from its 1993 location.

The goal of this study is to determine the role englacial and subglacial drainage system evolution plays in influencing summer ablation and discharge patterns at the terminus of Exit Glacier. We used ground-penetrating radar (GPR) to locate internal drainage features on the lower 100 meters of the glacier. GPR surveys were conducted in June and August to observe the development of the drainage systems over the course of an ablation season. Three antenna frequencies were used on a multi-frequency GPR so that various resolutions and depths in the ice could be viewed simultaneously.

Additionally, stream monitoring was conducted at the proglacial stream throughout the study period. Data loggers were installed at two locations to continuously record stream stage. These data were supplemented with velocity area measurements of discharge. Stream data were compared with weather records to differentiate noticeable meltwater releases from precipitation events.

Throughout the observation period, significant subglacial tunnels appeared, followed ice collapse above these features. A singular release event of subglacial water was also observed on July 4^th. After this event, drainage from that tunnel ceased. These observations indicate that the subsurface drainage systems near the terminus of Exit Glacier developed into efficient networks relatively early in the ablation season. The collapse of the roofs of subglacial tunnels contributed to over 20 meters of ice loss between June and August of 2011.