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Paraglacial landscape evolution in a rapidly deglaciating environment : a case study of Taan Fjord, Southeast Alaska, USA

University of British Columbia

The rapid thinning and retreat of Tyndall Glacier in Taan Fjord in Southeast Alaska has exposed 8 fluvial tributary watersheds to fast-acting paraglacial denudation processes. An average base-level fall of ~400 m has resulted in increased sediment yields and basin-averaged erosion rates in the watersheds over the first decades following the exposure of their outlets to the ocean. We used structure from motion photogrammetry to reconstruct the rate of surface thinning of Tyndall Glacier since 1957 and the rate of base-level fall for the tributaries. We modelled each fan-delta to obtain a minimum estimate of total sediment volume evacuated from each watershed and used geometric relations to determine sediment yields and erosion rates for each tributary through time. Between 1969 and 2014, the tributary basins contributed 165.7 ± 16.0 million m³ of total sediment to the fjord, or double the amount of sediment contributed by Tyndall Glacier. On average, the tributaries eroded their watersheds at a rate of 36.0 ± 5.7 mm yr-¹ and yielded three times more sediment annually than the glacier at 4.3 ± 0.3 million m³ yr-¹. Base-level fall led to knickpoint formation in most watersheds with an average rate of migration exceeding 20 m yr-¹. Those tributaries that have had the most time to respond to base-level fall have passed their peak sediment yields that are expected during the paraglacial period and have experienced the greatest landscape relaxation since the retreat of Tyndall Glacier. In contrast, the tributaries that have experienced base-level fall most recently at the head of the fjord are eroding their watersheds at a rapid pace, indicating that they are in the throes of the highly dynamic paraglacial period. In addition to the small-magnitude unravelling of the tributary basins, a large tsunamigenic landslide occurred in the fjord in 2015. This event as well as the high sediment yields observed in the tributary basins highlight the potential hazards associated with paraglacial landscapes. These rapidly changing environments are becoming more important to study in order to understand the changes that might occur in glaciated regions as climate continues to warm and glaciers continue to retreat.

Text

2018-08-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/66717

Geography

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/