Doctor of Philosophy in Resources, Environment and Sustainability (PhD)
Socio-Ecological Adaptation to Hydrological Change in High Mountains
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.
Bridge Glacier is a lacustrine calving glacier located in the southern Coast Mountains and terminates in a 6.2 km² proglacial lake. The glacier has retreated more than 3.55 km up-valley since 1984, the majority of the retreat having occurred since 2003. While surface melt may have contributed to the retreat, calving allowed for an additional annual volume of ice loss. The relative contributions from surface melt and calving to the total volume of ice loss is examined for the 2013 melt season. Surface melt is quantified using on-glacier meteorological data to drive a distributed energy balance model. The calving flux is quantified using field measurements of lake bathymetry, terminus area change, and ice thickness. Calving flux estimates are completed by daily measurements of terminus surface velocity derived from manual feature tracking using oblique time lapse camera imagery. Calving accounts for 23% of the total ice loss in the 2013 melt season, suggesting that surface melt is the main driver of mass loss at Bridge Glacier.Data from the 2013 field season is used to inform historical calving flux and surface melt estimates from 1984 to 2013. The calving flux is minor until 1991, at which point the glacier terminus achieves flotation, and begins to discharge large tabular icebergs. Calving was characterized by large, multi-annual retreats, alternating with periods of relative stability. The calving flux peaked from 2005 to 2010, when it was roughly equal to the mass loss due to surface melt. Calving was a much smaller contributor of mass loss from Bridge Glacier, except for a transient high-calving period in the late 2000s. Looking forward, Bridge Glacier will retreat into shallower water where the terminus will no longer float, and calving losses should decrease substantially. Although calving losses will become an increasingly minor portion of the mass balance, future retreat is expected at Bridge Glacier due to a legacy of dynamic thinning brought about by its transient calving phase.