Evolution of glacier-dammed lakes through space and time; Brady Glacier, Alaska, USA

Abstract

Glacier-dammed lakes and their associated jökulhlaups cause severe flooding in downstream areas and substantially influence glacier dynamics. The goal of this dissertation is to identify and characterize the evolution of glacier-dammed lakes in order to predict their future behaviour using ground-truthed remote sensing techniques and dendrochronology. Brady Glacier in southeast Alaska is particularly well suited for a study of these phenomena because it presently dams ten large (> 1 km 2 ) lakes and many smaller ones. This dissertation comprises three studies. First, I used interferometric synthetic aperture radar (InSAR) to identify and characterize three previously unknown subglacial lakes. InSAR allowed me to quantify the vertical displacement and volume of water discharged from the three lakes through time. From the fall of 1995 to the spring of 1996, subsidence ranged from 4 to 26 cm/day and the volume of water discharged ranged from 22,000 ± 2000 to 243,000 ± 14,000 m 3 /day. Subsidence and discharge rates declined significantly during the winter and continued at a lesser rate through March. Second, I used dendrochronology and precise elevation-constrained mapping to date glacially overridden and drowned trees at the glacier margin. Brady Glacier impounded Spur Lake to an elevation of 83 m a.s.l. around AD 1830 and 121 m a.s.l. around 1839. The glacier continued to advance, thickening by at least 77 m between ca. 1844 and 1859 at a site down-glacier of Spur Lake on the opposite glacier margin. Farther down-glacier, North Trick Lake began to form by 1861 and reached its highest elevation at approximately 130 m a.s.l. when Brady Glacier reached its maximum extent around 1880. Third, I georeferenced a variety of maps, airphotos, and optical satellite imagery to characterize the evolution of the glacier and lakes and also created five bathymetric maps. The main terminus of Brady Glacier has changed little since 1880. However, it downwasted at rates of 2-3 m/yr between 1948 and 2000, more than the regional average. The most dramatic retreat (2 km) and downwasting (123 m) occurred adjacent to glacierdammed lakes. These lakes will continue to evolve and play a pivotal role in the evolution of Brady Glacier. If downwasting and retreat continue at rates comparable to the past, the glacier may return to a tidewater regimen and retreat catastrophically until it stabilizes in shallow water.

Rising ELA and expanding proglacial lakes indicate impending rapid retreat of Brady Glacier, Alaska

“…Water depth is an important factor affecting the calving rate of glaciers in lacustrine environments; velocity and calving rate increase with water depth by a factor of 3.6 (Skvarca et al ., ). Capps () determined the bathymetry and calving depths of five of the lakes at Brady Glacier. Water depths increase toward the calving fronts at Abyss Lake, Bearhole Lake, Oscar Lake and Trick Lake; only at North Deception Lake does the water not currently become deeper towards the calving front; however, it almost certainly will as the east margin moves into the main Brady Glacier valley (Table ).…”

“…Approximately two thirds of the ice in the valley flows SSE towards Taylor Bay and one third flows NNW into Lamplugh and Reid glaciers and Glacier Bay (Bengtson, ; Derksen, ). The divide between south‐and north‐flowing ice lies at approximately 820 m a.s.l., based on the 2000 Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), which has a 30 m spatial resolution and 6 m vertical resolution (Capps, ).…”

“…The massive outwash plain at the terminus is primarily responsible for Brady Glacier maintaining an advanced position (Bengtson, ) while other glaciers in the Glacier Bay region have retreated substantially (Molnia, ). In the 1960s, the glacier slowly advanced, and during the 1970s, the terminus was stable (Bengtson, ; Molnia, ; Capps, ). By the late 1980s, the glacier was thinning, and some of the distributary termini were retreating (Molnia, ; Capps, ).…”

“…In the 1960s, the glacier slowly advanced, and during the 1970s, the terminus was stable (Bengtson, ; Molnia, ; Capps, ). By the late 1980s, the glacier was thinning, and some of the distributary termini were retreating (Molnia, ; Capps, ). By 2010, the main terminus had begun to retreat, but the retreat to date is less than 200 m. Subsidiary termini ending in proglacial lakes have retreated considerably more than the main terminus, ranging from 300 to 500 m in Abyss Lake and Trick Lake, to 1300–1800 m in North Deception Lake, Dixon and Bearhole Lake (Capps, ).…”

The Role of Englacial Hydrology in the Filling and Drainage of an Ice‐Dammed Lake, Kaskawulsh Glacier, Yukon, Canada

“…An estimated 24% of 1,348 recorded outburst flood events have resulted in some form of societal impact on downstream populations and infrastructure, with 12,445 recorded fatalities caused by these events (Carrivick & Tweed, 2016). As tributary glaciers thin and detach, new lakes may form at the margins of valley glaciers, and ongoing glacier thinning will likely result in more frequent drainage events (Clague & Mathews, 1973;Capps & Clague, 2014). In addition to being a hazard, outburst floods serve as a natural experiment that can be used to study the state and evolution of internal hydrological systems of glaciers (e.g., Björnsson, 1998;Bartholomaus et al, 2011).…”