Deglacierization of a Marginal Basin and Implications for Outburst Floods, Mendenhall Glacier, Alaska

Abstract

Suicide Basin is a partly glacierized marginal basin of Mendenhall Glacier, Alaska, that has released glacier lake outburst floods (GLOFs) annually since 2011. The floods cause inundation and erosion in the Mendenhall Valley, impacting homes and other infrastructure. Here, we utilize in-situ and remote sensing data to assess the recent evolution and current state of Suicide Basin. We focus on the 2018 and 2019 melt seasons, during which we collected most of our data, partly using unmanned aerial vehicles (UAVs). To provide longer-term context, we analyze DEMs collected since 2006 and model glacier surface mass balance over the 2006-2019 period. During the 2018 and 2019 outburst flood events, Suicide Basin released ∼ 30 × 10 6 m 3 of water within approximately 4-5 days. Since lake drainage was partial in both years, these ∼ 30 × 10 6 m 3 represent only a fraction (∼ 60%) of the basin's total storage capacity. In contrast to previous years, subglacial drainage was preceded by supraglacial outflow over the ice dam, which lasted ∼ 1 day in 2018 and 6 days in 2019. Two large calving events occurred in 2018 and 2019, with submerged ice breaking off the main glacier during lake filling, thereby increasing the basin's storage capacity. In 2018, the floating ice in the basin was 36 m thick on average. In 2019, ice thickness was 29 m, suggesting rapid decay of the ice tongue despite increasing ice inflow from Mendenhall Glacier. The ice dam at the basin entrance thinned by more than 5 m a-1 from 2018 to 2019, which is approximately double the rate of the reference period 2006-2018. While ice-dam thinning reduces water storage capacity in the basin, that capacity is increased by declining ice volume in the basin and longitudinal lake expansion, with the latter process challenging to predict. The potential for premature drainage onset (i.e., drainage before the lake's storage capacity is reached), intermittent drainage decelerations, and early drainage termination further complicates prediction of future GLOF events.

Toward Numerical Modeling of Interactions Between Ice-Marginal Proglacial Lakes and Glaciers

“…Vertical displacements of an ice surface have been interpreted to signify passage of energetic and pressurized lake water through/ beneath a glacier (e.g., Walder et al, 1996;Anderson et al, 2005;Tsutaki et al, 2013). Lake depth and longitudinal expansion of the lake are key to explaining glacier response to sudden lake drainage events, but are themselves a product of glacier dynamics, thereby complicating process understanding (Kienholz et al, 2020).…”

User Engagement in Developing Use-Inspired Glacial Lake Outburst Flood Decision Support Tools in Juneau and the Kenai Peninsula, Alaska

“…Suicide Basin is an approximately 0.7 km 2 ice-covered basin that sits roughly ∼3 km up-glacier from the terminus of Mendenhall Glacier in the Mendenhall Valley (Kienholz et al, 2020). The first reported GLOF from Suicide Basin was in 2011.…”

“…The first reported GLOF from Suicide Basin was in 2011. Since then, Suicide Basin has annually released one or more outburst floods into Mendenhall Lake via Mendenhall Glacier, raising water levels in both Mendenhall Lake and Mendenhall River to varying degrees (Morgan et al, 2013;Kienholz et al, 2020). Although the largest GLOF recorded thus far was in 2016, this research (2018-2019) came at an opportune time, given that 2018 was the third largest GLOF on record (National Weather Service Advanced Hydrologic Prediction Service, n.d.).…”

Dam type and topological position govern ice-marginal lake area change in Alaska and NW Canada between 1984 and 2019

“…As one of the largest, yet understudied, glacierized mountain regions in the world, characterizing Alaska's ice-marginal lakes and their evolution has important implications for local hazards (e.g., remote roads, the Trans Alaskan Pipeline), ecosystem dynamics, and comparable environments around the world. Previous studies in Alaska looked specifically at ice-dammed lakes (Post and Mayo, 1971;Wolfe et al, 2014), a subset of lakes (Field et al, 2021), individual case studies (e.g., Sturm and Benson, 1985;Anderson et al, 2003;Kienholz et al, 2020), or a regional subset within a global study (Shugar et al, 2020), none of which comprehensively characterize regional trends. This is the first study to systematically inventory and characterize all ice-marginal lakes in Alaska using Landsat-era satellite imagery between 1984 and 2019.…”

“…The largest lakes occur in tributary valleys, dammed by main branch ice, while many small lakes are found in pockets between glacier margins and valley walls. The decrease in ice-dammed lake number and area is likely due to the down-wasting of glacier surfaces (e.g., Larsen et al, 2015;Jakob et al, 2021), decreasing the height of the ice dam and therefore decreasing maximum area (and volume) of the lake (e.g., Kienholz et al, 2020). The formation of new conduits alongside or beneath the glacier could also be influencing ice-dammed lake drainage (Post and Mayo, 1971).…”