Surficial geology and proglacial lake change at Sít’ Tlein (Malaspina Glacier), Wrangell-St. Elias National Park and Preserve, Alaska (map files)

Thompson, Anna; Loso, Michael; Mooneyham, Sydney; Tober, Brandon; Larsen, Christopher; Holt, John

These map files are associated with the Natural Resource Report available at https://doi.org/10.36967/2301689. That report and these map files concern Sít’ Tlein. Sít’ Tlein (Tlingit for “Big Glacier”) is the traditional name for what has recently been called Malaspina Glacier, the largest glacier in Alaska. The piedmont terminal lobe of Sít’ Tlein is protected from the adjacent Pacific Ocean by a narrow, vegetated foreland dotted with proglacial lakes. Ice of the piedmont lobe is largely covered with debris and vegetation. These lakes and sedimentary deposits impact rates of melt and calving and therefore impact ongoing evolution of the glacier itself. To document these features, we present 1) a new surficial geology map for the foreland and piedmont lobe of Sít’ Tlein (an area of 3477 km2) at a scale of 1:24,000, and 2) a detailed time-series of proglacial lake extents. The surficial geology is referenced to a 2012 IFSAR Digital Terrain Model with 5-m resolution, supplemented with additional satellite images, maps, and digital elevation models. We visited the foreland in 2021 to ground-truth portions of the mapped area. Lake outlines were digitized from Landsat imagery, focusing on lakes adjacent to the central “Seward Lobe” of Sít’ Tlein. A majority of the mapping area is occupied by glacier ice, a sizable fraction of which is covered by supraglacial debris of varying thicknesses. Off glacier, in the foreland, glacial outwash is the most common mapping unit, followed by moraines of varying ages and finally by marine beaches, bars, and lagoons. Perhaps surprisingly, given significant changes in the glacier itself over the last half-century, these deposits have not changed dramatically since a similar map was produced by Plafker and Miller in 1958. The most significant changes we found are related to lake development. Other than Malaspina Lake, the largest and most persistent lake in the foreland, proglacial lakes were uncommon in the foreland in 1958. Our mapping shows that lake numbers on the Seward Lobe increased from 5 to more than 200 between 1972 and 2020. Most of the new thermokarst lakes are small, compared to Malaspina Lake, but may be having strong impacts on the future evolution of Sít’ Tlein. One of these new lakes, Sitkagi Lagoon, is ice-walled and receives input from the Pacific Ocean, portending the possible initiation of catastrophic tidewater glacier retreat.

Surficial geology and proglacial lake change at Sít’ Tlein (Malaspina Glacier), Wrangell-St. Elias National Park and Preserve, Alaska (map files)

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