Sít' Tlein (Malaspina Glacier): Mapping a Glacier Surrounded by Growing Lakes

Sít' Tlein is the traditional Tlingit name for Malaspina Glacier, a piedmont glacier and the largest glacier in Alaska. The glacier and its lakes lie on the traditional lands of the Yakutat Tlingit people, within Wrangell-St. Elias National Park and Preserve. Glaciers in Alaska and around the world are, with very few exceptions, retreating, and National Park Service scientists and colleagues set out to map how Sít' Tlein is changing.

Fluted moraines in the Sít' Tlein foreland just west of Malaspina Lake, looking northwest. NPS/Anna Thompson

The setting

The glacier's broad piedmont lobe is fed by the merged ice of three glaciers — the Agassiz, Seward and Marvine. Its end is shielded from the Pacific Ocean by a narrow strip of vegetated land dotted with lakes, and much of the ice of the lower lobe is itself covered by debris and plants. Both the lakes and the vegetation affect how fast the ice melts and calves, and so how the glacier evolves.

The area mapped (red) on the piedmont lobe of Sít' Tlein, fed by the Agassiz, Seward and Marvine Glaciers. National Park Service

Comparing with 1958

The project was conceived partly as a follow-up to surficial mapping by George Plafker and Don Miller published in 1958, which covered a similar area of the foreland and the glacier's melting zone. Because their map was based on aerial photographs from 1948, the new work captures well over half a century of change.

Glacier ice covers nearly 80% of the mapped area, and well over half of that is covered with debris, mostly thick. Surprisingly, apart from the area around Malaspina Lake, the ice edge today is within 1 km of where it was mapped in 1958. In some places it is further forward and in others further back, which suggests most of the differences come from mapping error rather than real retreat. The debris on the glacier surface has not changed dramatically either.

The new surficial geologic map of Sít' Tlein. National Park Service

Lakes everywhere

The one big, progressive change is in the lakes. In 1958, apart from Malaspina Lake — the largest and most persistent — lakes were rare in the foreland. Since then Malaspina Lake has grown substantially, accounting for the larger retreat of the ice nearby, and many other lakes have formed or expanded. On the Seward Lobe, the number of lakes rose from 5 to more than 200 between 1972 and 2020.

Most of the new ones are thermokarst lakes, formed as buried ice melts and the ground subsides. Some lie within the glacier margin rather than in front of it and are more properly called supraglacial; they often sit in areas of heavy thermokarst, ringed by steep, debris-covered ice cliffs grown over with alders. Altogether, the amount of water held along the glacier's edge is large and growing fast.

Lake outlines on the eastern Seward Lobe and at Malaspina Lake, 1972–2020. National Park Service

Sitkagi Lagoon

Sitkagi Lagoon shows how small lakes can merge into large ones. Many small thermokarst lakes formed there independently between 1972 and about 2013, then began to join together. Such mergers explain why, in places, a shoreline appears to retreat even as a lake grows overall.

The lagoon is ice-walled and sits right beside the Pacific, which now sends saltwater into it from time to time. That could speed melting and calving and potentially set off catastrophic tidewater glacier retreat. The mapping, in short, records a recent period of relative stability — and trends in lake growth that may foreshadow a far more dynamic future.

Lake outlines in the Sitkagi Lagoon area, 1972–2020. National Park Service

Sources

• National Park Service, "The Rapidly Changing Sít' Tlein (Malaspina Glacier)" (Wrangell-St. Elias National Park and Preserve): https://www.nps.gov/articles/000/changing-sit-tlein.htm
• The article summarizes A. C. Thompson, M. G. Loso, S. A. Mooneyham, B. S. Tober, C. F. Larsen and J. W. Holt, Surficial geology and proglacial lake change at Sít' Tlein (Malaspina Glacier), Wrangell-St. Elias National Park and Preserve, Alaska, Natural Resource Report NPS/WRST/NRR—2024/2620, National Park Service, 2024.