POST LITTLE ICE AGE ICE MARGINAL LAKE DEVELOPMENT AT BERING GLACIER, ALASKA

By 1905, Bering Glacier reached its Little Ice Age (LIA) maximum position. Its ~30 x 50 km bulbous piedmont lobe, with an area of ~900 km², ended ~3 km from the Gulf of Alaska. The piedmont lobe consisted of the Bering and Steller ice lobes, separated by an up to 20 km wide medial moraine band. Ice-penetrating radar studies showed that the piedmont lobe occupied a bedrock basin with depths as much as ~400 m below sea level, suggesting that future ice retreat might expose a mountain-bound lake with an area >400 km². Up-glacier, the basin floor was below sea level for as much as ~70 km from the glacier’s LIA maximum position.

Early 20^th century maps show many streams draining directly from the glacier margin into the Gulf of Alaska. By the early 1920s, several small lakes formed along the Bering Lobe’s southern margin, as had 5 small lakes (Berg Lakes 1-5), at the northwestern margin of the Steller Lobe. Aerial photography (1937- 2020) and satellite imagery 1964 - 2024), show the expansion and merging of numerous ice-marginal lakes along the southern margin of the Bering Lobe; continued Steller Lobe retreat resulting in the merging of the 5 Berg Lakes into one; and formation of three lakes on the SW side of the Steller Lobe.

A mid-1960s Bering Lobe surge filled most of the southern ice marginal lake basin with thick ice. Subsequently, in the 26 years between 1967 and 1993, this ice retreated up to 10.7 km forming Vitus Lake, with an area of ~70 km².

The 1993 - 1995 surge reduced Vitus Lake’s area to <50 km². Seismic profiling prior to the surge showed that Vitus Lake had water depths >160 m and sediment thicknesses >150 m. A sub-glacial outburst flood that began on Vitus Lake’s eastern margin in July 1994 continued for >1 yr and filled parts of the eastern basin with ~50 m of sediment containing large entrained blocks of ice.

By mid-1996, the Bering terminus in Vitus Lake began to retreat. After 12+ yr, it occupied essentially the same position that it had prior to the 1993 surge. The latest surge (late 2008 - mid-2012), resulting in a maximum of ~4 km of ice advance in the western part of Vitus Lake.

Currently, Bering Lobe’s terminus is rapidly retreating and Vitus Lake, with an area of ~200 km², is now the largest it has been in recorded history. Since 2014, the Steller Lobe terminus in Berg Lake has thinned, retreated, and changed to draining subglacially, flooding the coastal plain to the south >10 times, with as much as 0.5 km³ of water.