TIMING AND RATE OF GLACIALLY DRIVEN OUTWASH PLAIN AGGRADATION: POTHOLE LAKE, SOUTH-CENTRAL ALASKA

The prevailing view on the fluvial response to glacier activity is that outwash plains generally aggrade during ice advance and incise during retreat. However, the timing and behavior of glacio-fluvial aggradation is nuanced. The sediments of Pothole Lake (60.37°N, 150.04°W) in south-central Alaska provide a unique lens into the timing and rate of aggradation relative to Late Holocene glacier advance and retreat. Dammed by an outwash plain, Pothole Lake is positioned in a small side valley adjacent to the outwash plain of Skilak River, which drains Skilak Glacier, an outlet of the Harding Icefield on the Kenai Peninsula. We retrieved sediment cores from three sites within Pothole Lake, which has two primary subbasins and a maximum water depth of 24 m. Subbottom acoustic profiling revealed that the sediments in the subbasin proximal to Skilak River are 7 m thick compared to only 2 m thick in the distal subbasin. The sediment cores from Pothole Lake terminate in a buried forest floor deposit, which is overlain by at least 130 couplets that resemble the varved sediments of Skilak Lake, ranging in thickness from 0.4 to 8 cm. Pothole Lake’s cores are capped by 3 cm of organic-rich surface mud that was deposited between ~1950, when Skilak River apparently last flowed into Pothole Lake, and present. This suggests that the entire 2-km-wide valley floor of Skilak River was at least 30 m below its present elevation before aggrading the outwash plain that dams Pothole Lake, possibly as recently as a few hundred years ago (¹⁴C ages currently pending). Since 1950, Skilak Glacier has retreated from its Little Ice Age moraine and now calves into a moraine-dammed lake. The trapping of sediment by the moraine-dammed lake has caused Skilak River to downcut 3 m into its outwash plain near Pothole Lake’s dam. Preliminary analysis indicates that Pothole Lake was likely dammed during Skilak Glacier’s Little Ice Age advance, supporting the prevailing view that outwash plains aggrade during glacier advance.