Portage Glacier and Portage Pass, Alaska: Little Ice Age dynamics and the chronology of glacial retreat

Abstract

Portage Glacier currently acts as a lacustrine calving glacier discharging icebergs into Portage Lake, Alaska. This glacier advanced during the Little Ice Age, and from 1799 to 1911 completely filled the lake basin and deposited three large moraines downvalley of the modern lakeshore. During this time the glacial ablation regime was dominated by melting and a large outwash stream flowed from the glacier to Turnagain Arm. Portage Lake, approximately 5 km long, 2 km wide and 200 m deep, began to form in 1914 after the glacier retreated off the 1852 AD moraine. The deep water of the lake caused calving to become the dominant ablation process. From 1799 to 1914, the retreat rate averaged 3.8 m/year when the ablation regime was dominated by melting, but this rate increased to 56.3 m/year from 1914 to 1994 as the ablation became dominated by calving into the deep lake. Since 1994, the terminus of Portage Glacier has become stabilized where it rests on the bedrock shoreline at the east end of the lake. The mass balance is such that ice still flows to the terminus and calves into the lake, and the terminus has remained in approximately the same location.