Quo vadis, Alsek? Climate-driven glacier retreat may change the course of a major river outlet in southern Alaska

Climate change-induced glacier retreat can have substantial localized impacts that often go unnoticed in the sparsely populated regions where they occur. Here we predict that retreat of Grand Plateau Glacier, in southern Alaska, USA, will reroute the outlet of a major river with consequences for human activity in this remote region. The glacier terminus separates Alsek Lake and the present Alsek River outlet from Grand Plateau Lake. In response to thinning and retreat of that terminus, both lakes have more than doubled in size since 1958. Laser altimetry shows that terminus thinning continued at rates of up to 10 m/yr from 2017 to 2020. Radar soundings show that the bed of the thinning glacier terminus extends to >400 m below sea level, and that the two lakes will become conjoined within at most a few decades as the terminus further retreats. We predict that Alsek River will then abandon its present Dry Bay outlet channel in favor of the much steeper outlet of Grand Plateau Lake, 28 km to the southeast. Anadromous fish and associated predators in the lower Alsek will need to adapt to this change. Traditional and modern human activities centered on Dry Bay include commercial fishing, subsistence and sport hunting and fishing, and the finishing point for a world-renowned wilderness rafting expedition. Under present management guidelines, those activities cannot be relocated to the predicted future outlet, which sits within the federally designated wilderness of Glacier Bay National Park.