SURFACE ELEVATION CHANGE ON DEBRIS-COVERED EMMONS GLACIER, MOUNT RAINIER, WA

Emmons Glacier is located on Mount Rainier, one of the Cascade Volcanoes in Washington State. Much of the ablation area of Emmons Glacier is blanketed by a thick debris cover, which reduces ablation rates and has led to relative ice thickening in past decades, even as the terminus position retreated. To measure how glacier surface elevation has changed in time, we use elevation transects collected from the glacier surface between 2016 and 2021, and compare these field measurements to a digital elevation model based on a 2007 and 2008 LiDAR survey. Elevation change appears to be dependent on the thickness of the debris cover, which we have estimated from digging 40 pits to the depth of glacial ice, or until an obstruction was met or the walls of the pit failed and digging was no longer possible. The thickest debris cover occurs at the margins of the glacier, where pits reached depths of 53 to 85 cm, but did not reach the buried ice surface. The debris cover is thinnest near the centerline of the glacier where the ice surface was located beneath 2 to 22 cm of debris. A 2018 elevation transect, perpendicular to ice flow, showed lower surface elevations near the centerline of the glacier, where the debris is thinnest, including a 32 m deep supraglacial drainage channel. When compared to the 2007/2008 digital elevation model, the 2018 transect shows 35 m of surface lowering at the bottom of this drainage feature, and significant thinning of ~ 20 to 30 m across the transect, even in areas of thick debris cover. Our results suggest multiple influences on the evolution of the surface of Emmons Glacier, including supraglacial meltwater stream development, as well as spatial and temporal variations in the thickness of the debris cover including redistribution of debris on the surface.