SOURCES OF SUPRAGLACIAL DEBRIS ON EMMONS GLACIER, MOUNT RAINIER, WA

Debris covering glacier ice has a significant impact on the surface mass balance. Understanding the regional sources of debris, especially from any rockfall and rock avalanches, as well as understanding the local character of the debris cover, are necessary in order to understand patterns of glacier melt and retreat. We use satellite imagery and field measurements of clast size and angularity to help define and describe debris units on Emmons Glacier on Mount Rainier, WA. Emmons Glacier is the largest glacier by area on Mount Rainier, with a thick and extensive debris cover over the lower glacier that may include contributions from a 1963 rockfall off Little Tahoma Peak, which at the time covered most of the lower glacier with debris. Using satellite imagery we identified seven different supraglacial sediment units using color and texture differences that are visible at the scale of meters to tens of meters. Field measurements of clast angularity and size were collected at 51 sample sites across the debris cover between 2016 and 2021. These field data were used to test the remotely sensed debris-unit boundaries, and to evaluate the sources of the debris within supraglacial sediment units. Sediment units closer to the glacier margins are more angular, more weathered and include a higher proportion of fine-grained sediment than units located closer to the glacier centerline, suggesting deposition of rockfall on the glacier surface; past measurements of glacier surface velocities suggest that ice beneath these units, which make up ~ 60% of the debris cover by area, flows very slowly if at all. Supraglacial sediment in the center of the glacier is less weathered, includes less fine-grained material, and likely originates from glacial erosion and transport of bedrock material that is ablating out at the surface. Our work suggests ongoing glacier flow and delivery of sediment in the central portions of the glacier, but most of the debris cover across the lower glacier is dominated by highly weathered, rockfall-derived debris that shows little indication of continued delivery of supraglacial sediment by glacier flow.