Growth of retrogressive thaw slumps in the Noatak Valley, Alaska, 2006?2023

Retrogressive thaw slumps (RTS) are dramatic features of the Arctic landscape caused by thaw and subsidence of permafrost, followed by downslope flow of liquified sediment and water. RTS expose bare soil that can be transported into nearby water bodies. The National Park Service, Arctic Inventory and Monitoring Network monitors the growth of selected RTS in the Noatak Valley of northwestern Alaska. We studied the growth in area, the rate of advance of the main thaw escarpment, and the amount of subsidence using three-dimensional models created from small-format (35 mm) aerial photographs in multiple years from 2010 to 2020, supplemented with high-resolution satellite images from 2006 to 2023. We studied RTS growth in relation to thaw degree-days at the nearest weather station to account for the irregular time intervals between samples. For the thirteen study slumps that were present before 2016, all except one grew fastest prior to 2012. Most of these slumps were initiated in the years 2004-2005. Eight slumps had largely stabilized by our previous sample date (2014 or 2016) and grew little from then to our most recent sample date (2019 or 2020). Four slumps continued growing rather uniformly from 2016 through 2020, with scarp migration rates of 7 to 37 m yr-1 in 2019-2020. Just one showed markedly increased growth in recent years. Sediment transport into nearby water bodies from these slumps was greatly reduced relative to the early years in the slumps? lives because they were either growing very little, or the growth had progressed hundreds of meters from adjacent water bodies and the sediment mobilized at the advancing scarp was being deposited on the slump floor, usually in the form of a small ridge within 200 m of the scarp. The rather constant growth of the slumps over the past 6 to 8 years can be explained by the uniform geologic conditions at the sites plus the fact that slump growth is apparently related mainly to summer warmth (which has varied without much trend in recent years) as opposed to mean annual temperatures (which have increased in recent years). The most striking development was the re-initiation of slump activity in the vicinity of stabilized slump GAAR008. Four new slumps were first visible on 2016 satellite images near Douglas Creek and grew rapidly through 2023 across an area of previously stabilized slumps. The scarps advanced at rates of up to 60 m yr-1 and elevation loss at the scarp was up to 10 m. Approximately 137,000 m3 of material was lost from the slump footprints of three of the slumps as of 2020, of which an estimated 26,000 m3 accumulated in the adjacent floodplain, forcing the creek to migrate laterally up to 80 m. The missing 110,000 m3 consists of melted ice and sediment that was transported further by Douglas Creek. This outbreak of slumping at Douglas Creek appears to have been a localized event triggered by migration and erosion of the creek into ice-rich material, and not a widespread climate-driven event like the one that triggered many slumps in 2004-2005.