Sediment production and transport processes in an arctic watershed undergoing climate change

Arctic landscapes are among the most vulnerable on Earth to climate change, largely due to the degradation and thawing of permafrost. In steeper bedrock-dominated terrains, slope instability from warming permafrost leads to larger and more frequent rockfall and frost cracking events, which in turn increases the production and delivery of sediment to hillslopes and channel networks by debris flow and fluvial processes. However, there is a fundamental lack of data on past and current rates of sediment production and transport in Arctic watersheds. Without an understanding of these phenomena, it is impossible to predict the transient responses, rates, and directions of periglacial processes in response to future climate change. To begin to address this knowledge gap, we conducted a field-based study of the Black Mountain catchment in the Aklavik Range (Northwest Territories, Canada). This site was chosen due to its position within a zone of continuous permafrost and the presence of an alluvial fan at the base of the catchment, providing a closed system.

In the summer of 2019, after a summer storm event, we observed several debris flows that initiated from ice-filled gullies, as well as fluvial sediment transport from snowmelt. We documented flow and sediment transport conditions on the fan, yielding modern-day fluvial transport rates of 0.2–2 m³/hr for water runoff rates of 0.01–0.2 mm/hr. However, less-frequent mass flow events can rapidly deposit large amounts of sediment. For example, we estimate that a mass flow event that occurred in 2016 delivered ~1.5*10⁵ m³ of sediment to the fan—equivalent to ~8–85 years of continuous fluvial sediment transport. Based on our surficial and sedimentological mapping, the fan has likely been forming under a periglacial climate over the last ~13,000 years from a combination of mass flow and fluvial processes. Most of the fan (~67%) was deposited fluvially, but the upper, steeper portion of the fan was deposited by coarse granular debris flows. We hypothesize that accelerated warming has increased sediment supply due to frost cracking, leading to aggradation, increased debris flow activity, and upper fan steepening.