Signatures of Permafrost Processes in Fluvial Network Morphology and Change on the Seward Peninsula, Western Alaska, USA

Permafrost soils and hillslopes are sensitive to changes in hydrology and temperature, leading to rapid landscape change at high latitudes in response to warming. However, landscape response to high-latitude climate and vegetation feedbacks are poorly understood, making predictions of sediment and carbon release difficult. Sediment delivery from hillslopes may be controlled by topography, which is now readily available at high resolution across the Arctic; by thawing ancient ground ice, the distribution of which is unconstrained; or by heterogeneous soil and vegetation properties. Channel incision is similarly complicated by seasonal variation in thaw depth and timing of precipitation, as well as vegetation patterns. To address this knowledge gap, we studied soil-mantled hillslopes on the Seward Peninsula, western Alaska, where discontinuous permafrost is susceptible to thaw, and a range of sediment transport processes (creep, solifluction, catastrophic movement) coexist on the same hillslopes. Here, landscape dynamics related to thaw must be separated from erosion signals unrelated to permafrost thaw such as other stochastic erosion events and slope and channel response to base-level fall. In analyzing high-resolution topography, we find that Seward Peninsula hillslopes exhibit particularly low drainage density compared to temperate landscapes with comparable soil and bedrock. We attribute this pattern to efficient filling of hillslope concavities by freeze-thaw induced soil movement, as well as limited fluvial incision into frozen mineral soil under a permeable tundra vegetation mat via flow through water tracks, zero-order channels with minimal incision. Although Seward Peninsula drainage networks may be expanding and steepening in response to past lower sea level, water tracks unaffected by this perturbation are experiencing warming-induced subsidence and incision, observed in the field and InSAR-derived subsidence patterns. We predict fluvial networks on permafrost hillslopes to coalesce and incise upslope under future climate and vegetation change, which would instigate erosion and downstream sediment release as well as alter hillslope ecohydrology.