Hillslope‐Channel Transitions and the Role of Water Tracks in a Changing Permafrost Landscape

Abstract

The Arctic is experiencing rapid climate change, and the effect on hydrologic processes and resulting geomorphic changes to hillslopes and channels is unclear because we lack quantitative models and theory for rapid changes resulting from thawing permafrost. The presence of permafrost modulates water flow and the stability of soil‐mantled slopes, implying there should be a signature of permafrost processes, including warming‐driven disturbance, in channel network extent. To inform understanding of hillslope‐channel dynamics under changing climates, we examined soil‐mantled hillslopes within a ∼300 km2 area of the Seward Peninsula, western Alaska, where discontinuous permafrost is particularly susceptible to thaw and rapid landscape change. In this study we pair high‐resolution topographic and satellite data to multi‐annual observations of InSAR‐derived surface displacement over a five‐year period to quantify spatial variations in topographic change across an upland landscape. We find that neither basin slope nor the presence of knickzones controls the magnitude of recent surface displacements within the study basin, as may be expected under conceptual models of temperate hillslope evolution. Rather, the highest displacement magnitudes tended to occur at the broad hillslope‐channel transition zone. In this study area this zone is occupied by water tracks which are zero‐order ecogeomorphic features that concentrate surface and subsurface flow paths. Our results suggest that water tracks, which appear to occupy hillslope positions between saturation and incision thresholds, are vulnerable to warming‐induced subsidence and incision. We hypothesize that gullying within water tracks to outpace infilling by hillslope processes, resulting in growth of the channel network under future warming.

Permafrost extent sets drainage density in the Arctic

“…The proposed mechanism is consistent with field ( 40 , 41 ) and remote-sensing observations ( 27 ) of surface displacement on hillslopes with water tracks. Increased temperatures and rainfall can also generate active-layer detachments in convergent zones on hillslopes ( 42 ), an advective process that can drive higher sediment discharges for decades by creating new incised, contiguous channels ( 43 ).…”

“…In the future, less precipitation will fall as snow and more precipitation will fall as rain across the Arctic ( 47 , 48 ). Areas of hillslopes occupied by water tracks are likely to be geomorphically dynamic in the future; deeper and earlier thaw, combined with more rain and less snow, may cause sustained subsidence of the ground under water tracks ( 27 , 38 ). Inter-track water tables would respond by falling, potentially sapping shallower water tracks of interflow and lead flowpaths to coalesce ( Fig.…”

“…These linear zones of enhanced soil moisture can occur in the absence of well-defined channel valleys ( 18 , 23 ) because timing of historically peak discharge from winter snow storage and spring melt ( 24 ) coincides with minimal ground thaw on hillslopes ( 18 , 23 , 24 ). Previous Arctic studies ( 18 , 25 – 27 ) indicated soil-mantled Arctic hillslopes experience relatively limited channel development, but contrasting conceptual frameworks hypothesize that hillslopes underlain by permafrost should exhibit longer channel networks due to the limited capacity for thawed soils to store water ( 28 , 29 ). This competition between permafrost-modulated erodibility of channels and subsurface water storage, coupled with the importance of thaw-mediated sediment diffusivity, will determine whether permafrost landscapes exhibit a higher density of channels compared to their temperate counterparts.…”