High Arctic channel incision modulated by climate change and the emergence of polygonal ground

Domain: earth_science
Published: 2023-09-12
Editorial outcome: minor_revision

Abstract
Stream networks in Arctic and high-elevation regions underlain by frozen ground (i.e., permafrost) are expanding and developing in response to accelerating global warming, and intensifying summertime climate variability. The underlying processes governing landscape dissection in these environments are varied, complex and challenging to unravel due to air-temperature-regulated feedbacks and shifts to new erosional regimes as climate change progresses. Here we use multiple sources of environmental information and physical models to reconstruct and understand a 60-year history of landscape-scale channelization and evolution of the Muskox Valley, Axel Heiberg Island. A time series of air photographs indicates that freeze-thaw-related polygon fields can form rapidly, over decadal timescales. Supporting numerical simulations show that the presence of polygons can control how surface runoff is routed through the landscape, exerting a basic control on channelization, which is sensitive to the timing, duration and magnitude of hydrograph events, as well as seasonal air temperature trends. These results collectively highlight that the occurrence and dynamics of polygon fields modulate channel network establishment in permafrost-rich settings undergoing changes related to a warming climate.

Introduction
Global warming and the intensity of warming variability have been amplified in the Arctic since the 1980s. These features of climate change are manifest through a recent acceleration in the seasonal frequency of new thermokarst features, and the spatial expansion of stream networks. In temperate climates, channels incise the landscape and form stream networks that expand by erosion of river bed and bank sediments related to surface runoff and groundwater sapping. Contributions to these water flows include rainfall, seasonal snowmelt and glacial melt in seasonally cold and mountainous regions. In Arctic and high-elevation permafrost environments, contributions to surface and groundwater also include seasonal inputs from thawing ground ice and permafrost. However, how these hydrologic processes interact to influence channel development through erosion of frozen or partially frozen sediment particles substrates is unclear. In particular, summer surface water runoff is modulated by snowmelt, glacier and ice cap melt, intermittent lake outburst floods, and meltwater from seasonal pore ice in active layer soils and from permafrost. These hydrologic sources deliver water to the surface over distinct temporal and spatial scales that change during single melting seasons. Furthermore, the relative water contributions may change as active layer soil depths increase with rising seasonal mean temperatures, and in relation to the intensity of warming variability for climate change. As a result, we generally understand the sources of surface waters within permafrost landscapes, but not how these sources collectively govern the timing and pace of channelization and landscape change.

The flow pathways and erosive properties of meltwater sources are affected by the character and permeability of periglacial soils, and the dynamic growth and evolution of polygon fields (hereafter referred to simply as polygons) related to processes including thermal contraction, ice wedging and ice lens growth. The occurrence of active layer detachment slides and retrogressive thaw slumps of soil-ice-meltwater mixtures also influences hydrologic flow pathways by modifying the landscape topographically, and by providing point sources of potentially erosive meltwater flows together with fine and coarse sediments. As a result, and in marked contrast to temperate settings, channel initiation and evolution in Arctic and high-elevation environments depend on how the seasonal production and flow of meltwaters are influenced by periglacial landforms and the spatial distribution of seasonal ground ice and relatively long-lived permafrost. In particular, the strength and erodibility of the river bed and bank sediments by surface flows depend on seasonally varying volume fractions of pore ice, which can bind active layer soils depending on the thermal state of the material.

Here we characterize the consequences of these effects for stream network development and growth over an approximately decadal time scale. We explore whether and how periglacial landforms influence channelization at the basin scale, noting that the overall process is complex, due to the influence of numerous co-evolving environmental factors. Key remaining knowledge gaps are the underlying mechanics of sediment particle erosion under frozen, or partially frozen conditions, as well as the relative contributions to erosion and channel development from seasonally varying water sources. Indeed, increased rates of channel bed erosion may contribute to a positive climate feedback by enhancing carbon release through increasing the availability of thawed organic soil carbon for microbial decomposition. Consequently, and in light of accelerated Arctic warming, an acute knowledge gap at the intersection of climate change science and geomorphology is a clearer understanding of channel initiation and stream network development in permafrost environments. Here we combine air photographs from 1959 with field observations and LiDAR data collected in 2019 to characterize a remarkable 60-year landscape evolution in the Muskox Valley, Axel Heiberg Island. Details of this landscape change history are further explored using physical models, which combined highlight basic controls over stream network development, including how lake outburst floods can influence channel erosion and deposition, mediated through polygons and permafrost melting responses. We integrate our findings and propose a framework for describing landscape change, and to help better understand underlying processes governing the channelization responses of Arctic and high-elevation permafrost landscapes, as a result of a warming climate marked by intensifying summertime variability.

Results
We conducted fieldwork during the summer of 2019 within the Qikiqtani Region of Axel Heiberg Island, NU, located in the Canadian Arctic Archipelago ("Methods"). Our primary field site in Muskox Valley offers a unique opportunity to examine landscape change and channel network development from the early stages of response within an environment characterized by spatially continuous permafrost and a valley shaped by past glaciation. Muskox Valley is situated east of the Müller Ice Cap and has a polar desert climate with an annual mean air temperature of −19 °C ("Methods"). The relatively cold regional climate implies that widespread thaw and melt within this arid environment could be a recent response to climate change, and its amplification in the Arctic over the past several decades, as well as an important driver of basin channelization.

The current expression of surface channels along Muskox Valley is diverse, depending on the presence or absence of polygons. Low-altitude photographs and our high-resolution LiDAR-based digital elevation model (DEM) show that the character and extent of erosional channel incision are spatially discontinuous. Whereas well-developed channels are common around the edges of large polygons and within interconnected troughs, channelization is diffuse or non-existent where relatively poorly developed polygons occur in wetlands. Consistent with previous observations, we find that polygon geometry can influence and, in some places, govern the channel path or paths both across and down topographic gradients at the valley scale. Hydrodynamic modeling using the LiDAR-based DEM of a roughly 400 m segment of Muskox Valley ("Methods") shows that where polygons are present, water runoff for steady flow conditions is organized along interconnected polygon troughs, with larger locally averaged downstream velocities compared to wetland regions. These observations and results suggest topographic guiding of surface flows within polygon troughs enhances the likelihood of erosion there, and possibly increases the seasonal or event-driven rate of erosion. Furthermore, the spatial and discrete probability distribution of surface flows is strongly dependent on flow magnitude, and the presence or absence of polygons.

Valley floor topographic cross-sections from our LiDAR-based DEM provide insightful context for the differing characters of polygon-channel interactions in Muskox Valley. In the uppermost part of the basin at Station 200 m, the topographic cross-section is characterized by a V-shaped channel set within a polygon field. Downstream at Station 720 m, the cross-section is relatively flat across the valley bottom. Farther down the valley at Station 1120 m, there are two channels that occur within the troughs of polygons. The occurrence of multiple channels at this location is one example of a strong local control on the path of water runoff by polygons. At Station 1390 m, by contrast, the valley bottom is flat and flanked by raised polygons to the north. Runoff at this location has a tendency to spread across the valley floor, which is characterized by low topographic variability, and relatively high surface roughness.

Observations from the 1959 photographic records and our LiDAR-based DEM from 2019 show that sediment redistribution by fluvial erosion and deposition is actively reshaping the upper basin topographic profile. Fluvial transport is presently influenced by approximately isotropic and periodic polygon fields with a characteristic scale of ~10 m that is independent of the local valley slope ("Methods"). Channelized and unchannelized segments of the valley floor are distinguished neither by the predominant topographic character of the polygon fields nor on the basis of either the absolute profile slope or its changes down-valley. This slope independence of the channelization response is surprising because the local profile steepness for channelized and unchannelized segments are similar (1–8%), which would conventionally be interpreted to mean that erosion potential by flowing water should be broadly equivalent. In marked contrast to conventional views of the main controls over channel incision in mid-latitude temperate regions, the spatial pattern of channelization in Muskox Valley is arguably independent of channel steepness and grain size distributions, raising the importance of addressing critical knowledge gaps related to the erosion mechanics of river bed and bank sediments in permafrost settings.

A further conventional test for incision processes driven primarily by the erosive power of surface flows is the valley Width-Depth (W/D) ratio ("Methods"). Erosion and hence channelization is more likely within locations of smaller ratios because narrower valley profiles cause surface flows to converge into comparatively deeper and faster flows with relatively high erosive power. Width-Depth ratios within the upper part of the basin range from 5 to 100 ("Methods"). Width-Depth ratio values ≥30 are within or adjacent to unchannelized segments, and W/D ratio values ≤30 are associated with channels. Furthermore, Width-Depth ratios calculated for Muskox Valley generally fall within the range of values reported for a wider range of alluvial rivers located in the Pacific northwest, and desert southwest of North America. Significantly, however, Width-Depth ratios in Muskox Valley are unrelated to local valley slope, or to erosive power.

Although the mechanical controls (i.e., channel steepness and substrate erodibility) on channelization in Muskox Valley are inconsistent with conventional sediment transport analyses of erosion by surface water runoff, channel incision and architecture development are vigorous, nevertheless. Gravel bar-like features are, for example, present along Muskox Valley, indicating that peak seasonal streamflows over the past six decades or more were capable of transporting relatively coarse sediment particles. However, the valley profile has convex and concave segments, suggesting that erosional and depositional processes presently occur at specific locations along the valley floor, respectively, and that sediment transport potential has strong spatial signatures along the valley floor. These observations lead to a question: What processes govern the production and discharge of water with a sufficient flow rate to enable extensive channelization within Muskox Valley, and to what extent are these processes modulated by the seasonally varying thermal state of the substrate? This question directly addresses a critical knowledge gap related to sediment transport mechanics in permafrost and cold regions, which at present, has been little explored compared to more temperate landscapes.

We propose that the supply of water to emergent channel segments in Muskox Valley is governed by both point and spatially diffuse sources. The low elevation of the lake surface in 2019 relative to the channel head and the positions of beach terraces above the lake surface height suggest that intermittent floods from the lake (a point source) contribute to channel evolution, a topic we discuss below. Furthermore, our data suggests that post-glacial incision of Muskox Valley involves seasonal water-delivery processes acting in concert to varying extents over the full length of the basin (distributed sources). For example, several prominent tributaries merge to the main valley at locations where unchannelized wetland segments occur. Tributaries act as seasonal surface and subsurface water reservoirs by storing and (re-)releasing water to downstream reaches throughout the summer season. Over multiple seasons, presumably depending on the intensity of summertime warming as well as rain and/or snowfall events, there are evident effects of drainage from the fronts of earthflows on the south side of the valley and solifluction lobes on both sides of the valley.

An aerial photograph from 1959 reveals important clues regarding the general pace, time-dependence and spatially patchy nature of periglacial landform development and channelization within Muskox Valley. In 1959, one channelized segment occurred downstream of the lake up and until the point of strong valley narrowing. The pre-existing channel segment had a rounded and entrenched geometry with near-constant channel width. These characteristics are compatible with existing studies of collapse features related to the melting of spatially discontinuous ground ice in periglacial soil profiles, as well as the melting of ice wedges along polygon troughs. Areas upstream and downstream from this pre-existing channel segment in Muskox Valley were mostly unchannelized and polygons were generally absent. Over the next six decades, valley floor topography was profoundly altered through the development of multiple polygon fields, additional channelized segments, and activation of one earth flow. For example, the 350 m long valley segment immediately downstream of the lake developed well-defined polygons and channelized during the intervening 60 years. This polygon control over channel incision and evolution is less evident in the 1959 image data, and as a result, we hypothesize that coupling between polygons and enhanced valley channelization emerged with changes in the Muskox Valley hydrologic system related to the acceleration in Arctic warming evident since the 1980s. Such a sensitivity of polygon growth to, for example, enhanced groundwater flow is expected from physical models of frost heave applied to Arctic environments.

In addition to increased production and release of meltwater, a comparison of the 1959 and 2018 base imagery suggests that at least one lake outburst flood occurred. We infer that the emergence of polygons alongside the downstream margin of the lake contributed to this flood due to lake leakage along contraction cracks, with ultimate failure and channel initiation in response to rising lake levels associated with climate change. Differences between the post-1959 and 2019 lake level positions suggest that approximately ~104–105 m3 of water was released into the valley during the flood ("Methods"). Assuming a single outburst flood occurred, simple calculations on the basis of the approximate transport properties of sediment particles associated with plausible flow geometries suggest that the flood was capable of transporting gravel and cobble-sized sediment, resulting in gravel and cobble imbrication observed at various locations along the channelized reaches ("Methods"). As a result, the flood event likely enhanced channelization and deposition patterns downstream of the lake, yet did so without destroying or modifying the isotropic pattern of the polygons.

Methods
Capturing and processing of LiDAR point clouds
We collected surface topography data in Muskox Valley during a one-day field campaign on July 7, 2019, using the Akhka R4DW backpack mobile laser scanning system. The portable unit collects high-precision 3D topographic data kinematically at a maximum spatial resolution of 0.1–1 cm. The system operation is based on Global Navigation Satellite System–Inertial Measurement Unit (GNSS-IMU) positioning and observes GPS and GLONASS constellations for position. Precise platform movements are observed by the near-navigation grade IMU to produce attitude data for precise georeferencing of the laser scanner and image data. 3D data collection is carried out by two profiling laser scanners, each operating at different wavelengths. The trajectory data was post-processed with data from a base station located at our camp roughly 60 km south of Muskox Valley at coordinates 8813520, 482220, UTM projection, Zone 16.

We mapped roughly 2 km of the valley floor extending downstream from the relict glacial lake, with a valley floor width of approximately 200 m. Mapping occurred by walking across the valley floor, roughly following the polygon contour and other terrain features to collect seamless data over the valley. Within channelized reaches, we followed the channel centerline and then mapped each bank by climbing in and out of the channel along collection lines with a spacing of about 5 m. From median elevations binned and resampled onto a 2 cm grid, we use LASTOOLS (LAStools, Efficient LiDAR Processing Software [version 191018, academic], obtained from http://rapidlasso.com/LAStools) to interpolate the data to produce DEMs with a 4 cm resolution.

Powerspectral estimation
We first extract ~20–100 down-valley and cross-valley profiles from each of the LiDAR image tiles that are combined to produce the Hillshade in Fig. 2. A 1D power spectrum is estimated to a 95% confidence for each profile using a Thompson multitaper method. As additional checks, spectral results for the fundamental mode discussed in this paper are then compared with estimates from conventional Welch and autocorrelation-based methods. Average down-valley and cross-valley spectra are then binned and smoothed to produce the results in Fig. 2. Down-valley and cross-valley polygon field isotropy is at the well-resolved largest scale of the ~10 m fundamental mode. Power spectral analysis of the ground topography is useful because it characterizes the dominant scale(s) of the topographic ground structure(s), or physical patterns. In the present case, the power spectra reveal that polygon fields along the valley floor have a similar dominant length scale measured both down and across the valley. Furthermore, and depending on how polygons are spatially organized, this dominant length scale imparts a relative and consistent unit flow path length as surface flows incise apparently stochastically along interconnected polygon troughs.

Calculation of width-depth ratio and local channel gradient
We acquired valley cross-section profiles on LiDAR elevation data to calculate width-depth ratios. We used ArcGIS 10.8.1 to clip the LiDAR data in four 600 m wide tiles along valley, measuring 1–2 km each. We then extracted 20 cross-sections spaced regularly every 70 m down valley, which were then loaded onto MATLAB to calculate cross-section slope and curvature (Supplementary Fig. 6). To minimize uncertainty related to qualitative measurements, we measured top width as the distance between curvature minima, and bottom width (valley floor) as the distance between curvature minima located within the previously identified valley width. Note that some profiles have multiple channels. These were also measured following the same technique. Associated depth along each transect was measured as the deepest point from the top width line.

Channel gradient calculated from the processed LiDAR point cloud as a centered average for a window length of 20 m. Averaging window does not overlap, and represents the gradient for discrete 20 m channel segments, beginning at the upstream extent of valley channelization.

Calculation of the outburst flood volume, flow rate and mobility analysis
The ESRI 30 cm Hi Resolution World Imagery (ESRI Map Service, ID: 10df2279f9684e4a9f6a7f08febac2a9) and the 2019 LiDAR data were used to estimate the volume of water associated with a proposed lake outburst flood event that occurred sometime between 1959 and 2019. The pre-event lake surface area measured approximately 95,000 m2: 750 m × 150 m × 0.85, where 0.85 is a shape factor correction from a rectangle. The post-event lake surface area measured approximately 60,000 m2: 700 m × 100 m × 0.85. The differential area associated with the lake outburst flood is approximately 35,000 m2. Our LiDAR and photographic data suggest between 1959 and 2019, the lake water surface elevation dropped by approximately 3 m (Fig. 4b). If this entire depth loss is associated with the lake outburst flood, the estimated flood volume is within the range of 104–105 m3. It is also possible that all or a proportion of this volume was lost due to slow leakage at the downstream lake margin. Here we focus on the lake outburst loss mechanism as it provides a plausible explanation for the balance of our field and photographic observations, and inferences—in particular, the imbrication of gravels and cobbles along channelized segments.

We used an empirical relationship to estimate the outburst flood peak flow rate as Q p~3.1W*H1.5*[A/(A+τ*H0.5)]3, where W=W b+Z b*H with W b the breach width, Z b a parameter with a value of 1.0, H the height of water above the breach, τ the elapsed time since breach development, A=23.4S a/W with S a the surface area of the lake at the breach elevation (Supplementary Table 3). For the calculations, we assume an average lake surface area of 72,500 m2 (average of 95,000 and 60,000 m2), a breach width of 2 m, a breach hydraulic head of 1 m and a breach development time of 0.167 h. Calculations are done using imperial units and converted to metric.

Field photographs suggest that the peak flood depths of the proposed lake outburst flood could have achieved 2 m; we also recognize that this is difficult to discern in the field as incision likely occurred during the passage of the flood peak and thus depths could be overestimated from field evidence. Despite the uncertainty associated with the thermal state of bed sediments when the proposed flood may have occurred, we use a simple Shield’s analysis to estimate conditions necessary to mobilize gravel and cobble size sediment particles observed on the streambed in Muskox Valley for flow depths of 1 and 2 m (Supplementary Figs. 2 and 3). The visual and photographic estimated 50th and 90th percentile grain sizes on the streambed in the valley were 20 and 96 mm. The slope-corrected critical Shield’s condition using an average valley slope of 0.03 suggests flow depths of 2 m are sufficient to transport both the 50th and 90th percentile grain sizes, and flow depths of 1 m are sufficient to transport the 50th percentile size (Supplementary Table 1). We assumed top flow widths of 2 and 3 m in both cases and applied a cross-section shape adjustment factor of 0.5 to calculate the hydraulic radius.

Calculation of estimated active layer thaw depth
Active layer thaw depth estimate made using a solution to the Stefan problem scaled for the volumetric water content: L_skin=λ*[(2kΔT s t)/(θρ w L)]0.5 where λ is a correction factor with a value less than 1, k is the thermal conductivity of the bulk unfrozen substrate which is assumed to be a saturated, vertically homogeneous gravelly sand, ΔT s is an air temperature step change at the surface above 0 °C, t is the time since the start of air temperature step change, θ is the volume fraction of water in the substrate, ρ w is the bulk density of water and L is the mass-based latent heat for the fusion of water. Calculations based on k=1.1 Wm−1 C−1 per Dalla Santa et al., θ=0.40, ρ w=1000 kg/m3, L=3.34*105 J/kg, and λ has a value of 0.95 based on Fig. 5 of Kurylyk and Hayashi. In choosing the thermal conductivity of the gravelly sand substrate, we assume no appreciable shallow groundwater flow. In the context of Muskox Valley and similar landscapes, the time scale t is that for a lake draining event V f/Q a, where V f is the estimated volume of water released during the outburst flood event (discussed above), and Q a is the average flow rate (m3/s) during the flood event. The time scale V f/Q a gives the time over which a thermal perturbation is delivered to the active layer substrate. See the online repository for a Jupyter Notebook documenting the calculations and plotting of Fig. 6.

Hydrodynamic modeling
The simulations were conducted using the LiDAR-derived DEM, and BASEMENT-Basic Simulation Environment, ETH Zurich. We focused the modeling on a portion of Muskox Valley which is channelized and also supports a wetland. This choice was made in order to examine how differing landform properties influence the routing of surface flows. The input unstructured mesh was calculated using a plugin for QGIS for BASEMENT, with cell sizes ranging from 0.10–5 m. The mesh was delineated into two regions of channel and floodplain, or overbank. The hydrodynamic module solves the shallow water equations, where the system unknowns are the water surface elevation and the two components of the streamflow per unit width. The hydraulic friction closure was calculated using the Manning-Strickler relation, with standard roughnesses set for the channel and the floodplain. Eddy viscosity was calculated with the constant model. The simulations assume no sediment transport and hence a fixed topographic boundary because we presently lack theory to estimate sediment particle entrainment of partially or completely frozen channel bed surfaces.

Three different steady flows were assigned at the upstream boundary of 0.25, 2.5 and 5 m3/s. The input mesh and model build files have been provided in the supporting online repository.

Data availability
The processed topography data and the hydrodynamic model setup input mesh file have been deposited at figshare: https://doi.org/10.6084/m9.figshare.19790491.v1. The repository includes a README.txt file to navigate the contents.

Code availability
The Python scripts used to produce the inset to Fig. 1 and the main part of Fig. 6, and the hydrodynamic model setup files used to produce Fig. 3 and Supplementary Fig. 1 with the BASEMENT open access software are also provided in the above-referenced data repository at figshare. The calculations for Supplementary Tables 2 and 3 are provided as Supplementary Data 1 in the online supplemental information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I have reviewed the paper entitled ‘Interplay among climate, polygonal terrain and channel\nformation in the Canadian High Arctic’. This study tries to reveal the interactions among climate\nchange, hydrologic processes governing water delivery, and consequently the timing and pace of\nchannelization and landscape change in a high Arctic permafrost valley setting. Personally, I think\nthis is an important contribution to the existing literature as there is a critical knowledge gap in\nterms of measuring the sedimentary and geomorphic responses to modern climate change (East et\nal., 2022, Earth's Future), as well as the possible impacts of rapid erosion on carbon release and\nthus climate feedbacks (Zhang et al., 2022, Nat. Rev. Earth Environ.). The findings of this study\nwould be of interest to a range of research communities including climatologists,\ngeomorphologists, as well as hydrologists and permafrost scientists. Although some of the\nstatements in this study seem speculative and lack more robust in-situ observations (details\nbelow), it will likely inspire and stimulate more follow-up studies to enhance observations in the\nArctic and develop new generations of more rigorous physical models of channel network\ndevelopment in the strategically important cold regions.\nOverall, I think the manuscript is likely suitable for publishing in Nature Communications, after\nclarification of the major questions and several aspects listed below.\n(1) The study seems lake quantitative findings and some statements are vague. For example, in\nLines 24-26: this sentence seems the key message of this study, but the readers are left unclear\nabout the specific findings. e.g., what are the specific major controls over channel incision in\npermafrost landscapes? what are the plausible positive feedbacks among the climate, periglacial\nlandforms and channelization processes? Lines 26-29: again, it is not clear here and seems\noverstated. What is the new generic landscape change model (it is still unclear after reading\nthroughout the paper)? what does the ‘intensifying variability’ represent? Do you mean the\n'intensifying variability of climate, runoff, or sediment supply'?\n(2) Introduction (Lines 35-55): it does summarize the existing knowledge gaps in a concise and\ncomprehensive way and does highlight the importance of the seasonal dynamics of climate, ice,\nthe active layer, different water sources and sediment supply in the permafrost environment in\ndriving landscape change and channel initiation, but this study lacks quantitative seasonal field\nobservations of these key factors to support the underlying process. In particular, the seasonal\nwater dynamics (multiple water sources in cold regions) and maybe also sediment supply can\nexert important controls on channel initiation and development. Maybe the data from similar\npermafrost landscapes would be helpful to explain some of the statements.\n(3) The study concludes that it “offers a local framework to develop new generations of rigorous\nphysical models of valley-scale channel network development……” However, the readers are left\nunclear about the framework and also the process-based positive feedback with climate warming.\nCould you please explicitly explain process-based positive feedback by inserting a conceptual\nframework in Figure 4? The current Figure 4 is nice and helpful, and it can be improved by\nhighlighting the interactions of different processes.\n(4) Similar to the High Mountain Asia (Li et al., 2021, Science), the Arctic climate has become\nwarmer and wetter since the 1990s, with more summer rainfall that will likely change the\nhydrological processes and cause more permafrost disturbances and related erosion and channel\nnetwork evolution. The changing rainfall patterns and their impacts on landscape changes (e.g.,\npermafrost thermokarst development and channelization) should also be explicitly discussed (after\na quick search, the word ‘rainfall’ only appears once in the main paper).\nAdditional comments:\n(1) Line 79-81. This statement seems not supported by the data. Can you prove that the polygon\ngeometry can influence or govern the channel path or paths both across and down topographic\ngradients at the valley scale?\n(2) Line 137-139. Is it possible to provide more quantitative statements, such as the relative\ncontributions of different water sources to emergent channel segments? How frequent is the\noutburst floods from the upstream lake?\n(3) Line 287-290. It is not entirely clear how steeper hydraulic gradients provide increased surface\nareas for heat exchange.\n(4) Movie: unfortunately, the youtube link does NOT work and I did not see it from the manuscript\ntracking system.\n(5) Method-Calculation of Width-Depth Ratio. It is not easy to follow here. Can you provide a\nsketch diagram for the calculation of width and depth? For example, how do you determine the\naverage depth and width for the cross-sections in Figure 2?\n(6) Method-Calculation of the Mobility Analysis and Table ED2. I am not exactly working on\nincipient sediment motion. Here are a few questions for your consideration. Is the Shield’s analysis\nsuitable to estimate sediment particle mobilization conditions here? how about other methods that\nare more suitable for pebble-cobble sized sediment particle mobilization? For Table ED2, the\nauthors seem to treat the riverbed sediment as a uniform bed when calculating the grains'\nmobilization velocity for a certain D50 or D90. However, the actual channel is a non-uniform bed,\nand how to consider it? The study selects 4 sets of experimental calculations (Table ED2). Are they\nrepresentative and can they truly reflect the critical flow conditions for riverbed surface particles?\nWhat about the different ice conditions that could affect sediment mobility?\n(7) Figs. 1-3. Please add the scale bars in these images. Would it be good to highlight the\ngeographical information on the figure instead of describing it in the captions (Fig. 1 and Fig. 3).\n(8) Fig. 2. Could the fonts in this figure be a bit larger?\n(9) Line 33: change ‘mountain regions’ to ‘cold regions’?\n(10) Fig. 4: nice figure. Maybe the long caption could be shortened a bit. I did not see the\nframework from this figure.\nReferences mentioned:\nEast, A. E., Warrick, J. A., Li, D., Sankey, J. B., Redsteer, M. H., Gibbs, A. E.,... Barnard, P. L.\n(2022). Measuring and Attributing Sedimentary and Geomorphic Responses to Modern Climate\nChange: Challenges and Opportunities. Earth's Future, 10(10), e2022E-e2983E.\nLi, D., Lu, X., Overeem, I., Walling, D. E., Syvitski, J., Kettner, A. J.,... Zhang, T. (2021).\nExceptional increases in fluvial sediment fluxes in a warmer and wetter High Mountain Asia.\nScience, 374(6567), 599-603.", "reviewer_2": "Joel Rowland\nLos Alamos National Laboratory\nThe manuscript provides a detailed field study of an evolving channel system in the Canadian High\nArctic. Based on these observations that manuscript puts forward conceptual model for the relative\ncontrols of fluvial and thermal controls on erosion and channel development in permafrost settings\ncharacterized by ice wedge polygons. The paper is well written and the graphics are excellent. I\nthink the study is of broad value and highlights future data needs in the field I reviewed an earlier\nsubmission of this manuscript and believe the paper is acceptable for publication in its present\nform. I only have a few minor points.\nFigure 2: It is stated that the cross-section elevations are normalized by the maximum elevation.\nThe axis reports elevation in m, but ranges from 0-1, are all of the cross sections only 1 m in\nheight or should they be unitless? As mentioned in the prior review, it would be helpful if the\nmanuscript explained what is learned by the power spectra and how it relates to the valley and\nchannel evolution.\nLine 165-170: As written, it sounds as though the authors infer that the polygons have formed in\nthe valley bottom since 1959. Is it possible that the polygons pre-date 1959 but did not have a\nvisible surface expression until warming begin to melt the ice wedges and causes local ground\nsubsidence that made the polygons visible in imagery and in the topography? I had always\nassumed, perhaps incorrectly without reading more background, that ice wedge polygon\ndevelopment was a slower longer-term process than time for them to appear at this study site.\nDo the authors see any evidence for the development or pipes through the polygon network.\nElsewhere melting of ice wedges by flowing water beneath intact ground surfaces is not\nuncommon (Fortier et al 2007). Collapse of soil into these tunnels can lead to channel\ndevelopment that is very distinct from gullying and headcutting commonly observed in more\ntemperate systems. Do the authors see any evidence of this at this study site?\nIn Fig ED2 The caption notes that gravel bed material show imbrication, it might be worth\nmentioning this in the main text to indicate evidence that the gravels were transported by flowing\nwater rather than just being lag deposits from loss of fine sediments and ice.\nI was not able to access the movie. The link provided still required a password to sign in.", "reviewer_3": "Notes with Nature Communications Review, Jan 2023\nThis study uses field observations of a small valley river system in the Canadian high Arctic to\ninvestigate what are the major controls on channel incision and morphological change.\nThe study site is located in the continuous permafrost region, meaning that the entire landscape is\n(still) dominated by frozen substrate and a seasonally active topsoil layer only. Ongoing and future\nprojections of rapid warming will provoke rapid change in these systems and documentation of\nchange in landscape processes is important and timely.\nThis paper is original and of relevance for anyone interested in evolution of permafrost landscapes.\nThis topic is important because of the permafrost carbon feedback system which allows for\npreviously frozen carbon to possibly decompose and release carbon dioxide or methane upon\nmodern enhanced thaw. How that process plays out across the landscape is not well understood,\nand hydrological controls and channel corridors are thought to be especially complex due to\ngeomorphic feedbacks of sediment transport and bank erosion. In addition, fundamental process\nunderstanding as presented in this paper applies to much wider zones of landscapes across the\nNorthern hemisphere during the Quaternary glacial and interglacial cycles.\nNoteworthy results include the detailed geomorphic descriptions of channel networks in a unique\nhigh-latitude cold region setting. These areas are extremely data sparse, and this study provides\nhigh-quality modern topographical data. In addition, analysis over 60+ years documents how\nsedimentation patterns are reshaping the topographic profiles under the present-day warming.\nThe study recognizes that well-developed channels are common around the edges of large\npolygons and within inter-connected troughs. But the data also indicates that channelization is\ndiffuse or non-existent where relatively poorly developed polygons occur in wetlands. Thus, the\npaper provides as a finding how presence or absence of permafrost polygon geometry can\ninfluence or even control the channel pathways. This control has been hypothesized by other\npapers, and the authors do reference appropriate previous work. One exception: please reference\nto the work on polygon to hydrological connections by Liljedahl et al.,\nhttps://www.nature.com/articles/ngeo2674, which appears perhaps more relevant than some of\nthe work on Martian features.\nThe analysis of the topographic data also notes remarkable slope independence of the\nchannelization process and demonstrates that W/D ratios do not necessarily follow erosive power\nor valley slope.\nOne major recommendation for improvement is to place these W/D ratios in perspective. The\nfinding can be compared to lower latitudinal settings from literature, and to other Arctic settings.\nThe Arctic DEM data will allow for this analysis. I think such analysis deserves a figure.\nThen, the authors ask what processes or factors control the production and discharge of water with\na sufficient flow rate to help understand channelization within Muskox Valley? This the novel part\nof the manuscript!\nQuantitiative modeling of sediment transport processes and substrate thaw is invoked to provide\nunderstanding of entrainment of coarse sediment for frozen environments. This analysis is a new\ncontribution, and my second major recommendation is to present this modeling more upfront in\nthe paper. The model suggests that to transport frozen gravel, stream or river water needs to add\nheat first to thaw out the gravel from its matrix, and provides estimates of the time that thaw\nprocess would take. If stream discharge is cold and episodic, the duration of thawing out the bed\nmay have inhibited transport even at hydraulically sufficient conditions. The description of these\nmodel results and Figure ED3 suggest that in frozen environments critical threshold sediment\ntransport theory needs to be adapted. This can be better integrated into the main manuscript.\nThis study discusses implications of observations and available data, and provides suggestions for\nmechanisms based on field observations, high resolution topographic data and historical imagery\nand climate change, there is no direct evidence for some of the mechanisms. But this is typical for\nstudies in such data sparse regions, the authors are careful to discuss the limited data, and the\nquantitiative modeling further supports suggested controls.\nWork is well documented in methods section. I applaud the authors for their intent to share the\nanalysis as open-source workflows and for provided jupyter notebooks. The supplementary\nmovies, figshare and jupyter notebooks are not viewable at this stage, even using the private links\nprovided. So, I cannot evaluate this question properly and leave this question to the editor to\nverify.\nI annotated the PDF with more minor suggestions to reword the abstract slightly, to make minor\nedits to figures etc.\nIrina Overeem\nUniversity of Colorado"}, "author_response": "Response to Comments for Chartrand et al – NCOMMS-22-53478-T\nTitle: Interplay among climate, polygonal terrain and channel formation in the Canadian High Arctic\nMay 11, 2023\n_____________________________________________________________________________________________________________________________\nResponse to Reviewer 1 Comments\nReviewer 1 Introductory Remarks:\nI have reviewed the paper entitled ‘Interplay among climate, polygonal terrain and channel formation in the\nCanadian High Arctic’. This study tries to reveal the interactions among climate change, hydrologic processes\ngoverning water delivery, and consequently the timing and pace of channelization and landscape change in a high\nArctic permafrost valley setting. Personally, I think this is an important contribution to the existing literature as\nthere is a critical knowledge gap in terms of measuring the sedimentary and geomorphic responses to modern\nclimate change (East et al., 2022, Earth's Future), as well as the possible impacts of rapid erosion on carbon release\nand thus climate feedbacks (Zhang et al., 2022, Nat. Rev. Earth Environ.). The findings of this study would be of\ninterest to a range of research communities including climatologists, geomorphologists, as well as hydrologists and\npermafrost scientists. Although some of the statements in this study seem speculative and lack more robust in-situ\nobservations (details below), it will likely inspire and stimulate more follow-up studies to enhance observations in\nthe Arctic and develop new generations of more rigorous physical models of channel network development in the\nstrategically important cold regions.\nOverall, I think the manuscript is likely suitable for publishing in Nature Communications, after clarification of the\nmajor questions and several aspects listed below.\nResponse: Thanks for your constructive and supportive feedback. We appreciate your time and energy in\nreviewing and thinking about our work. Your comments have helped us prepare a more clear and impactful\ncommunication of our work, including development of new analysis (hydrodynamic modelling) and expansion of\nthe thaw front calculations to highlight a particular knowledge gap we are presently pursuing.\nReviewer 1 Substantial Comments:\n(1) The study seems lake quantitative findings and some statements are vague. For example, in Lines 24-26: this\nsentence seems the key message of this study, but the readers are left unclear about the specific findings. e.g., what\nare the specific major controls over channel incision in permafrost landscapes? what are the plausible positive\nfeedbacks among the climate, periglacial landforms and channelization processes? Lines 26-29: again, it is not\nclear here and seems overstated. What is the new generic landscape change model (it is still unclear after reading\nthroughout the paper)? what does the ‘intensifying variability’ represent? Do you mean the 'intensifying variability\nof climate, runoff, or sediment supply'?\nResponse: Thanks for pointing out the need for clarifications in the summary paragraph. We have addressed these\ncomments in several ways:\n1. At lines 24-26 of the original submission, we added the following phrase: “related to a proposed lake\noutburst flood event.”\n2. At lines 26-29 of the original submission, we added the following phrase: “such as enhanced rates of\npermafrost degradation.”\n3. We have also modified the last sentence of the summary paragraph as follows: “We integrate our\nobservations and propose a new generic landscape change framework to better understand model for the\nunderlying processes governing the channelization response...”\n4. The “intensifying variability” phrase is with respect to the warming climate, as the sentence intends. No\nchange made here.\n(2) Introduction (Lines 35-55): it does summarize the existing knowledge gaps in a concise and comprehensive\nway and does highlight the importance of the seasonal dynamics of climate, ice, the active layer, different water\nsources and sediment supply in the permafrost environment in driving landscape change and channel initiation, but\nPage 1\nNCOMMS-22-53478-T Response to Comments\nthis study lacks quantitative seasonal field observations of these key factors to support the underlying process.\nIn particular, the seasonal water dynamics (multiple water sources in cold regions) and maybe also sediment supply\ncan exert important controls on channel initiation and development. Maybe the data from similar permafrost\nlandscapes would be helpful to explain some of the statements.\nResponse: To date, high elevation, Arctic and High Arctic research which notes and discusses channelization in\nbroad terms does so in the context of suspended, or fine sediment. This is understandable because relatively fine\nsediment loads and dynamics are used as a proxy for watershed scale erosion conditions, and fine sediment is\neasier to measure and quantify compared to coarse, or bedload sediment. We see the the later problem as an\nemerging key to help advance landscape change science in these regions; however, we are unaware of any\nsubstantial records of bedload sediment measurement, particularly in High Arctic polar desert climates (we are\naware of and have read Micheal Church’s PhD research on Baffin Island – a wetter climate than the High\nCanadian Arctic). As a result, we have not attempted to incorporate data from other publications into the research\nwe present. We have, however, addressed the comment with a few editorial changes, including the addition of the\nfollowing sentence on line 39 of the original submission:\n“The occurrence of active layer detachment slides and thaw slumps also influences hydrologic flow pathways by\nphysically modifying the landscape, and providing point sources of meltwaters which can enhance down gradient flow\nchannelization, introducing new sources of fine and coarse sediment to drainage basins9,19.”\nAnd we have added additional references at the end of two sentences within the range of line numbers of the\noriginal submission noted in the comment. We were remiss in not citing the noted publications in our original\nsubmission, and focused our referencing to research which relates directly to water sources known to increase fine\nsediment flux in permafrost landscapes, and as a result an apparent increase in watershed scale erosion.\n(3) The study concludes that it “offers a local framework to develop new generations of rigorous physical models\nof valley-scale channel network development……” However, the readers are left unclear about the framework and\nalso the process-based positive feedback with climate warming. Could you please explicitly explain process-based\npositive feedback by inserting a conceptual framework in Figure 4? The current Figure 4 is nice and helpful, and it\ncan be improved by highlighting the interactions of different processes.\nResponse: Thanks for this important comment. We have carefully expanded Figure 4 (Figure 5 in the revised\nmanuscript) to address the comment, while also being mindful that additional content could make the figure\ndifficult to understand and therefore mask or hide the intent to focus on aspects of permafrost and periglacial\nlandscapes which collectively contribute to drainage basin establishment and channelization. The principal\nadditions to Figure 4 include:\n1. Addition of a flow hydrograph, sediment flux and a drainage channel from a hypothetical glacier located\nat the left, mid-watershed part of the depicted basin. This links, in particular, to important research by Li et\nal. (2021) and Zhang et al. (2022) which highlights that glacier melt due to climate change (and\ndownstream induced streamflow driven channel margin erosion due to thermal and mechanical effects)\nincreases downstream fine sediment fluxes (and coarse sediment depending on the grain size distribution\nof eroded deposits and regolith.\n2. Addition of a tributary channel originating from a thaw detachment slide (vignette G), that causes a fan at\nthe valley margin, and a channel which then connects to the main channel at the valley center. This\nrepresents research by Walvoord et al. (2016), and Lafrenière and Lamoureux (2019) which highlights the\nimportance of thaw slumps to basin sediment budgets. We also observed during our fieldwork that a large\nthaw slump located on a plateau above the valley floor resulted in the development of a new tributary\nchannel which connected down to the valley trunk channel system --> providing a direct input of seasonal\nwater and sediments to the receiving basin.\n3. Addition of two new features to vignette D: (a) a second permafrost boundary line (dashed, thinner line\nabove the permafrost boundary line) which indicates that as the adjacent channel incises and deepens, the\nlocal permafrost boundary/active layer thickness increases and deepens, (b) a zoomed in snippet which\nshows a packed sediment bed with organics within the interstices of the sediments and which are subject\nto decomposition by microbes as the permafrost boundary deepens and the sediments seasonally dry. The\nPage 2\nNCOMMS-22-53478-T Response to Comments\nDecomposition and carbon respiration is dependent on the occurrence of buried organic carbon, and\nwhether sediments/soil moisture content (e.g. Natali et al., 2015).\n4. Addition of colorized relative sediment supply size distribution keys to various vignettes in the figure to\nindicate respective roles in delivery of sediment to the basin from the depicted processes, and as a result an\noverall/general indication of how each process is important for drainage basin establishment. Relative\nsediment supply is shown as being fine (suspended transport), or coarse (bedload transport). This is\nimportant because alluvial channels in mountainous landscapes are shaped predominately by bedload\ntransport, but can be initiated through erosion that is predominately within fine, coarse or a size mixture of\nsediments. Several publications (referenced in the figure caption) were consulted in developing the\ncolorized keys. The addition of the keys to the various vignettes (which previously focused on\nimplications for water sourcing and movement through and within the landscape) and additional revisions\ndiscussed above provide a clearer idea of how different periglacial and permafrost landscape processes\ncouple, or ‘interact’ to give rise to landscape development due to a warming climate.\nLast, as we understand a ‘framework’ should (a) ask or motivate the development of impactful research questions;\n(b) provides an organizing structure for general understanding, and for research questions motivated by the\nframework, and (c) provides a general basis for interpreting how ‘things’ are understood (in the present case\nperiglacial and permafrost landscapes responding to a warming climate), and as a basis for developing new\nhypotheses, etc. When we use ‘framework’ in the presentation this is what we mean; we believe the modifications\nmade to Figure 4 based on the comments above helps in understanding the figure as a general framework.\n(4) Similar to the High Mountain Asia (Li et al., 2021, Science), the Arctic climate has become warmer and wetter\nsince the 1990s, with more summer rainfall that will likely change the hydrological processes and cause more\npermafrost disturbances and related erosion and channel network evolution. The changing rainfall patterns and\ntheir impacts on landscape changes (e.g., permafrost thermokarst development and channelization) should also be\nexplicitly discussed (after a quick search, the word ‘rainfall’ only appears once in the main paper).\nResponse: Thanks for this important comment. We have incorporated a direct reference to rainfall at several\nadditional locations throughout the main body of the manuscript:\n1. Lines 151 of the Original Submission: “Over multiple seasons, presumably depending on the intensity of\nsummertime warming and as well as rain and/or snowfall precipitation events4-6,...”;\n2. Lines 211-212 of the Original Submission: “...subject to the magnitude of the seasonal water budget and\nthe potential for proportionally more rainfall under a warmer Arctic35 (Fig 5).” With this addition we have\nalso incorporated an important new reference concerning future modelled precipitation trends in the Arctic\n(McCrystall et al., 2021, Nature Communications).\nWe also note that rainfall and its importance to the processes discussed was significantly incorporated within\nFigure 4 of the original submission.\nReviewer 1 Additional Comments:\n(1) Line 79-81. This statement seems not supported by the data. Can you prove that the polygon geometry can\ninfluence or govern the channel path or paths both across and down topographic gradients at the valley scale?\nResponse: This was an important question which we considered carefully. We have completed additional analysis\nusing a 2-dimensional hydrodynamic model to examine how polygons and the lack thereof influences surface flow\nrouting along one part of Muskox Valley (what we call Panel C, of the LiDAR data). The simulations were\nconducted using BASEMENT-Basic Simulation Environment, ETH Zurich. The results are summarized in the new\nFigure 3, as well as supplemental material Figure ED1 and Table ED3. Figure 3 illustrates that polygons influence\nhow surface flow is routed down valley for a surface flow of 2.5 cms, which is within the a range of flows that\ncould have occurred during the hypothesized lake outburst flood (Table ED3). Supplemental Figure ED1 illustrates\nadditional results from an plausible late summer, early fall flow (0.25 cms), to a an additional flow that could have\noccurred during the hypothesized lake outburst flood (5 cms) (Table ED3).\nPage 3\nNCOMMS-22-53478-T Response to Comments\nFurthermore, the distribution of downstream velocities differs from polygon regions to wetland regions, as\nexpected. The map of surface flow routing in Figures 3 and ED1 also shows that water can move down the\npredominate topographic gradient, as well as across gradients as higher situated polygon troughs become flow\npaths for surface flow. In our original submission we inferred this behavior based on visual observations of similar\nflow behavior from our trip to Devon Island in the summer 2022; preparation of the model provides more concrete\nevidence of the inferred influence and represents a substantial addition to our overall presentation and supporting\nanalysis. We note that the simulations were run for a fixed land surface boundary condition because we presently\ndo not know how to model the entrainment of channel bed sediments which may be frozen, or partially frozen (as\nwe discuss toward the end of the main body of the manuscript, and as elaborated by Zhang et al., 2022). All model\nset-up files have been uploaded to the manuscript data repository to facilitate reproduction by others.\n(2) Line 137-139. Is it possible to provide more quantitative statements, such as the relative contributions of\ndifferent water sources to emergent channel segments? How frequent is the outburst floods from the upstream\nlake?\nResponse: We presently lack the necessary data to make general statements about relative contributions of\ndiffering water sources to emergent channel segments, as well as how frequent outburst floods may be at our study\nsite, or for similar landscapes. This is part of the focus of future research that we are seeking funding to complete.\nWe believe the present sentence is true to what we do not know/understand, in the context of framing an important\nquestion.\n(3) Line 287-290. It is not entirely clear how steeper hydraulic gradients provide increased surface areas for heat\nexchange.\nResponse: As a channel incises or deepens, there is more surface are exposed at the channel margins, provided the\nabandoned floodplain or adjacent surface changes elevation in a relatively minor way compared to the channel.\nWe believe the source of confusion lies in how we worded the original sentence. We have modified the subject\nsentence as follows (additions in red text):\n“Channel cross-section development within polygon troughs locally steepens hydraulic gradients, and a deepening channel\nprovides increased surface areas at the channel margins for heat exchange, which can, in turn, increase rates of local\npermafrost thaw.”\n(4) Movie: unfortunately, the Youtube link does NOT work and I did not see it from the manuscript\ntracking system.\nResponse: We apologize for the movie not being accessible. We have now made the movie public.\n(5) Method-Calculation of Width-Depth Ratio. It is not easy to follow here. Can you provide a sketch diagram for\nthe calculation of width and depth? For example, how do you determine the average depth and width for the cross-\nsections in Figure 2?\nResponse: Thanks for the clarifying comment/question. We have rewritten the section in the Field Site, Movies\nand Methods Summary section. The revised text is as follows:\n“We acquired valley cross-section profiles on LiDAR elevation data to calculate width-depth ratios. We used ArcGIS\n10.8.1 to clip the LiDAR data in four 600 m wide tiles along valley, measuring 1-2 km each. We then extracted 20 cross-\nsections spaced regularly every 70 m down valley, which were then loaded onto MATLAB to calculate cross-section slope\nand curvature (Fig. ED6). To minimize uncertainty related to qualitative measurements, we measured top width as the\ndistance between curvature minima, and bottom width (valley floor) as the distance between curvature minima located\nwithin the previously identified valley width45. Note that some profiles have multiple channels. These were also measured\nfollowing the same technique. Associated depth along each transect was measured as the deepest point from the top width\nline.”\nWe have also prepared a new supplemental figure to illustrate an example of the cross-section slope and curvature\n(Figure ED6).\n(6) Method-Calculation of the Mobility Analysis and Table ED2. I am not exactly working on incipient sediment\nmotion. Here are a few questions for your consideration. Is the Shield’s analysis suitable to estimate sediment\nPage 4\nNCOMMS-22-53478-T Response to Comments\nparticle mobilization conditions here? how about other methods that are more suitable for pebble-cobble sized\nsediment particle mobilization? For Table ED2, the authors seem to treat the riverbed sediment as a uniform bed\nwhen calculating the grains' mobilization velocity for a certain D50 or D90. However, the actual channel is a non-\nuniform bed, and how to consider it? The study selects 4 sets of experimental calculations (Table ED2). Are they\nrepresentative and can they truly reflect the critical flow conditions for riverbed surface particles? What about the\ndifferent ice conditions that could affect sediment mobility?\nResponse: Thanks for the questions and thoughts. We have conducted the originally presented sediment mobility\nanalysis using the most generally accepted approach in fluvial sediment transport research. The calculations make\nno assumption of sediment size uniformity; on the contrary, a mixture of sizes is explicitly assumed in the use of a\nD50 and D90, as these representative sizes are drawn from a distribution of sizes. The question of whether the\nShield’s analysis is suitable or not is a fair question; until new theory is developed to a stage for making similar\ncalculations, the Shield’s analysis is the best science can offer when site information is somewhat limited.\nFurthermore, we present the outcomes of the calculations as ‘estimates’, noting there is uncertainty in the\npresented approach. We presently lack the necessary theory to make estimates of sediment mobility for partially or\ncompletely frozen channel beds-->this is the exact emphasis of the knowledge gap we present in the manuscript,\nand the focus of laboratory experiments we are presently building (within the introductory paragraph and in\ndiscussion of original submission Figure 4—i.e. the conceptual framework).\n(7) Figs. 1-3. Please add the scale bars in these images. Would it be good to highlight the geographical information\non the figure instead of describing it in the captions (Fig. 1 and Fig. 3).\nResponse: We have a several different responses here:\n It is difficult to insert a scale bar into Figure 1 because these are a variety of photographs taken from\ndifferent altitudes and perspectives. However, we inserted two approximate length scale indications within\nFigure 1 based on the GIS coverage and our LiDAR DEM.\n In our Figure 2 original submission the x- and y-axes are in units of meters, and we indicate stationing\nalong the channel profile line in meters (200 m increments). Based on the comments, we have added a\nscale bar to Figure 2a.\n In our Figure 3 original submission the top image provides an indication of approximate length scale,\nwhich is consistent between images. We have not added anything to Figure 3 in our revisions based on this\ncomment.\n(8) Fig. 2. Could the fonts in this figure be a bit larger?\nResponse: Sorry about that. We have increased the font size of the inset figures x- and y-axes labels, and the row\nlabels in the power spectra of topography inset image.\n(9) Line 33: change ‘mountain regions’ to ‘cold regions’?\nResponse: We inserted ‘seasonally cold and’ in front of ‘mountainous regions’\n(10) Fig. 4: nice figure. Maybe the long caption could be shortened a bit. I did not see the framework from this\nfigure.\nResponse: Thank you for the nice comment. We have left the caption as presented in the original submission (with\nsome minor edits and additions) so that potential readers can understand the figure without needing to refer to the\nmain text. We have addressed the concern regarding the idea of a ‘framework’ above, in our response to the earlier\ncomment.\nResponse to Reviewer 2 Comments:\nReviewer 2 Introductory Remarks:\nThe manuscript provides a detailed field study of an evolving channel system in the Canadian High Arctic. Based\non these observations that manuscript puts forward conceptual model for the relative controls of fluvial and\nPage 5\nNCOMMS-22-53478-T Response to Comments\nthermal controls on erosion and channel development in permafrost settings characterized by ice wedge polygons.\nThe paper is well written and the graphics are excellent. I think the study is of broad value and highlights future\ndata needs in the field. I reviewed an earlier submission of this manuscript and believe the paper is acceptable for\npublication in its present form. I only have a few minor points.\nResponse: Thanks for your constructive and supportive feedback. We appreciate your time and energy in\nreviewing and thinking about our work. We have addressed each of your comments through revisions, or further\nexplanation provided below.\nReviewer 2 General Remarks:\n(1) Figure 2: It is stated that the cross-section elevations are normalized by the maximum elevation. The axis\nreports elevation in m, but ranges from 0-1, are all of the cross sections only 1 m in height or should they be\nunitless? As mentioned in the prior review, it would be helpful if the manuscript explained what is learned by the\npower spectra and how it relates to the valley and channel evolution.\nResponse: Thanks for catching that mistake—yes, the elevations in the figure are unitless and we have corrected\nthis error. To answer your second question, in our original submission we provided the following text within the\nField Site, Movies and Methods Summary, under the Power Spectra Estimation section:\n“Power spectral analysis of the ground topography is useful because it characterizes the dominant scale(s) of the\ntopographic ground structure(s), or physical patterns. In the present case the power spectra reveal that polygon fields along\nthe valley floor have a similar dominant length scale measured both down and across the valley.”\nAnd we have provided the additional sentence following the above sentence within the same section:\n“Furthermore and depending on how polygons are spatially organized, this dominant length scale imparts a relative and\nconsistent unit flow path length as surface flows incise apparently stochastically along interconnected polygon troughs.”\n(2) Line 165-170: As written, it sounds as though the authors infer that the polygons have formed in the valley\nbottom since 1959. Is it possible that the polygons pre-date 1959 but did not have a visible surface expression until\nwarming begin to melt the ice wedges and causes local ground subsidence that made the polygons visible in\nimagery and in the topography? I had always assumed, perhaps incorrectly without reading more background, that\nice wedge polygon development was a slower longer-term process than time for them to appear at this study site.\nResponse: This is an important question and comment—thanks. Our discussion is based on visual interpretation of\nthe 1959 imagery, which could be prone to error. However, a careful review of the imagery suggests the\noccurrence of at least one set of polygons located roughly 350 m downstream of the lake at the head of Muskox\nValley. Furthermore, review of Figure ED4 indicates that polygons were widespread at different locations within\nand near to Muskox Valley at the time of the 1959 imagery. As a result, we infer that other areas interpreted to\nhave developed polygons at a later time, did not have polygons at the time of the 1959 imagery, or polygons were\nin the very early stages of development. Furthermore, if one assumes that polygons were widespread in Muskox\nValley in 1959, it would be necessary to explain why polygons in some parts of the valley are much more\nprevalent or obvious in the imagery than in other areas where they might be assumed to occur, but cannot be\nvisually inferred. We believe this is a more difficult position to maintain. Similar conclusions as proposed in the\nmanuscript can be reached by comparing Figure ED4 with recent aerial imagery of Muskox Valley. As implied in\nyour comment, this raises the issue that existing explanations of ice-wedge polygon development are incomplete,\nor are missing aspects that may introduce sensitivity to accelerated warming effects which can account for the\nrelatively rapid development we suggest. This is part of future research we plan to address.\n(3) Do the authors see any evidence for the development or pipes through the polygon network. Elsewhere melting\nof ice wedges by flowing water beneath intact ground surfaces is not uncommon (Fortier et al 2007). Collapse of\nsoil into these tunnels can lead to channel development that is very distinct from gullying and headcutting\ncommonly observed in more temperate systems. Do the authors see any evidence of this at this study site?\nResponse: This is a great question (one we asked ourselves many times), and one we evaluated carefully in the\nfield, as well as this past summer when we completed fieldwork on Devon Island. To be very clear, no, we saw no\nPage 6\nNCOMMS-22-53478-T Response to Comments\nevidence of piping-->nothing. As we mention in the original submission text, we cannot rule out that piping was/is\nnot a part of the channelization process in Muskox Valley, but we see no present day evidence that it is active in\nthe valley. We suspect that if it was active early on in the channelization process, most or all evidence was\ndestroyed with the hypothetical lake outburst flood event(s). But we have no evidence for this so we do not raise\nthis point in the manuscript. This is a major part of what makes the Muskox problem so intriguing.\nThe other aspect which we do not raise but hope to address with future experiments is the idea that piping can still\nbe active, but obscured by coarse material that fills introduced voids/pipes on a time scale that basically matches\nthe time scale of piping. In valley deposits rich with coarse sediments we hypothesize this is an alternate\nexplanation for the development of channels, but with the apparent absence of pipes and other voids related to ice\nwedge melting by surface flow.\n(4) In Fig ED2 The caption notes that gravel bed material show imbrication, it might be worth mentioning this in\nthe main text to indicate evidence that the gravels were transported by flowing water rather than just being lag\ndeposits from loss of fine sediments and ice.\nResponse: Thanks for this suggestion. We have modified the sentence at line 185 of the original submission to\ninclude reference to imbrication and Fig. ED2:\n“Assuming a single outburst flood occurred, simple calculations on the basis of the approximate transport properties of\nsediment particles associated with plausible flow geometries suggests that the flood was capable of transporting gravel and\ncobble sized sediment30.31, resulting in gravel and cobble imbrication observed at various locations along the channelized\nreaches (Methods; Figs. ED2 and ED3).”\n(5) I was not able to access the movie. The link provided still required a password to sign in.\nResponse: We apologize for the movie not being accessible. We have now made the movie public.\nResponse to Reviewer 3 Comments:\nReviewer 3 Introductory Remarks:\nThis study uses field observations of a small valley river system in the Canadian high Arctic to investigate what\nare the major controls on channel incision and morphological change. The study site is located in the continuous\npermafrost region, meaning that the entire landscape is (still) dominated by frozen substrate and a seasonally\nactive topsoil layer only. Ongoing and future projections of rapid warming will provoke rapid change in these\nsystems and documentation of change in landscape processes is important and timely.\nThis paper is original and of relevance for anyone interested in evolution of permafrost landscapes. This topic is\nimportant because of the permafrost carbon feedback system which allows for previously frozen carbon to\npossibly decompose and release carbon dioxide or methane upon modern enhanced thaw. How that process plays\nout across the landscape is not well understood, and hydrological controls and channel corridors are thought to be\nespecially complex due to geomorphic feedbacks of sediment transport and bank erosion. In addition, fundamental\nprocess understanding as presented in this paper applies to much wider zones of landscapes across the Northern\nhemisphere during the Quaternary glacial and interglacial cycles.\nNoteworthy results include the detailed geomorphic descriptions of channel networks in a unique high-latitude\ncold region setting. These areas are extremely data sparse, and this study provides high-quality modern\ntopographical data. In addition, analysis over 60+ years documents how sedimentation patterns are reshaping the\ntopographic profiles under the present-day warming.\nResponse: Thank you for the supportive and constructive review. We appreciate the time and energy you put into\nthinking about the presented research.\nReviewer 3 General Remarks:\n(1) The study recognizes that well-developed channels are common around the edges of large polygons and within\ninter-connected troughs. But the data also indicates that channelization is diffuse or non-existent where relatively\nPage 7\nNCOMMS-22-53478-T Response to Comments\npoorly developed polygons occur in wetlands. Thus, the paper provides as a finding how presence or absence of\npermafrost polygon geometry can influence or even control the channel pathways. This control has been\nhypothesized by other papers, and the authors do reference appropriate previous work. One exception: please\nreference to the work on polygon to hydrological connections by Liljedahl et al.,\nhttps://www.nature.com/articles/ngeo2674, which appears perhaps more relevant than some of\nthe work on Martian features.\nResponse: Thanks for pointing us to this publication and research. We were unaware of it, and agree on the\nrelevance to Muskox Valley and implications for our presented research. We have incorporated it at relevant\nlocations throughout the manuscript.\n(2) The analysis of the topographic data also notes remarkable slope independence of the channelization process\nand demonstrates that W/D ratios do not necessarily follow erosive power or valley slope. One major\nrecommendation for improvement is to place these W/D ratios in perspective. The finding can be compared to\nlower latitudinal settings from literature, and to other Arctic settings. The Arctic DEM data will allow for this\nanalysis. I think such analysis deserves a figure.\nResponse: Thanks for this suggestion. We have brought in a basic comparison within the main body of the\nmanuscript to alluvial rivers from the Pacific Northwest (large dataset), and from arid to semi-arid regions of the\nU.S. southwest (at line 122 of the original text submission). As expected, the width/depth from Muskox Valley\nfalls within the ranges from the Pacific Northwest and the U.S. southwest. The exception being braided river\nsegments where width/depth are commonly > 100. We have not developed a new figure, or completed a pan-Arctic\nanalysis of width-depth ratios. We believe this would be best as the focus of a different contribution. With this\nsaid, we do agree that the apparent independence between valley steepness and local width/depth is interesting and\nconfusing—we hope to address this issue with theory development which will explore how systems like Muskox\nValley evolve and respond to a warming climate.\n(3) Then, the authors ask what processes or factors control the production and discharge of water with a sufficient\nflow rate to help understand channelization within Muskox Valley? This the novel part of the manuscript!\nQuantitative modeling of sediment transport processes and substrate thaw is invoked to provide understanding of\nentrainment of coarse sediment for frozen environments. This analysis is a new contribution, and my second major\nrecommendation is to present this modeling more upfront in the paper. The model suggests that to transport frozen\ngravel, stream or river water needs to add heat first to thaw out the gravel from its matrix, and provides estimates\nof the time that thaw process would take. If stream discharge is cold and episodic, the duration of thawing out the\nbed may have inhibited transport even at hydraulically sufficient conditions. The description of these\nmodel results and Figure ED3 suggest that in frozen environments critical threshold sediment transport theory needs\nto be adapted. This can be better integrated into the main manuscript.\nResponse: Thanks for this important suggestion. We have addressed your suggestion in several ways:\n1. Revised thaw front analysis: We revised the simple thaw front analysis to include a constant surface\ntemperature forcing, interrupted by a hypothetical ‘heat wave’ at the surface, or heat transient. This\nrevision provides a realistic set of circumstances in Arctic environments for the present warming regime.\nThe main insight from this revised analysis is two-fold. (a) the thermal wave due to surface heat transients\nwill likely take on the order of days to reach thaw front depths set by the background conditions, at which\npoint thermal wave propagation will accelerate and penetrate more deeply (depending on local substrate\nand ground ice conditions). (b) the effects of a heat transient will be carried forward to the following thaw\nseason because a deeper thaw front means more soil/substrate volume to freeze, subject to water\navailability, etc. This can likely have the effect of deepening the active layer over time, and drying out the\nlandscape provided the seasonal precipitation regime is steady—this same effect was raised by Liljedahl et\nal. (2016), but due to differing processes related to polygon trough degradation. This highlights that\nmultiple processes can interact or are reinforced and led to similar outcomes. We have brought this\nanalysis into the main body of the manuscript including the revised figure. We also completed the revised\nanalysis using the thaw front relationship reported by Kurylyk and Hayashi (2016) because it provides a\nPage 8\nNCOMMS-22-53478-T Response to Comments\nmore direct way to represent the effects related to surface temperature events, and includes a simple\ncorrection factor for sensible heat effects.\n2. As discussed above, we also developed basic flow routing simulations using the 2-dimensional\nhydrodynamic model Basement (ETH Zurich). We completed the modelling to evaluate how polygons,\nand the lack thereof, influence the routing of surface flows down topographic gradients. Furthermore, we\nwanted to provide evidence for the direct coupling between polygons and surface flows (which we\nobserved in person during the summer 2022 on Devon Island-see new Movie detailed in the Field Site,\nMovie and Methods Summary) to support the proposal that polygons influence and likely accelerate rates\nof channelization where polygon troughs are interconnected and surface flows are large enough to\nmobilize channel bed sediments. As you point out in your comment, the next step is to develop a\ntheoretical basis for sediment mobilization from partially or completely frozen channel beds—the seasonal\ntiming of thawing such beds holds significant implications for sediment transport and channel evolution.\nThese model simulations and implications have also been moved and incorporated into the main body of\nthe manuscript (new Figure 3).\n(4) This study discusses implications of observations and available data, and provides suggestions for mechanisms\nbased on field observations, high resolution topographic data and historical imagery and climate change, there is\nno direct evidence for some of the mechanisms. But this is typical for studies in such data sparse regions, the\nauthors are careful to discuss the limited data, and the quantitative modeling further supports suggested controls.\nResponse: Thanks for this supportive and rationale comment. We hope that experiments we are presently planning\nand building, as well as future planned trips to the High Arctic will shed additional light on some of the hypotheses\nwe raise.\n(5) Work is well documented in methods section. I applaud the authors for their intent to share the analysis as\nopen-source workflows and for provided jupyter notebooks. The supplementary movies, figshare and jupyter\nnotebooks are not viewable at this stage, even using the private links provided. So, I cannot evaluate this question\nproperly and leave this question to the editor to verify.\nResponse: Apologies for the challenges in accessing the online repository of information. We have now corrected\nthe issues and all supporting online material to promote open science is accessible.\n(6) I annotated the PDF with more minor suggestions to reword the abstract slightly, to make minor edits to figures\netc.\nResponse: Thank you for taking the time to annotate the original submission. Here are our responses:\n1. Embedded Comment 1: We have left the phrase “This feature of” in the second sentence of the abstract.\nWe use this phrase to refer to the amplification presented in the opening sentence. No changes made.\n2. Embedded Comment 2: We believe the sentence reads okay, and captures the message we intend. No\nchanges made.\n3. Embedded Comment 3: Based on your suggestion, we have added the word “rising”. Thanks.\n4. Embedded Comment 4: We noted the site is located in the Qikiqtani Region of NU in the opening sentence\nof the section. No changes made.\n5. Embedded Comment 5: We do mention the power spectra in the original submission lines 98-103. We\nhave also added one sentence to the Field Site, Movie and Methods Summary to better explain what we\nlearn from the power spectra, based on a comment raised by Reviewer 2.\n6. Embedded Comment 6: Thanks for this suggestion. We have added colored lines indicating the location of\ncross-sections shown in Figure 2a.\n7. Embedded Comment 7: Thanks for catching that typo. Corrected to ‘later’ time.\nPage 9\nNCOMMS-22-53478-T Response to Comments\n8. Embedded Comment 8: Thanks. Noted and addressed above.\n9. Embedded Comment 9: Thanks and good catch. We have deleted the power ‘-1’ from the annual\nprecipitation value for Eureka, NU.\n10. Embedded Comment 10: We addressed this comment above in relation to your width/depth comment.\n11. Embedded Comment 11: Thanks.\n12. Embedded Comment 12: We agree that there are other possible explanations for the loss of lake volume.\nEvaporation over a 60-year time frame seems unlikely with seasonal inputs of water each year in the 60-\nyear period. We agree slow leakage is plausible. We have added the following sentences to the subject\nparagraph based on your comment:\n“Is it also possible that all or a proportion of this volume was lost due to slow leakage at the downstream lake\nmargin. Here we focus on the lake outburst loss mechanism as it provides the only plausible explanation for the\nbalance of our field and photographic observations and inferences–in particular the imbrication of gravels and\ncobbles along channelized segments.”\n13. Embedded Comment 13: Good point. However, given the lack of basis to calculate mobility fro partially\nor completely frozen bed sediments, use of the Shield’s estimate is the best available alternative. We have\nprefaced the sentence with the following introductory phrase:\n“Despite the uncertainty associated with the thermal state of bed sediments when the proposed flood may have\noccurred, we use a simple Shield’s analysis30,31 to estimate conditions necessary to mobilize gravel and cobble size\nsediment particles observed on the streambed in Muskox Valley for flow depths of 1 and 2 m (Figs. ED2 and ED3).”\n14. Embedded Comment 14: Correct, we were unable to complete pebble counts. As noted above we have\nmodified the text to indicate that the source of our grain size values is from visual and photographic\nestimates.\n15. Embedded Comment 15. Thanks. As discussed above and in response to another comment you raised, we\nhave incorporated this work within the main body of the revised manuscript.\nEND OF COMMENTS\nReferences:\nKurylyk, B. L., & Hayashi, M. Improved Stefan Equation Correction Factors to Accommodate Sensible Heat Storage during\nSoil Freezing or Thawing. Permafrost and Periglacial Processes, 27(2), 189–203 (2016).\nLafrenière, M. J. and Lamoureux, S. F.: Effects of changing permafrost conditions on hydrological processes and fluvial\nfluxes, Earth-Science Rev., 191, 212–223 (2019).\nLi, D., Overeem, I., Kettner, A. J., Zhou, Y., & Xixi, L. Air temperature regulates erodible landscape, water and sediment\nfluxes in the permafrost-dominated catchment on the Tibetan Plateau. Water Resources Research (2021).\nLiljedahl, A. K., Boike, J., Daanen, R. P., Fedorov, A. N., Frost, G. V., Grosse, G., et al. Pan-Arctic ice-wedge degradation in\nwarming permafrost and its influence on tundra hydrology. Nature Geoscience, 9(4), 312–318 (2016).\nNatali, S. M., Schuur, E. A. G., Mauritz, M., Schade, J. D., Celis, G., Crummer, K. G., Johnston, C., Krapek, J., Pegoraro, E.,\nSalmon, V. G., and Webb, E. E. Permafrost thaw and soil moisture driving CO2 and CH4 release from upland tundra. J.\nGeophys. Res. Biogeosci., 120, 525– 537 (2015).\nMcCrystall, M. R., Stroeve, J., Serreze, M., Forbes, B. C., & Screen, J. A. New climate models reveal faster and larger\nincreases in Arctic precipitation than previously projected. Nature Communications, 12(1), 6765 (2021).\nWalvoord, M. A. and Kurylyk, B. L.: Hydrologic Impacts of Thawing Permafrost—A Review, Vadose Zo. J., 15(6) (2016).\nZhang, T., Li, D., East, A. E., Walling, D. E., Lane, S., Overeem, I., et al. Warming-driven erosion and sediment transport in\ncold regions. Nature Reviews Earth & Environment (2022).\nPage 10\nNCOMMS-22-53478-T Response to Comments\nPage 11"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for the opportunity to review this revised manuscript. The authors have done well to\ncomprehensively address my previous concerns and comments on the manuscript, and I sincerely\nthank them for their efforts in that regard. In particular, the new analysis on the 2-d hydrodynamic\nmodel enhances the confidence of the study. The manuscript is, in my opinion, now acceptable for\npublication. I believe this paper has broad interests to the community of\npermafrost/hydrology/geomorphology and will stimulate more follow-up studies and funds.\nMy last comment is that in the beginning of the study the authors could also cite East et al., 2022,\nEarth's Future; the idea is to place this paper in a larger context and bigger framework of measuring\nthe sedimentary and geomorphic responses to modern climate change. In the discussion, the authors\ncould further enhance the comparison with other cold regions to expand its broad readership.\nEast, A. E., Warrick, J. A., Li, D., Sankey, J. B., Redsteer, M. H., Gibbs, A. E.,... Barnard, P. L. (2022).\nMeasuring and Attributing Sedimentary and Geomorphic Responses to Modern Climate Change:\nChallenges and Opportunities. Earth's Future, 10(10), e2022E-e2983E.\nDongfeng Li, Beijing", "reviewer_3": "This paper on “Interplay among climate, polygonal terrain and channel formation in the Canadian High\nArctic” has undergone significant revision and thoughtful responses to issues earlier raised by\nreviewers have improved the manuscript.\nThe overall objective is now better clarified; to better understand stream and channel incisions in\npermafrost environments.\nThis overarching goal is motivated by the fact that permafrost environments are rapidly changing and\nthat there are geomorphic and carbon cycle implications that are not fully constrained. But I also I find\nthe point the authors make based on their channel width/depth data analysis convincing: existing\ncommon hydraulic geometry relationships between W/D and either streampower or slope were not\nfound in Muskox valley. So, as the authors point out, it is this absence of relationships suggests\ndifferent processes being of importance in these permafrost environments.\nIn the earlier version of the manuscript the reader was left pondering: What then are the most\nimportant processes?\nOne possible control is the role of polygons on channel evolution. I value how the newly added\nBASEMENT simulation hydrodynamics simulation better illustrates the control of polygons on routing\nof water in the landscape at low flow (2.5 m3/s). Field observations also document thaw slumping and\nother abrupt thaw features. And simple but novel modeling hints at a control of freeze-thaw dynamics\non early season sediment transport. However, there remains some tension between the objective of\nunderstanding the interactions between climate change, seasonal permafrost dynamics and evolving\nchannel geometry, and the confounding catastrophic event of a lake outburst, likely of utmost\nimportance for this specific’s valley incision and the cause of major sediment transport, but somewhat\nunique for Muskox valley and operating at a very different time scale than the more seasonal thaw.\nThese discussions leads to one of the major conclusions; channel evolution in the Arctic is\ncomplicated, and many factors play a role. The proposed generic landscape evolution model provides\na starting point for future research to quantify these processes.\nThank you for adding several key references, this made the paper well embedded in current research\non this topic.\nFigures were good and illustrative to begin with and have still improved based on suggestions.\nI used the newly provided/fixed links to access the online repository of information. I confirm\nsupporting online material is accessible and complete."}, "author_response": "NCOMMS-22-53478-A Response to Comments\nResponse to Comments for Chartrand et al – NCOMMS-22-53478-A\nTitle: Interplay among climate, polygonal terrain and channel formation in the Canadian High Arctic\nJuly 18, 2023\n_____________________________________________________________________________________________________________________________\nResponse to Reviewer 1 Comments\nReviewer 1 Introductory Remarks:\nThank you for the opportunity to review this revised manuscript. The authors have done well to comprehensively\naddress my previous concerns and comments on the manuscript, and I sincerely thank them for their efforts in that\nregard. In particular, the new analysis on the 2-d hydrodynamic model enhances the confidence of the study. The\nmanuscript is, in my opinion, now acceptable for publication. I believe this paper has broad interests to the\ncommunity of permafrost/hydrology/geomorphology and will stimulate more follow-up studies and funds.\nResponse: Thanks for your constructive and supportive feedback. We appreciate your time and energy in\nreviewing and thinking about our work. Your comments have helped us prepare a more clear and impactful\ncommunication of our work.\nReviewer 1 Additional Comments:\nMy last comment is that in the beginning of the study the authors could also cite East et al., 2022, Earth's Future;\nthe idea is to place this paper in a larger context and bigger framework of measuring the sedimentary and\ngeomorphic responses to modern climate change. In the discussion, the authors could further enhance the\ncomparison with other cold regions to expand its broad readership.\nResponse: Thanks for pointing out the East et al., 2022 publication. We are familiar with it, as well as the\npredecessor East and Sankey, 2020 publication. Both publications are important contributions to the\ninterdisciplinary science of climate change and geomorphology. We believe, however, for the purposes of the\npresent work, the East and Sankey (2020)1 publication provides more direct connection and relevance to some of\nthe issues we discuss, notwithstanding the fact that both publications are much more broad in scope than we\nattempt here. Furthermore, East and Sankey (2020) setup much of the issues discussed in the East et al. (2022)\npublication. As a result, we have integrated East and Sankey (2020) into our work here.\nSpecifically, we cite their research in relation to (1) the idea that environmental change is in general complex due\nto the numerous co-evolving environmental factors, and (2) in relation to the knowledge gap related to\nchannelization and channel network development, which sits as a gap at the interface between climate change and\ngeomorphology. This last point was a main issue raised by East and Sankey (2020) as a critical area that\ngeoscientists should focus their research. The East and Sankey (2020) publication is the new reference #27 in the\nmanuscript. No other changes made in relation to other cold regions. Our present manuscript cites and discusses\nresearch from Antarctica, other locations in the western hemisphere Arctic, the Siberian Arctic and the Tibetan\nplateau. We believe this is sufficient for the focus of our presented research.\nResponse to Reviewer 3 Comments:\nReviewer 3 Introductory Remarks:\nThis paper on “Interplay among climate, polygonal terrain and channel formation in the Canadian High Arctic” has\nundergone significant revision and thoughtful responses to issues earlier raised by reviewers have improved the\nmanuscript.\n1. East, A. E., & Sankey, J. B. (2020). Geomorphic and Sedimentary Effects of Modern Climate Change: Current\nand Anticipated Future Conditions in the Western United States. Reviews of Geophysics, 58(4), e2019RG000692-\ne2019RG000692. https://doi.org/10.1029/2019RG000692.\nPage 1\nNCOMMS-22-53478-A Response to Comments\nThe overall objective is now better clarified; to better understand stream and channel incisions in permafrost\nenvironments.\nThis overarching goal is motivated by the fact that permafrost environments are rapidly changing and that there are\ngeomorphic and carbon cycle implications that are not fully constrained. But I also I find the point the authors\nmake based on their channel width/depth data analysis convincing: existing common hydraulic geometry\nrelationships between W/D and either streampower or slope were not found in Muskox valley. So, as the authors\npoint out, it is this absence of relationships suggests different processes being of importance in these permafrost\nenvironments.\nIn the earlier version of the manuscript the reader was left pondering: What then are the most important processes?\nOne possible control is the role of polygons on channel evolution. I value how the newly added BASEMENT\nsimulation hydrodynamics simulation better illustrates the control of polygons on routing of water in the landscape\nat low flow (2.5 m3/s). Field observations also document thaw slumping and other abrupt thaw features. And\nsimple but novel modeling hints at a control of freeze-thaw dynamics on early season sediment transport.\nHowever, there remains some tension between the objective of understanding the interactions between climate\nchange, seasonal permafrost dynamics and evolving channel geometry, and the confounding catastrophic event of\na lake outburst, likely of utmost importance for this specific’s valley incision and the cause of major sediment\ntransport, but somewhat unique for Muskox valley and operating at a very different time scale than the more\nseasonal thaw.\nThese discussions leads to one of the major conclusions; channel evolution in the Arctic is complicated, and many\nfactors play a role. The proposed generic landscape evolution model provides a starting point for future research to\nquantify these processes.\nThank you for adding several key references, this made the paper well embedded in current research on this topic.\nFigures were good and illustrative to begin with and have still improved based on suggestions.\nI used the newly provided/fixed links to access the online repository of information. I confirm supporting online\nmaterial is accessible and complete.\nResponse: Thank you for the supportive and constructive review. We appreciate the time and energy you put into\nthinking about the presented research. We agree with you that future research should emphasize more concrete\nexamination of “interactions between climate change, seasonal permafrost dynamics and evolving channel\ngeometry”. We intend to focus on this issue, in part, with future numerical simulations and fieldwork. No changes\nmade based on Reviewer 3 comments.\nPage 2"}]

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