Subglacial Drainage Patterns of Devon Island, Canada: Detailed Comparison of Rivers and Subglacial Meltwater Channels

The Cryosphere, 12, 1461–1478, 2018 https://doi.org/10.5194/tc-12-1461-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Subglacial drainage patterns of Devon Island, Canada: detailed comparison of rivers and subglacial meltwater channels Anna Grau Galofre1, A. Mark Jellinek1, Gordon R. Osinski2, Michael Zanetti3, and Antero Kukko4 1Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver (BC), Canada 2Department of Physics and Astronomy, University of Western Ontario, London (ON), Canada 3Department of Earth Sciences and Centre for Planetary Science and Exploration, University of Western Ontario, London (ON), Canada 4Center of Excellence in Laser Scanning Research, Department of Remote Sensing and Photogrammetry, Finnish Geospatial Research Institute Geodeetinrinne 2, Masala, Finland Correspondence: Anna Grau Galofre ([email protected]) Received: 18 October 2017 – Discussion started: 23 November 2017 Revised: 7 March 2018 – Accepted: 8 March 2018 – Published: 20 April 2018 Abstract. Subglacial meltwater channels (N-channels) are thermore, our findings are consistent with theoretical expec- attributed to erosion by meltwater in subglacial conduits. tations drawn from analyses of flow driven by gradients in They exert a major control on meltwater accumulation at the effective water pressure related to variations in ice thickness. base of ice sheets, serving as drainage pathways and modi- Our field and remote sensing observations represent the first fying ice flow rates. The study of exposed relict subglacial high-resolution study of the subglacial geomorphology of the channels offers a unique opportunity to characterize the ge- high Arctic, and provide quantitative and qualitative descrip- omorphologic fingerprint of subglacial erosion as well as tions of subglacial channels that revisit well-established field study the structure and characteristics of ice sheet drainage identification guidelines. Distinguishing subglacial channels systems. In this study we present detailed field and remote in topographic data is critical for understanding the emer- sensing observations of exposed subglacial meltwater chan- gence, geometry, and extent of channelized meltwater sys- nels in excellent preservation state on Devon Island (Cana- tems and their role in ice sheet drainage. The final aim of this dian Arctic Archipelago). We characterize channel cross sec- study is to facilitate the identification of subglacial channel tion, longitudinal profiles, and network morphologies and networks throughout the globe by using remote sensing tech- establish the spatial extent and distinctive characteristics of niques, which will improve the detection of these systems subglacial drainage systems. We use field-based GPS mea- and help to build understanding of the underlying mechanics surements of subglacial channel longitudinal profiles, along of subglacial channelized drainage. with stereo imagery-derived digital surface models (DSMs), and novel kinematic portable lidar data to establish a detailed characterization of subglacial channels in our field study area, including their distinction from rivers and other melt- 1 Introduction water drainage systems. Subglacial channels typically clus- ter in groups of ∼10 channels and are oriented perpendic- Subglacial meltwater channels, often referred to as N- ular to active or former ice margins. Although their overall channels, are the erosional expression of turbulent flows in direction generally follows topographic gradients, channels pressurized subglacial channels. Together with subglacial can be oblique to topographic gradients and have undulating channels incised in overlying ice (R-channels), they mod- longitudinal profiles. We also observe that the width of first- ulate meltwater accumulation at the base of ice sheets and order tributaries is 1 to 2 orders of magnitude larger than in serve as highly efficient drainage pathways carrying melt- Devon Island river systems and approximately constant. Fur- water to the ice terminus (e.g., Röthlisberger, 1972; Weert- man, 1972; Nye, 1976; Sugden et al., 1991; Greenwood et al., Published by Copernicus Publications on behalf of the European Geosciences Union. 1462 A. Grau Galofre et al.: Subglacial drainage patterns of Devon Island 2007; Kehew et al., 2012). In particular, the transition from ited (Greenwood et al., 2007). Field identification of sub- distributed to channelized drainage leads to a reduction in ice glacial channels consists of (1) identification from fluvial flow rates, modifying ice loss rates and enhancing surging runoff (proglacial channels or river systems) and (2) distinc- (e.g., Schoof, 2010). Subglacial channelized drainage plays tion from other meltwater features, primarily lateral melt- a key role in deglaciation, and so their spatial characteris- water channels (e.g., Beaney and Shaw, 2000; Greenwood tics, density, and distribution can help explain the patterns of et al., 2007; Syverson and Mickelson, 2009; Margold et al., glacial retreat (e.g., Sugden et al., 1991; Greenwood et al., 2013). A set of subglacial channel identification criteria is 2007). In spite of their importance, some outstanding ques- presented by Greenwood et al.(2007) and summarized in Ta- tions remain: what are the typical length scales that charac- ble1. These guidelines are qualitative, and the characteristics terize subglacial drainage systems (i.e., what is the drainage listed here may or may not be all present in a set of subglacial area, how many individual valleys form, how many tribu- channels (e.g., Sugden et al., 1991; Beaney and Shaw, 2000). taries do they have, etc.)? Can we reliably identify subglacial To categorize and characterize the features presented as sub- channels from rivers and other meltwater channels by us- glacial meltwater channels in Dyke(1999), we conducted ing remote sensing techniques, including imagery and topo- fieldwork on central Devon Island. In Sect. 2 we provide graphic data? And how do the characteristics of remarkably a detailed description of our field data acquisition and pro- well-preserved channels compare with channels elsewhere? cessing. We acquired GPS-borne channel longitudinal pro- To answer these questions, here we perform a detailed geo- files along with stereo imagery and derived photogrammetry morphological study of exposed subglacial channels on De- digital elevation models (DEMs). We also used a portable von Island (Canadian Arctic Archipelago). This work repre- kinematic lidar scanning (KLS) system, a novel method of sents the first field and high-resolution remote sensing char- measuring ultra-high-resolution topography (< 2 cm pixel−1 acterization of subglacial channels in the high Arctic, one of DEMs), which is described in more detail in Sect. 2.1.2. In the areas with the best exposures of such features worldwide. Sect. 3 we present a quantitative characterization of the ob- Well-preserved exposed subglacial channels are rare. Dur- served channel networks, which we apply to distinguish sub- ing glacial recession, meltwater released from the ice sheet glacial channels from lateral meltwater channels and rivers accumulates at the ice marginal area and erodes the chan- in Sect. 4. In Sect. 5 we present a detailed qualitative de- nel, with post-glacial sediment accumulation causing burial scription of subglacial channel morphology and the shape of or partial burial (Le Heron et al., 2009). Vegetation overprint subglacial drainage networks, which serves to further char- and fluvial incision makes the detailed study of channel ge- acterize and distinguish subglacial channel networks. ometry and morphology difficult (e.g., Walder and Hallet, 1979). Exceptions include areas with polar desert climate in Field site: Devon Island the Antarctica Dry Valleys (e.g., Sugden et al., 1991) and the Canadian High Arctic (e.g., Dyke, 1993, 1999). The re- Devon Island was covered by an extensive Innuitian ice sheet duced rainfall conditions of these sites, recent ice retreat, and that reached its maximum extent during the last glacial max- null or minimal vegetation cover are key for the preservation imum (e.g., England, 1987; Dyke, 1999; England et al., of these features. The morphology of subglacial channels at 2006). Shortly after the Younger Dryas, around 10 000 ra- our field study area on Devon Island (Fig.1), the second- diocarbon years BP, the margin of this ice sheet began re- largest of the Queen Elizabeth Islands in the Canadian Arctic treating towards the current coast line, and the final rem- Archipelago, is consequently well preserved. The retreat of nants in central Devon Island vanished around 8800 radiocar- the Devon Island ice cap, in addition, offers a unique op- bon years BP (Dyke, 1999), leaving a landscape of plateaus, portunity to compare recently exposed subglacial channels fiords and deeply incised canyons. We refer the reader to with systems incised during the Younger Dryas Innuitian de- work by Dyke(1999) and England et al.(2006) for a deglaciation. The systems identified by Dyke(1999) as sub- tailed discussion on the glacial history of the island dur- glacial meltwater channels on Devon Island are approx. 10– ing and since the Innuitian ice sheet. Since deglaciation, the 20 m wide and 3–6 m deep, which is consistent with sub- landscape evolution is mainly the result of periglacial proglacial channels observed elsewhere, and 1 to 2 orders of cesses and erosion by ephemeral seasonal streams (e.g., Mc- magnitude smaller than tunnel valleys

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