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Chapter 19: Old Crow Flats: Thermokarst lakes in the forest-tundra transition

Pascale Roy-Leveillee and Christopher R. Burn

Abstract

Old Crow Flats, in northern , is a 5,600 km2 Arctic wetland surrounded by mountains. It contains thousands of thermokarst lakes. The area was not glaciated during the Wisconsinan but was submerged by a glacial lake that drained catastrophically 15,000 years ago. Today the glacilacustrine plain is underlain by continuous permafrost and is within the forest-tundra ecotone of northern Yukon. Lakes cover approximately 35% of the plain area. Many lakes have rectilinear shores and are oriented northeast-southwest or northwest-southeast where tundra vegetation dominates the ground cover. Where taiga and tall shrubs dominate the vegetation cover, lakeshores tend to be irregular. Drained lake basins are abundant in the Flats. Overlapping basins indicate that several generations of thermokarst lakes have formed and drained over the last 15,000 years. In tundra areas, drained basins generally have wet, depressed margins surrounding a slightly elevated centre. Ice-wedge polygon are ubiquitous in the tundra and are often strikingly orthogonal near lakes and drained basins. The Flats are incised by Porcupine and Old Crow rivers which meander 20 to 50 m below the plain level. Effects of climatic warming on the Flats may threaten the traditional activities and food security of the Vuntut Gwitch’in.

Keywords Oriented lakes, patterned ground, permafrost, thermokarst

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19.1 Introduction

Old Crow Flats (OCF) is a 5,600 km2 Arctic wetland in eastern Beringia that is surrounded by mountains (Fig. 19.1). Thousands of shallow lakes cover more than one third of the surface and form a freshwater landscape recognized as a wetland of international significance (The Secretariat of the Convention on Wetlands 2014). Many of the lakes of OCF have rectilinear shorelines, and are oriented either northeast-southwest, or northwest-southeast. The lakes of OCF exhibit key features of the thermokarst lake cycle, such as lake expansion and drainage followed by permafrost recovery and lake reinitiation. Signs of lake expansion by thawing of ground ice include steep undercut banks with dangling peat curtains, spruce trees leaning towards the water on bank tops, and partly submerged shrubs and trees at the edge of the lakes (Roy-Leveillee and Burn 2010). There are many drained lake basins throughout the Flats, commonly with deeply incised outlets, suggesting that catastrophic lake drainage may occur. Old Crow Flats is within the forest-tundra transition of northern Yukon, and forms a patchy mosaic of water, forest, and tundra incised by entrenched meandering rivers. It has long been a refuge for Arctic wildlife (Wolfe et al. 2011). The river bank bluffs have yielded a rich fossil record, including possible indications of human presence in the area as long as 40,000 to 35,000 years ago (Morlan 2003). Today Old Crow Flats are a key part of the Traditional Territory of the Vuntut Gwitchin First Nation (VGFN), and local observations indicate that the Flats are undergoing pronounced changes in temperature, precipitation, vegetation cover, lake- water levels, and wildlife distribution (Wolfe et al. 2011). VGFN members are concerned that these environmental changes are linked to climatic warming and threaten the traditional activities and food security of their citizens (Wolfe et al. 2011).

19.2 Physiography and climate OCF is located within a bowl-shaped structural depression formed as the surrounding topography was uplifted, during orogenic events in the Devonian and late Cretaceous to Tertiary Periods (Lane 2007). To the north, OCF is separated from the Arctic coast by British (max. elevation 1655 m) and Barn mountains (max. elevation 1109 m). To the east, Richardson Mountains (max. elevation 1671 m) rise between OCF and Mackenzie Delta. OCF is bordered by Old Crow Range Page 3 of 18

(max. elevation 1270 m) to the west, and the Keele and Dave Lord ranges extend to the south (max. elevation 1395 m). The mountainous terrain to the north separates OCF from the maritime influence that moderates air temperatures at the Arctic coast (Wahl 2004; Burn 2012). Mean January temperature at Old Crow is −29 ºC, the lowest in Yukon, and mean air temperature in July is 15 ºC, with mean daily maximum temperatures ≥ 20 ºC in June, July and August. Approximately 60% of the annual precipitation falls between May and September and on average only 108 mm of precipitation falls during the snow season, between October and April. In winter and spring, coniferous trees absorb more solar radiation than snow-covered shrubs and grasses, resulting in significant differences in atmospheric heating between areas dominated by taiga and tundra (Roy-Léveillée et al. 2014). The resulting difference in air temperature increases with incoming solar radiation and continues to increase in May for a short period after disappearance of the snow cover, due to persistence of lake ice until early June on large lakes in tundra areas (Roy- Léveillée et al. 2014). A regional temperature composite record indicates that, between 1930 and 2000, winter, spring, and summer temperatures increased by 1.9, 1.6, and 1.2 ºC respectively, which is consistent with local observations of climatic and environmental change (Wolfe et al. 2011; Porter and Pisaric 2011). Simulations by coupled general circulation models for the 21st century generally predict increases of 4 to 5 ºC in summer temperature and 9 to 12 ºC in winter temperature near Old Crow and precipitation is also predicted to increase. Thermokarst landscapes such as OCF, which are shaped by interactions between permafrost and surface water, may be particularly sensitive to such changes in air temperature and precipitation.

19.3 Surficial deposits and permafrost Old Crow Flats was not glaciated during the Wisconsinan glacial stage, but advances of the Laurentide Ice Sheet about 30 ka BP and again about 17 ka BP blocked eastward drainage at McDougall Pass and in Bonnet Plume Basin (Fig. 19.1). Glacially dammed water in Bonnet Plume Basin was diverted through the Eagle River meltwater channel towards Glacial Lake Old Crow, which formed in the Bell-Old Crow-Bluefish basins (Fig. 19.1) (Zazula et al. 2004). The glacial lake had a maximum extent of approximately 13,000 km2, and the elevation of paleo- shorelines around OCF indicate water depths ranging from a few meters to more than 50 m Page 4 of 18

(Rocheleau 1997). These paleoshorelines constitute ridges of poorly sorted local material, indicating that Glacial Lake Old Crow only persisted for short periods of time (Rocheleau 1997). The glacial lake deposited up to 10 metres of unfossiliferous glacilacustrine silts and clays over the thick layered sands and silts which had accumulated in the basin during the Pleistocene (Lichti-Federovich 1973). It finally drained westward into basin at The Ramparts between 16.4 and 14.9 ka BP, establishing the present-day westward drainage of (Zazula et al. 2004). Beneath the glacilacustrine deposits, ice-wedge pseudomorphs and involutions are visible in river bank exposures in a stratigraphic unit that is possibly >2.48 Ma. This suggests that permafrost was present in OCF during the late Pliocene (Pearce et al. 1982; Burn 1994). It is likely that permafrost degraded completely beneath Glacial Lake Old Crow and aggraded again in the exposed glacilacustrine sediments following lake drainage (Lauriol et al. 2009). Measurements in a geotechnical borehole drilled at Old Crow, in the Porcupine River valley, indicated a permafrost thickness of 63 m. In OCF, permafrost temperatures at the depth of zero annual amplitude are −3.1 to −4.0 ºC in the taiga along Porcupine and Old Crow rivers, and vary between −5.1ºC beneath low shrubs and −2.6 ºC beneath tall shrubs on the Flats (Roy-Léveillée et al. 2014). River-bank exposures indicate that the distribution of excess ground ice in the upper 40 m of deposits is limited to the glacilacustrine sediments deposited by Lake Old Crow (Matthews et al. 1990). Surface features indicative of ice-rich permafrost in the Flats include thermokarst ponds with rapidly receding lake shores, drunken forest, and extensive ice-wedge polygon networks.

19.4 Thermokarst lakes and drained basins Old Crow Flats includes over 2500 thermokarst lakes. Thermokarst lakes form as thawing of ice- rich permafrost leads to settlement of the ground surface and formation of depressions where water tends to accumulate. Many of the lakes of OCF likely formed during the early Holocene climate optimum (Mackay 1992; Burn 1997), when July temperatures in OCF were approximately 6 ºC higher than today (Lauriol et al. 2002a). However some of the earliest lakes may have developed from remnant ponds after the drainage of Glacial Lake Old Crow (Ovenden 1985). The lakes of OCF are generally flat-bottomed and shallow, with an average depth of 1 to 1.5 m and a maximum depth of 4 m. Over 80% of the lakes are smaller than 10 hectares in area Page 5 of 18 but large lakes, up to 36 km2, occur throughout the Flats. The lakes tend to expand laterally because conduction of heat from the lake into the bank sediment results in the progressive subsidence of shore banks. Shore recession is accelerated by thermo-mechanical erosion where wave action reaches exposed permafrost and rates of shore recession up to 3.9 m yr-1 have been observed under such conditions (Roy-Leveillee and Burn 2010). Many of the lakes and drained basins of OCF are oriented either parallel or perpendicular to the dominant wind direction, NE-SW or NW-SE, respectively. Lakes between 1 ha and 1 km2 are commonly oriented parallel to the dominant wind direction, whereas lakes larger than 10 km2 are almost exclusively oriented NW-SE. The proportion of oriented lakes increases with lake size, indicating that lake orientation is controlled by processes associated with shore erosion and lake expansion. In areas where the vegetation cover is dominated by low shrubs and grasses, the shorelines of oriented lakes tend to be rectilinear due to wave erosion and redistribution of sediment by longshore currents (Fig. 19.2a, c, and e) (Roy-Leveillee and Burn 2015). Where taiga and tall shrubs dominate the landscape, vegetation can remain anchored in the sediment after subsidence of shore banks beneath water and form a barrier protecting the shore from wave action and ice push, thereby hindering the redistribution of sediment by wind-generated currents. In these areas, shorelines are generally irregular and thaw subsidence is the dominant process affecting lake expansion (Fig. 19.2b, d, and f). Despite the expansion of the lakes, residents of Old Crow report decreasing lake water levels and increasing frequency of catastrophic lake drainage (Wolfe et al. 2011). These observations are consistent with remote-sensing analyses, which indicate that there was a 3.5% decrease in lake surface area in OCF between 1972 and 2001, associated with the catastrophic drainage of two large lakes and a negative water balance from 1988 to 2001 (Labrecque et al. 2009). The hydrological regime of the OCF lakes differs depending on the characteristics of the surrounding vegetation (Turner et al. 2010). Where taiga and tall shrubs dominate the vegetation cover, the lake water balance is dominated by input from snowmelt until mid-summer and the lakes are susceptible to catastrophic drainage due to overflow and erosion of ice wedges (Lantz and Turner 2015). In tundra areas where the vegetation cover is dominated by dwarf shrubs and grasses, the snow cover is redistributed by wind during winter, and the main water input for the lakes is rainfall. Lakes in the latter category, as well as lakes with smaller catchments, are particularly vulnerable to water level draw down by evaporation during summer and many have Page 6 of 18 decreased in size since the 1950s (Bouchard et al. 2013; Turner et al. 2014). The frequency of catastrophic drainage in OCF increased between 1951 and 2009 and may reflect recent increases in precipitation (Lantz and Turner 2015). Increased precipitation levels may increase the intensity of thermo-mechanical erosion of shores or lead to overflow and drainage initiation by ice-wedge erosion (Fig. 19.3). Drained lake basins abound throughout OCF and, in certain areas, overlap of basins and lakes suggest that several generations of thermokarst lakes have developed and drained since Glacial Lake Old Crow. In parts of Old Crow Flats dominated by tundra vegetation, the centre of drained basins is generally slightly raised and better drained than the depressed margins, where ponding is common (Fig 19.4). This topography results from the transport of fine sediment away from eroding lake margins during lake expansion. Over time, this transport results in the deposition of more sediment in the central parts of lakes, where the lake has been submerged for longer and has subsided below wave-base, than at the shallow margins, where resuspension by wave action occurs frequently (Roy-Leveillee and Burn 2016) (Fig 19.5a). This difference in sediment volume is revealed after drainage, when permafrost aggrades in the lake-bottom sediment and underlying talik (Fig. 19.5b).

19.5 Ice-wedge polygons In the tundra of OCF, patterned ground resulting from the development of ice wedges is omnipresent and clearly visible from the air (Fig. 19.6). Ice wedges are massive bodies dominantly composed of foliated ice which form through the repeated infiltration and freezing of snowmelt in thermal contraction cracks (Mackay 1974). They are typically interconnected in a network forming a polygonal pattern at the surface. In Old Crow Flats, ice wedges are occasionally exposed in cross-section in rapidly eroding lakeshore banks. The ice-wedge networks are strikingly orthogonal over extensive areas around the lakes and drained lake basins (Fig. 19.6). Orthogonal ice-wedge networks develop where a source of heat in the landscape, such as a water body, reduces the thermal stress in one direction and cause ground cracking to occur at 90º to the edge of the heat source (Lachenbruch 1962). In OCF, ice-wedge networks are widespread in areas where tundra vegetation dominates, but they are rare in areas dominated by tall shrubs and taiga (Roy-Leveillee and Burn 2010). This difference in ice-wedge abundance Page 7 of 18 between taiga and tundra likely reflects difference in snow cover characteristics and ground temperature (Roy-Léveillée et al. 2014).

19.6 Meandering rivers The Flats are incised by Porcupine and Old Crow rivers which meander 20 to 50 m below the plain level (Fig. 19.7). Intense fluviglacial discharge from the Laurentide Ice sheet led to the rapid incision of the Porcupine and Old Crow river valleys approximately 12,000 years ago. During this period the Porcupine River attained a depth of 50 m and a width of 5 km over a distance of 30 km in less than 1,000 years (Lauriol et al. 2002b). Today the Porcupine and Old Crow rivers run along a fill-cut terrace formed by the aggradation of 5 to 10 m of sediment, between 8 and 4 ka BP, and subsequent channel incision in the deposit. River migration across the valley bottoms formed oxbows lakes and meander scars on the terrace providing evidence of former river courses (Lauriol et al. 2002b). Porcupine and Old Crow river bank bluffs contain geological records of environmental change spanning over one million years and have yielded the richest ice-age fossil record in Canada, including bones of woolly mammoths, steppe bison, horses, lions, ground sloths and camels (Harington 1977).

19.7 Conclusion Protected by its mountainous surroundings, Old Crow Flats forms a striking freshwater landscape where thermokarst lakes, drained lake basins, ice-wedge polygon networks, and deeply entrenched meandering rivers form intricate patterns that vary through the forest-tundra transition. The area provides important habitat for over 500,000 waterbirds and numerous other species including moose, wolverine, and muskrats which are important to the traditional subsistence activities of the Vuntut Gwich’in. Local residents report that the Flats are undergoing pronounced changes in temperature, precipitation, vegetation cover, lake-water levels, and wildlife distribution (Wolfe et al. 2011). The Vuntut Gwitchin First Nation is concerned that these environmental changes are linked to climatic warming and threaten the traditional activities and food security of their citizens. Despite its multi-layered significance, sensitive environment, and rapidly changing climate, OCF remains an understudied area as it is remote and difficult to access. Page 8 of 18

The authors Pascale Roy-Leveillee is Assistant Professor of Geography at the Laurentian University School of Northern and Community Studies in Sudbury, Canada. Her research focuses on permafrost conditions and thermokarst processes in central and northern Yukon. Christopher Burn is Professor of Geography at Carleton University in Ottawa, Canada. His research concerns relations between climate and permafrost in northwest Canada, where he is committed to long- term investigations of frozen ground.

Acknowledgements Our research on permafrost in the Old Crow Flats has been funded by the Natural Science and Engineering Research Council of Canada, the Canadian International Polar Year Program, the Polar Continental Shelf Program (Natural Resource Canada), and the Northern Scientific Training Program (Aboriginal Affairs and Northern Development Canada). Essential logistical support has been provided by the Vuntut Gwitchin First Nation Government, the Yukon Field Unit of the Parks Canada Agency, and the Aurora Research Institute, Inuvik. Several field assistants contributed to data collection including A.J. Jarvo, A.L. Frost, B. Brown, D. Charlie, E. Tizya-Tramm, K. Tetlichi, L. Nagwan, M. Frost Jr., S. Njoutli, S. Frost, and C.Z. Braul.

Bibliography Bouchard F, Turner KW, MacDonald LA, Deakin C, White H, Farquharson N, Medeiros AS, Wolfe BB, Hall RI, Pienitz R, Edwards TWD (2013) Vulnerability of shallow subarctic lakes to evaporate and desiccate when snowmelt runoff is low. Geophys Res Lett 40:2013GL058635. doi: 10.1002/2013GL058635

Burn CR (2012) Climate. In: Burn CR (ed) Herschel Island Qikiqtaryuk: A Natural and Cultural History of Yukon’s Arctic Island. Wildlife Management Advisory Council, Whitehorse, pp 48–53

Burn CR (1994) Permafrost, tectonics, and past and future regional climate change, Yukon and adjacent Northwest Territories. Can J Earth Sci 31:182–191. doi: 10.1139/e94-015

Burn CR (1997) Cryostratigraphy, paleogeography, and climate change during the early Holocene warm interval, western Arctic coast, Canada. Can J Earth Sci 34:912–925. doi: 10.1139/e17-076 Page 9 of 18

Harington CR (1977) Pleistocene mammals of the Yukon Territory. Ph.D. Thesis, University of Alberta

Labrecque S, Lacelle D, Duguay CR, Lauriol B, Hawkings J (2009) Contemporary (1951-2001) evolution of lakes in the Old Crow Basin, Northern Yukon, Canada: Remote sensing, numerical modeling, and stable isotope analysis. Arctic 62:225–238. doi: 10.14430/arctic134

Lachenbruch AH (1962) mechanics of thermal contraction cracks and ice-wedge polygons in permafrost. Geological Society of America, Special Paper 70, 66pp.

Lane LS (2007) Devonian–Carboniferous paleogeography and orogenesis, northern Yukon and adjacent Arctic . Can J Earth Sci 44:679–694. doi: 10.1139/e06-131

Lantz TC, Turner KW (2015) Changes in lake area in response to thermokarst processes and climate in Old Crow Flats, Yukon. J Geophys Res Biogeosciences 2014JG002744. doi: 10.1002/2014JG002744

Lauriol B, Cabana Y, Cinq-Mars J, Geurts M-A, Grimm FW (2002a) Cliff-top eolian deposits and associated molluscan assemblages as indicators of Late Pleistocene and Holocene environments in Beringia. Quat Int 87:59–79. doi: 10.1016/S1040-6182(01)00062-3

Lauriol B, Duguay CR, Riel A (2002b) Response of the Porcupine and Old Crow rivers in northern Yukon, Canada, to Holocene climatic change. The Holocene 12:27–34. doi: 10.1191/0959683602hl517rp

Lauriol B, Lacelle D, Labrecque S, Duguay CR, Telka A (2009) Holocene evolution of lakes in the Bluefish Basin, northern Yukon, Canada. Arctic 62:212–224. doi: 10.14430/arctic133

Lichti-Federovich S (1973) Palynology of six sections of Late Quaternary sediments from the Old Crow River, Yukon Territory. Can J Bot 51:553–564. doi: 10.1139/b73-066

Mackay JR (1992) Lake stability in an ice-rich permafrost environment: examples from the western Arctic coast. In: Robarts RD, Bothwell ML (eds) Aquatic Ecosystems in Semi- arid Regions: Implications for Resource Management, August 27-30, 1990, Saskatoon, Canada. National Hydrology Research Institute, Environment Canada, Saskatoon, pp 1– 25

Mackay JR (1974) Ice-wedge cracks, Garry Island, Northwest Territories. Can J Earth Sci 11:1366–1383. doi: 10.1139/e74-133

Matthews JV, Schweger CE, Janssens JA (1990) The last (Koy-Yukon) interglaciation in the northern Yukon: evidence from Unit 4 at Ch’ijee’s Bluff, Bluefish Basin. Géographie Phys Quat 44:341–362. doi: 10.7202/032835ar

Morlan RE (2003) Current perspectives on the Pleistocene archaeology of eastern Beringia. Quat Res 60:123–132. doi: 10.1016/S0033-5894(03)00070-X Page 10 of 18

Ovenden L (1985) Hydroseral Histories of the Old Crow Peatlands, Northern Yukon. Ph.D. Thesis, University of Toronto

Pearce GW, Westgate JA, Robertson S (1982) Magnetic reversal history of Pleistocene sediments at Old Crow, northwestern Yukon Territory. Can J Earth Sci 19:919–929. doi: 10.1139/e82-077

Porter TJ, Pisaric MFJ (2011) Temperature-growth divergence in white spruce forests of Old Crow Flats, Yukon Territory, and adjacent regions of northwestern North America. Glob Change Biol 17:3418–3430. doi: 10.1111/j.1365-2486.2011.02507.x

Rocheleau M (1997) Sédimentologie des paleoplages de la plaine d’Old Crow, Territoire du Yukon, Canada. M.A., Université d’Ottawa

Roy-Leveillee P, Burn CR (2010) Permafrost conditions near shorelines of oriented lakes in Old Crow Flats, Yukon Territory. Proceedings of the sixth Canadian Permafrost Conference, Calgary, Canada, September 12-16, 2010. Canadian Geotechnical Society, Calgary, pp 1509–1516

Roy-Leveillee P, Burn CR (2015) Geometry of oriented lakes in Old Crow Flats, northern Yukon. Proceedings, sixty-eighth Canadian Geotechnical Conference and seventh Canadian Permafrost Conference, Quebec City, Canada, September 21-23, 2015. Canadian Geotechnical Society, Richmond, BC, 8 p

Roy-Leveillee P, Burn CR (2016) A modified landform development model for the topography of drained thermokarst lake basins in fine-grained sediments. Earth Surf Proc Land. doi:10.1002/esp.3918

Roy-Léveillée P, Burn CR, McDonald ID (2014) Vegetation-Permafrost Relations within the Forest-Tundra Ecotone near Old Crow, Northern Yukon, Canada: Permafrost Temperatures within the Forest-Tundra near Old Crow, YT. Permafr Periglac Process 25:127–135. doi: 10.1002/ppp.1805

The Secretariat of the Convention on Wetlands (2014) The List of Wetlands of International Importance. http://ramsar.org/ pdf/sitelist.pdf

Turner KW, Edwards TWD, Wolfe BB (2014) Characterising runoff generation processes in a lake-rich thermokarst landscape (Old Crow Flats, Yukon, Canada) using δ18O, δ2H and d- excess measurements. Permafr Periglac Process 25:53–59. doi: 10.1002/ppp.1802

Turner KW, Wolfe BB, Edwards TWD (2010) Characterizing the role of hydrological processes on lake water balances in the Old Crow Flats, Yukon Territory, Canada, using water isotope tracers. J Hydrol 386:103–117. doi: 10.1016/j.jhydrol.2010.03.012

Wahl HE (2004) Climate. In: Smith CAS, Meikle JC, Roots CF (eds) Ecoregions of the Yukon Territory: Biophysical properties of Yukon landscapes. Agriculture and Agri-Food Canada, Summerland, pp 19–23 Page 11 of 18

Wolfe BB, Humphries MM, Pisaric MF, Balasubramaniam AM, Burn CR, Chan L, Cooley D, Froese DG, Graupe S, Hall RI, others (2011) Environmental change and traditional use of the Old Crow Flats in northern Canada: an IPY opportunity to meet the challenges of the new northern research paradigm. Arctic 64:127–135

Zazula GD, Duk-Rodkin A, Schweger CE, Morlan RE (2004) Late Pleistocene chronology of Glacial Lake Old Crow and the north-west margin of the Laurentide ice sheet. In: J. Ehlers and P.L. Gibbard (ed) Quaternary Glaciations-Extent and Chronology Part II: North America. Elsevier, London, pp 347–362

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Figures

Fig. 19.1 Location map of Old Crow Flats showing (a) northern Yukon with extent of Glacial Lake Old Crow in Bell, Bluefish, and Old Crow basins. The approximate maximum limit of the Laurentide Ice Sheet is shown in light grey (after Zazula et al. 2004, Fig. 1 and Lauriol et al. 2009, Fig. 1) and (b) Landsat ETM orthoimage of Old Crow Flats and surrounding areas acquired on August 30 2001. Waterbodies are in black (©Department of Natural Resources Canada. All rights reserved)

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Fig. 19.2 Lake geometry and shore erosion in tundra and taiga areas. a) Oriented lakes with rectilinear shorelines in an area where the vegetation cover is dominated by low shrubs and grasses; b) lakes with irregular shorelines in an area where the vegetation cover is dominated by taiga; c) rectilinear shoreline affected by thermo-mechanical erosion; d) irregular shore affected by thaw subsidence where trees have remained anchored in the sediment after submergence; e) shore section with overhanging peat curtains and thermo-erosional niche; f) shore section protected from thermo-mechanical erosion by partly submerged trees and tall shrubs. Photographs © P. Roy-Leveillee Page 14 of 18

Fig. 19.3 Zelma Lake (5,300 km2, in the background) and two smaller adjacent lakes, drained catastrophically in June 2007. Photograph © T. Pretzlaw, used with permission

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Fig. 19.4 Landsat ETM orthoimage of a part of Old Crow Flats dominated by low-shrub tundra, near Johnson Creek, where several drained lake basins are visible. Areas with high moisture contents generally appear in darker shades of grey, and waterbodies are in black (©Department of Natural Resources Canada. All rights reserved)

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Fig. 19.5 Conceptual model of sedimentation patterns near receding thermokarst lakeshores (left) and resulting topography after lake drainage and permafrost agradation (right). A light grey pattern indicates sediments that are in permafrost (Roy-Leveillee and Burn 2016)

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Fig. 19.6 Orthogonal ice-wedge polygons near oriented lakes. Note that alders and willows grow almost exclusively on the ice-wedge ridges, where the ground is slightly elevated and better drained than the polygon centres. Photograph © C.R. Burn

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Fig. 19.7 Meanders on the Old Crow River near the centre of Old Crow Flats. Note the vegetation patterns inside the first meander indicating previous channel positions. The eroding bluffs are approximately 30 m high. At this location, the width of the Old Crow River valley varies between 1.2 and 2.5 km. Photograph © K. Turner