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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 Yukon, 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, lakewater 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
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(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 (RoyLéveillée 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 paleoshorelines around OCF indicate water depths ranging from a few meters to more than 50 m
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(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 Yukon River basin at The Ramparts
between 16.4 and 14.9 ka BP, establishing the present-day westward drainage of Porcupine River
(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 icerich 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
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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
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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
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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.
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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 longterm 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.
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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
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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