Old Crow Flats: Thermokarst Lakes in the Forest–Tundra Transition

Abstract

Old Crow Flats, in northern Yukon, is a 5600 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 polygons 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 20–50 m below the plain level. Effects of climatic warming on the Flats may threaten the traditional activities and food security of the Vuntut Gwich’in.