Boundaries

This region encompasses the Yukon River drainage and covers the forested area of Alaska and the Yukon, extending upstream of the Tanana River on the west to the Richardson Mountains in the Yukon Territory on the east, and bounded by the Brooks Range on the north and the Alaska Range on the south.

Topography

The headwaters of the Yukon River are in the glaciated coastal mountain ranges of British Columbia. From there to Dawson City, the river has a continuous chain of islands. In some areas of the Yukon River volcanic ash, from a major eruption in southern Yukon more than 1200 years ago, has been deposited up to 30 cm thick in a white band of soil along the river banks. Much of the Yukon River has deeply incised tributary channels, slumping and undercut river banks, sand and gravel bluffs, and constantly changing gravel bars. The smooth, rolling terrain of the Klondike Plateau is carved by deep valleys. Permafrost is sometimes visible on north-facing slopes.

Freshwater habitats

The Yukon River begins in a chain of lakes in British Columbia and has tributaries feeding it from several mountain chains. From its mountainous beginning, the Yukon meanders through the boreal forest of Yukon’s central lowlands, and is slow-moving and shallow except during spring runoff. It has a very low gradient with few rapids except for those at Miles Canyon. The river has many wooded islands and is often bordered by mountains. As the Yukon enters Alaska, it widens into the broad interior plateau of the Yukon Flats. The “Thirty Mile Section” of the Yukon River was designated a Canadian Heritage River in 1991. Lake Laberge is a riverine lake with the Yukon River serving as both its inflow and outflow. This lake was formed by glacial activity during the last Ice Age and is ultra-oligotrophic as a result of low nutrient levels and low light penetration.

Terrestrial habitats

Tundra and taiga/boreal forest are the primary biomes within the ecoregion. Along the upper course of the Yukon River characteristic species include white and black spruce (Picea glauca and P. mariana), with a small amount of quaking aspen (Populus tremuloides), dwarf willow (Salix spp.), birch (Betula spp.), balsam poplar (Populus balsamifea), and ericaceous shrubs (Ericaceae). These forests are usually open and extensive. Willow, dwarf birch, Labrador-tea (Ledum decumbens), and bush cinquefoil (Potentilla fruticosa) characterize bottomland bogs and other extremely wet areas (Ricketts et al. 1999). Alpine tundra at higher elevations consists of lichens, mountain avens (Dryas hookeriana), ericaceous shrubs (Ericaceae), sedge (Carex spp.), and cottongrass (Eriophorum spp.) (ESWG 1995).

Description of endemic fishes

There are no known endemic species.

Ecological phenomena

Many species in this ecoregion exhibit anadromy including Pacific salmons (Oncorhynchus spp.), chars (Salvelinus spp.) and whitefishes (Coregonus spp.). The Yukon is one of the most important salmon-breeding rivers in the world. Huge chinook salmon (Oncorhynchus tshawytscha) migrate more than 2,000 km from the Bering Sea, returning to spawn in tributary creeks.

Justification for delineation

The ecoregions of Canada were identified based on the faunal similarity of 166 major watersheds based on a cluster analysis of freshwater fish occurrences in these watersheds. The Upper Yukon ecoregion is largely comprised of the upper Yukon River watershed, which drains into the Bering Sea. A portion of the ecoregion was not glaciated during the last Ice Age and formed part of the Beringian refugium. However, given the depauperate nature of this refugium and its northern location, this ecoregion has a depauparate freshwater fish fauna primarily comprised of species with some saltwater tolerance.

Level of taxonomic exploration

Fair /  Poor