Boundaries

This region encompasses the Lower Yukon River drainage below the confluence with the Tanana River. It extends up to, but does not include the Lower Mackenzie River in the east. It is bounded by, the Beaufort Sea in the north, the Bering Sea in the west, and the Gulf of Alaska in the south. The ecoregion includes the Aleutian Islands and Bristol Bay drainages.

Topography

This ecoregion ranges from flat coastal plains to the rugged, deeply dissected mountains of the Brooks and Alaska ranges. Much of the area is level to gently rolling with many lakes and rivers. In the Alaskan interior, broad valleys and basins are typical, along with some rolling hills and piedmont slopes. Elevations within the ecoregion range from sea level to over 2500 m (McNab & Avers 1994).

Freshwater habitats

The ecoregion includes meandering streams and side sloughs, as well as oxbow, thaw, and morainal lakes. Along the coast wetland vegetation is common, including fens, bogs, and marshes (McNab & Avers 1995). The primary types of rivers include those with headwaters in the arctic region and those with headwaters further south (Mecklenburg et al. 2002). Peak flow in Arctic rivers occurs in spring and autumn, and is influenced by rain, as well as melting snow and ice. These rivers remain cool throughout the short summer, and freeze during winter, unless they are fed by perennial springs. Rivers south of the Arctic region have maximum flows between May and June, and may or may not freeze during winter, depending on their location and size. Most lakes are of glacial origin.

Terrestrial habitats

The ecoregion is characterized by tundra along the coast and taiga within the interior. Tundra vegetation consists of mesic graminoid herbaceous communities dominated by sedges (Eriophorum spp. and Carex spp.) and low scrub communities. Protected, well-drained valley bottoms may contain coniferous forests dominated by white spruce (Picea glauca) and balsam poplar (Populus balsamifera), or mixed forests characterized by white spruce and paper birch (Betula papyrifera). Black spruce (Picea mariana) occurs in bottomlands and other wet areas with poorly drained soils (Ricketts et al. 1999).  

Description of endemic fishes

The only known endemic species is the Angayukaksurak char (Salvelinus anaktuvukensis), restricted to streams on the crest of the Brooks Range.

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. Chinook salmon (Oncorhynchus tshawytscha) migrate more than 2,000 km from the Bering Sea, returning to spawn in tributary creeks (Mecklenburg et al. 2002).

Justification for delineation

This ecoregion includes a tier of river systems along the western and northern coasts of Alaska, including the drainages of Bristol Bay, the lower Yukon River, and the coastal drainages up to the watershed divide at the MacKenzie River delta on the Arctic slope. The region incorporates most of the unglaciated areas from the Wisconsin Ice Age that contain relict species, most prominently the Alaskan blackfish (Dallia pectoralis). The Lower Yukon River is separated from Upper Yukon because of a high dissimilarity between their fish faunas and the history of the Lower Yukon as a glacial refuge zone (Lindsey and McPhail 1986). The dividing line between the Lower and Upper Yukon coincides with the farthest upstream records of Dallia at Tanana and Fairbanks (C. Lindsey, pers. comm.).

Level of taxonomic exploration

Fair / Poor