The assessment area includes three ecoregions in interior Alaska: Yukon River Lowlands, Kuskokwim Mountains, and Lime Hills. Included in these ecoregions are two major river systems (Yukon and Kuskokwim) as well as approximately thirty small communities, all located along rivers or streams. Although none of the communities can be reached by road, Galena, McGrath, Aniak, and Illiamna serve as air-transportation hub communities for the region. The assessment boundary, following BLM guidelines, constitutes the three component ecoregions and any 5th level hydrologic units that intersect the ecoregion boundaries. Four additional 5th level hydrologic units were included in the study area to close two gaps in the boundary. This was agreed to by the BLM to facilitate seamless integration with neighboring REA efforts and to ensure that regionally important resources that may exist just outside of the ecoregion boundaries are included in the analysis. The descriptions below are paraphrased from Unified Ecoregions of Alaska (Nowacki et al. 2001).
Yukon River Lowlands
The Yukon River Lowlands are an expansive wetland system associated with the lower stretches of the Yukon and Koyukuk Rivers in west-central Alaska. Although this area was unglaciated, meltwater floods deposited vast quantities of sediment within these riverine corridors during glacial retreat. Deep deposits of undifferentiated sediments underlie these floodplains and adjacent lowlands. A seasonally moist, continental climate prevails with cool, moist summers and cold, dry winters. Permafrost is absent along the younger floodplains, but is thin, discontinuous, and relatively “warm” on the abandoned floodplains in the adjacent lowlands. Poor drainage caused by permafrost contributes to the prevalence of wet, organic-rich soils. Collapse-scar features from thawing permafrost are common. Water levels drop in the Yukon River and its tributaries in early fall during freeze-up and remain low until spring breakup when substantial ice-jam flooding can occur. The vegetation along the major rivers is highly productive and supports vigorous stands of white spruce and balsam poplar. Active floodplains and riverbars support tall stands of alders and willows. Robust wet sedge meadows and aquatic vegetation reside in infilling sloughs and oxbow ponds. The adjacent permafrost-dominated lowlands support black spruce woodlands, and birch-ericaceous shrubs and sedge-tussock bogs. Many flat organic surfaces are pockmarked with dense concentrations of lakes and ponds. These areas support large populations of moose and black bear. The oxbow sloughs and thaw ponds support abundant waterfowl. Lowland forests are important to furbearers. The large rivers support important runs of chinook, chum, and coho salmon.
This terrain is comprised of old, low rolling mountains that have eroded largely without the aid of recent glaciations. A continental climate prevails with seasonal moisture provided by the Bering Sea during the summer. Mountains are composed of eroded bedrock and rubble, whereas intervening valleys and lowlands are composed of undifferentiated sediments. Thin to moderately thick permafrost underlies most of the area. Boreal forests dominate, grading from white spruce, birch, and aspen on uplands to black spruce and tamarack in lowlands. Tall willow, birch, and alder shrub communities are scattered throughout, particularly where forest fires burned in the recent past. Rivers meander through this undulating landscape following fault lines and highly eroded bedrock seams. These mountains support abundant moose, bears, beavers, and scattered caribou herds.
The Lime Hills are glacially dissected mountains extending from the west side of the Alaska Range. The effects of substantial glaciation are etched in the surface topography through a repeated sequence of sharp mountain ridges with steep headwalls and broad U-shaped valleys. The ridges and mountainsides are covered with colluvial rubble, while the valleys contain glacial moraines and outwash with some alluvial deposits along rivers. The continental climate is moderated somewhat by maritime influences of the Bering Sea and North Pacific Ocean. The area is underlain by isolated masses of permafrost. Vegetation is predominately tall and low shrub communities of willow, birch, and alder. Spruce forests and woodlands are confined to valley bottoms and mountain toeslopes. These habitats support moose, bears, caribou, and various furbearers.