Chapter 8
Ecological Subregions of the United States



Province 131--Yukon Intermontane Plateaus Tayga

Two Sections have been delineated in this Province:

These Sections are located in central Alaska; their area is about 56,100 mi2 (145,300 km2)

Section 131A--Upper Kobuk Valley

Geomorphology. Diverse topography which includes scattered groups of hills and low mountains surrounded by irregular lowlands and broad flat divides. Elevation ranges from 1,300 to 3,940 ft (400 to 1,200 m).

Lithology and Stratigraphy. Most of the section consists of alluvial fan and basin deposits of later Mesozoic to Quaternary age. The Koyukuk terrane, consisting of island arc volcanic and volcaniclastic rocks, occupies the southwest corner of the Section.

Soil Taxa. The dominant soils are Aquepts and Ochrepts, formed in a cryic or pergelic temperature regime.

Potential Natural Vegetation. Closed forests of spruce, birch, and aspen occur on moderately drained to well drained sites. In wetlands, open black spruce forests are often interspersed with willow thickets and treeless bogs.

Fauna. The lakes and wetlands associated with the Kobuk, Koyukuk, and Selawik Rivers support breeding populations of common loons, horned grebes, and red-necked grebes. Beaver and snowshoe hare are common. Lynx occur along the Koyukuk River. Northern pike are found in this Section. Sheefish and chum salmon are common in the Kobuk and Selawik Rivers. Wood frogs have been reported.

Climate. Average annual precipitation ranges from 12 to 14 in (300 to 360 mm). Average annual temperature ranges from 16 to 24 oF (-9 to -4 oC). The growing season lasts approximately from May 15 through September 10.

Surface Water Characteristics. The Kobuk River and tributaries of the Upper Koyukuk River drain the area. Numerous thaw lakes dot valleys and broad flats. Wetlands occupy 39 percent to 76 percent of the area.

Disturbance Regimes. Most wildfires, which occur frequently, are ignited by lightning strikes.

Land Use. Primary human uses are subsistence and recreational hunting and fishing.

Cultural Ecology. The Inupiaq Eskimos are the principal residents of this Section.

Compiled by Alaska Region.


Section 131B--Yukon-Kuskokwim Bottomlands

Geomorphology. This Section represents a collection of flat bottomlands along the larger rivers of interior Alaska. Although nearly level, broad valleys and basins are typical, some low rolling hills and piedmont slopes do occur. Riparian features, such as meandering streams and side sloughs, are prevalent. Oxbow, thaw, and morainal lakes are abundant. Elevation generally ranges from 400 ft (120 m) in the west to 1,640 ft (500 m) in the east.

Lithology and Stratigraphy. Alluvial fan and basin fill of late Tertiary and Quaternary age are most common. This Section is underlain mainly by postaccreted Cenozoic deposits and various terranes.

Soil Taxa. The dominant soils are Aquepts that have pergelic temperature and aquic moisture regimes. Specifically, Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochepts, Typic Cryochepts, and Typic Cryofluvents predominate. Most soils were formed by loess and alluvial materials.

Potential Natural Vegetation. Dominant vegetation communities span a moisture gradient from mesic to hydric and include spruce-poplar forests, open black spruce forests, floodplain thickets of willow and alder, and graminoid marsh.

Fauna. The lakes and wetlands associated with the Yukon and Kuskokwim Rivers support breeding populations of common loons, horned grebes, red-necked grebes, and common goldeneyes. Ruffed grouse, belted kingfishers, alder flycatchers, and Hammond's flycatchers also frequently breed in the forests of these river valleys. The best habitat in interior Alaska for furbearers such as mink, marten, muskrat, and river otter occurs in this Section. Red squirrels, northern bog lemmings, and yellow-cheeked voles also occur. The Yukon and Kuskokwim Rivers and their tributaries support northern pike, sheefish, chum salmon, and king salmon.

Climate. Average annual precipitation ranges from 10 to 24 in (250 to 610 mm). Average annual temperature ranges from 22 to 30 oF (-6 to -1 oC). The growing season is approximately from May 15 to September 10. The average freeze-free period is 70 to 120 days.

Surface Water Characteristics. Surface water in streams, lakes, and bogs is abundant in most of the area. Permafrost is widespread but discontinuous. Soils are poorly drained where permafrost exists. Wetlands occupy up to 61 percent of the area.

Disturbance Regimes. Wildfire is a very common event, averaging about 2,260 ha in size. A high frequency of lightning storms, coupled with prevailing warm and dry summers, promote fire occurrence. River flooding is frequent, particularly in the spring.

Land Use. Primarily interior settlements and agriculture occur in this Section. Subsistence and recreational hunting and fishing are popular.

Cultural Ecology. Residents are mainly Koyukon, Tanana, and Kuskokwim Athabaskans.

Compiled by Alaska Region.