Section M332B Bitterroot Valley
Geomorphology. This area includes high, glaciated mountains with alpine ridges and cirques at higher elevations and glacial and lacustrine basins at lower elevations. Steep slopes, sharp crests, and narrow valleys are characteristic. Elevation ranges from 2,500 to 6,000 ft (763 to 1,830 m) in basin areas; the range is 3,000 to 8,000 ft (915 to 2,440 m) in mountains, with some alpine areas up to 10,000 ft (3,050 m). This Section is within the Northern Rocky Mountains physiographic province.
Lithology and Stratigraphy. Lower Tertiary, Mesozoic, and Precambrian granite and metasedimentary rock occur.
Soil Taxa. There are frigid and cryic Ochrepts, Boralfs, and Orthents, with significant areas of rockland and talus. Soils in basins are Borolls, Ochrepts, and Fluvents. Xeric intergrades probably occur. These soils are generally shallow to moderately deep, with loamy or sandy textures containing large amounts of rock fragments. Some soils at higher elevations are moderately influenced by volcanic ash accumulations.
Potential Natural Vegetation. Kuchler classified vegetation as Douglas-fir forest and western ponderosa forest (80 percent) and foothills prairie (20 percent), mostly in the lower valleys. The upper timberline occurs at about 8,800 ft (2,667 m). Common tree species include western larch, Douglas-fir, subalpine fir, and ponderosa pine. Grassland species are mainly bluebunch wheatgrass, Idaho fescue, and rough fescue.
Fauna. Birds are typical of the northern Rocky Mountains, including species such as black-capped chickadee, Steller's jay, and western tanager. The slightly drier and more open areas also provide habitat for dryland species such as sage grouse, black-billed magpies, and horned larks. Other species of note are the harlequin duck, flammulated owl, Lewis' woodpecker, black-backed woodpecker, American dipper, and Nashville warbler. Species near the edge of their range are pileated woodpecker, chestnut-sided chickadee, and Townsend's warbler. Typical herbivores and carnivores include white-tailed deer, mule deer, elk, moose, black bear, bobcat, and cougar. Smaller common herbivores include the snowshoe hare and the northern flying squirrel. Rare species include the grizzly bear, gray wolf, lynx, wolverine, and northern bog lemming. Herpetofauna typical of this Section are the spotted frog and long-toed salamander.
Climate. Precipitation ranges from 14 to over 80 in (360 to 2,030 mm). Most of the precipitation in fall, winter, and spring is snow. Climate is cool temperate with some maritime influence. Summers are relatively dry. Temperature averages 36 to 46 F (2 to 8 C). The growing season lasts 45 to 130 days.
Surface Water Characteristics. Many perennial streams with dendritic and structurally controlled patterns occur. Many third order drainages are deeply incised into narrow, V-shaped canyons as they leave the mountains. Major rivers include the Bitterroot and the Clark's Fork.
Disturbance Regimes. Fire, insects, and disease are the dominant natural sources of disturbance. Fires were generally low intensity, frequent ground fires prior to fire suppression efforts. Fuel accumulations have now set the stage for large, high-intensity fires.
Land Use. Land use is mostly timber harvest with some livestock grazing. Mining and recreational uses are also important.
