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Ecological Sections: Mono

Subsection 341Dd
Sweetwater Mountains - Pine Grove Hills

This subsection includes the Sweetwater Mountains, which are between the West Walker and East Walker Rivers; the Wellington and Pine Grove Hills, which are both contiguous with the Sweetwater Mountains on the northwest and northeast edges of the mountains; and Bridgeport Valley on the upper East Walker River.  It has a cold, semi-arid to subhumid climate.  MLRA 26f.

Lithology and Stratigraphy.  The bedrock is mainly Mesozoic granitic and late Cenozoic volcanic and tuffaceous sedimentary rocks.  Much of the volcanic rock is Pliocene basalt and Pliocene or Quaternary andesite.  Also, there are Mesozoic granitic and Pre-Cretaceous sedimentary and metamorphic rocks, Quaternary alluvium, and Quaternary glacial till.

Geomorphology.   Steep mountains and moderately steep to steep hills predominate in this subsection.  Also, there are some gently to moderately sloping pediments and alluvial fans.  Glacial moraines and floodplains are present along the West Walker River and in Bridgeport Valley.  Most ridges in the subsection are oriented north-south, but a trend is not apparent in some areas of late Cenozoic volcanism that still have topography related to that volcanism.  The elevation range is from about 5600 feet up to 9559 feet on Bald Mountain.  Mass wasting, volcanism, fluvial erosion and deposition, and freeze-thaw are the main geomorphic processes.

Soils.  The soils are mostly shallow Xerollic Haplargids, shallow Aridic Haploxerolls, Aridic Argixerolls, shallow Typic Xeropsamments, and shallow Typic Cryopsamments  on granitic rocks.  There are shallow Xeric Torriorthents; shallow Xerollic Haplargids; Lithic Xerollic Haplargids; Lithic Ultic and Aridic Haploxerolls; Lithic Ultic, Aridic, Typic, and Pachic Argixerolls; Typic Durixerolls; and  Pachic Cryoborolls on other kinds of rocks.  Aridic Argixerolls and shallow Abruptic Xerollic Durargids are common on alluvial fans.  Soils on moraines are mostly Typic Haploxerolls and Ultic Haploxeralfs.  There are Typic Haplaquolls and Aquic Argixerolls on floodplains and basin floors.  The soils are well drained, except for somewhat poorly to poorly drained soils on floodplains and basin floors.  Soil temperature regimes are mostly frigid, and cryic at higher elevations.  Soil moisture regimes are aridic, xeric, and aquic.

Vegetation.   The predominant natural plant communities are mostly Big sagebrush series, much Singleleaf pinyon series, and some Curlleaf mountain-mahogany series.  Jeffrey pine series and Limber pine series occur on higher mountain slopes, and Low sagebrush series on shallow soils.  Sedge meadow alliances occur on floodplains and basin floors.

Characteristic series by lifeform include:

Grasslands: Alpine habitat, Ashy ryegrass series, Breaked sedge series, Creeping ryegrass series,  Indian ricegrass series, Nebraska sedge series,  Needle-and-thread series, Rocky Mountain series, Short sedge series, Tufted hairgrass series.

Shrublands: Big sagebrush series, Bitterbrush series,  Low sagebrush series, Parry rabbitbrush series, Rubber rabbitbrush series.

Forests and woodlands: Curlleaf mountain-mahogany series, Jeffrey pine series, Limber pine series Singleleaf pinyon series, Singleleaf pinyon - Utah juniper series, Utah juniper series.
Climate.  The mean annual precipitation is about 10 to 30 inches.  Much of the precipitation is snow.  Mean annual temperature is about 35 to 45 F.  The mean freeze-free period is in the range from 50 to 100 days.

Surface Water.  Runoff is rapid in mountains, hills, pediments, and fans and slow from floodplains and basin floors.  It drains to the Walker River.  Streams from the Sierra Nevada are permanent, but those originating in the mountains of this subsection are dry much of each year.  There are some closed depressions in volcanic terrain, but they are generally dry. 

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