Subsection M261Ej
Tahoe - Truckee

Lithology and Stratigraphy.   Cenozoic volcanic rocks predominate in this subsection. There are some Mesozoic granitic rocks, Jurassic marine sedimentary rocks, and Jurassic and older metavolcanic rocks.  The Cenozoic volcanic rocks are mostly Pliocene andesite, basalt and pyroclastic rocks and Pleistocene basalt.  Pleistocene glacial till and outwash are common and there are small areas of Quaternary alluvial and lacustrine deposits.

Geomorphology.   This subsection is on a gently sloping to moderately steep plateau with some steep mountains that is dissected by streams in canyons that have steep to very steep sideslopes.  Most of the canyons on the west side of the subsection have been modified by glacial erosion and are U-shaped there.  The elevation ranges from about 5000 along the Truckee River and adjacent to Sierra Valley up to 9143 feet on Mt. Lola.  Faulting, mass wasting, and fluvial erosion and deposition are the main geomorphic processes.

Soils.  The soils are mostly Ultic and Mollic Haploxeralfs; Lithic Ultic Haploxerolls; Ultic and Lithic Ultic Argixerolls; Andic Xerochrepts; and Andic Xerumbrepts.  There are some Cryumbrepts at the highest elevations.  Soils on granitic rocks are mostly Entic Haploxerolls and shallow Typic Xeropsamments, but they are not extensive.  Soils on glacial till are mostly Pachic, Entic, and Typic Xerumbrepts.  The soils are mostly well drained, but some on basin floors are somewhat poorly drained.  Soil temperature regimes are mostly frigid, but some are cryic.  Soil moisture regimes are mostly xeric.  Soils with aquic moisture regimes are present in glaciated terrain and small valleys,  but they are not extensive.

Vegetation.   The predominant natural plant communities are Ponderosa pine series, Mixed conifer series, White fir series, Red fir series, and Big sagebrush series.  Jeffrey pine series is common in drier areas on shallow and rocky soils.  There are a few small patches of Mountain hemlock series at the highest elevations.  Lodgepole pine series prevails on many wet soils and on drier soils where cold air drainage and frost limit the regeneration of other trees.  Sedge meadow communities are common, but they are not extensive.  Black cottonwood series is common in riparian areas and there is some Aspen series.

Characteristic series by lifeform include:
Grasslands: Alpine habitat, Ashy ryegrass series, Breaked sedge series, Fen habitat, Idaho fescue series, Montane meadow habitat, Needle-and-thread series, Rocky Mountain sedge series, Shorthair reedgrass series, Shorthair sedge series.
Shrublands: Big sagebrush series, Bitterbrush series, Low sagebrush series, Parry rabbitbrush series, Rubber rabbitbrush series.
Forests and woodlands: Aspen series, Jeffrey pine series, Lodgepole pine series, Mixed conifer series, Mountain hemlock series, Ponderosa pine series, Red fir series, Washoe pine series, Western white pine series, White fir series, Whitebark pine series.

Surface Water.  Runoff is rapid from most of the area.   Most of it flows to the Truckee River, but runoff from the northern part of the subsection flows to Sierra Valley, which is at the head of the Middle Fork of the Feather River.  Maximum flow in these rivers is during spring when snow is melting rapidly.  There are many small and a few large natural lakes or ponds in glaciated terrain, and some reservoirs.