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Ecological Sections: Sierra Nevada - (MAP)

Subsection M261Em
Batholith and Volcanic Flows

This subsection comprises the mid-elevations of the western slope of the Sierra Nevada between the American River and Merced River.  It has a temperate and humid climate.  MLRA 22d.

Lithology and Stratigraphy.   Mesozoic granitic, mainly adamellite and granodiorite,  and post-batholith volcanic rocks predominate in this subsection, but there are some pre-batholith rocks, too.  The volcanic rocks are mostly Miocene and Pliocene andesitic lahars of the Mehrten Formation that include interbedded gravels and sands south of the Mokelumne River.  The pre-batholith rocks are mostly metamorphosed Paleozoic marine sedimentary rocks.

Geomorphology.   This subsection is on a gently sloping to moderately steep plateau with some steep hills on it.  Several large rivers from the American to the Merced Rivers cross the subsection.   These rivers and some of their tributaries flow in the bottoms of very steep sided canyons that are up to 2000 feet deep.  The elevation ranges from about 3000 to 6000 feet.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Dystric and shallow Dystric Xerochrepts; Ultic Haploxeralfs; and Entic and Pachic Xerumbrepts on granitic rocks.  They are mostly Andic and Entic  Xerumbrepts; Ultic Haploxeralfs; and Xeric Haplohumults on volcanic rocks.  The soils are mostly well drained.  Soil temperature regimes are mostly mesic.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant communities, from lower to higher elevations, are Ponderosa pine series, Mixed conifer series, and White fir series.  There is Jeffrey pine series in volcanic terrain at higher elevations.  Canyon live oak series is common on very steep rocky slopes.

    Characteristic series by lifeform include:
    Grasslands: Ashy ryegrass series, California oatgrass series, Montane meadow habitat, Needle-and-thread series.
    Shrublands: Brewer oak series, Deerbrush series, Greenleaf manzanita series, Interior live oak series, Tobacco brush series, Wedgeleaf ceanothus series.
    Forests and woodlands: Canyon live oak series, Douglas-fir series, Douglas-fir - Ponderosa pine series, Incense-cedar series, Jeffrey pine series, Knobcone pine series, Mixed conifer series, Ponderosa pine series, White fir series.
Climate.  The mean annual precipitation is about 40 to 60 inches; much of it falls as  snow.  Mean annual temperature is about 45 to 55 F.  The mean freeze-free period is in the range from 100 to 150 days.

Surface Water.  Runoff is rapid; it flows to the American, Cosumnes, Mokelumne, Stanislaus, Tuolumne, or Merced Rivers or tributaries of them.  Maximum flow in these rivers is during spring when snow is melting rapidly.  There are no natural lakes, but there are some reservoirs in the subsection. 

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