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.
Batholith and Volcanic Flows
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:
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.
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
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.
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.