Mojave River Valley Museum
Ecological Sections: Sierra Nevada - (MAP)
subsection is the intermediate elevations of the western slope of the Sierra
Nevada where it is crossed by the Yuba, American, Cosumnes, Mokelumne,
Stanislaus, and Tuolumne Rivers. It has a temperate and humid climate.
MLRAs 22c and 22d.
Upper Foothills Metamorphic Belt
Lithology and Stratigraphy. Metamorphosed Paleozoic
marine sedimentary rocks predominate in this subsection; they are in the
Shoo Fly and Calaveras Complexes. Also, Mesozoic granitic and late
Tertiary volcanic rocks are moderately extensive, and there are some Jurassic
- Triassic metavolcanic and Jurassic marine sedimentary rocks. The
volcanic rocks are mostly Miocene and Pliocene lahars. There is a
strip of ultramafic rocks along the west side of the Melones Fault Zone
and a few patches of them east of the fault zone.
This subsection is on a gently sloping to moderately steep plateau that
is crossed by the Yuba, American, Cosumnes, Mukelumne, Stanislaus, and
Tuolumne Rivers. These rivers and the north, south, and middle forks
of them run in the bottoms of very steep sided canyons that are as much
as 2000 feet below the plateau surface. The elevation ranges about
2000 to 7000 feet, but is mostly below 6000 feet. Fluvial erosion
and, in the river canyons, mass wasting are the main geomorphic processes.
Soils. The soils are mostly Typic Haploxerults and Dystric
and Dystric Lithic Xerochrepts on sedimentary and metamorphic rocks.
The Dystric Lithic Xerochrepts are mostly on steep canyon sideslopes.
Soils on granitic rocks are mostly Ultic Haploxeralfs; Dystric and shallow
Dystric Xerochrepts; and Entic Xerumbrepts. Soils on volcanic rocks
are mostly Andic and Lithic Xerumbrepts; Ultic Haploxeralfs; and Xeric
Haplohumults. Mollic Haploxeralfs predominate on ultramafic rocks.
The soils are well drained. Soil temperature regimes are mostly mesic.
Soil moisture regimes are xeric.
Vegetation. The predominant natural plant communities
are Mixed conifer series and, at lower elevations, Ponderosa pine series.
There is some White fir series at higher elevations, Canyon live oak series
on very steep, rocky canyon sideslopes, and Mixed chaparral shrublands
on steep south-facing slopes with shallow soils. The northernmost
grove of trees in the Giant sequoia series is in this subsection.
Characteristic series by lifeform include:
Climate. The mean annual precipitation is about 40 to 80 inches.
Much of the precipitation is snow; it is mostly snow above about 4000 feet.
Mean annual temperature is about 45° to 55° F. The mean freeze-free
period is about 100 to 150 days.
Grasslands: Ashy ryegrass series, California oatgrass series,
Idaho fescue series, Montane meadow habitat, Needle-and-thread series.
Shrublands: Brewer oak series, Chamise series, Deerbrush series,
Greenleaf manzanita series, Interior live oak series, Tobacco brush series,
Wedgeleaf ceanothus series.
Forests and woodlands: Birchleaf mountain-mahogany series, Black
oak series, Canyon live oak series, Douglas-fir series, Douglas-fir - Ponderosa
pine series, Douglas-fir - tanoak series, Incense-cedar series, Knobcone
pine series, Mixed conifer series, Ponderosa pine series, Sequoia series,
Tanoak series, White fir series.
Surface Water. Runoff is rapid from most of the area.
Most of the runoff flows to the Yuba, American, Cosumnes, Mukelumne, Stanislaus,
or Tuolumne Rivers or tributaries of them. Maximum flow in
these rivers is much later than that in tributaries that head within the
subsection, because of snow melt at higher elevation east of this subsection.
There are no natural lakes, but some reservoirs.