Lithology and Stratigraphy
This subsection contains mostly late Quaternary alluvium. The alluvium is predominantly from granitic rock sources in the southern Sierra Nevada.
This subsection is on very gently to gently sloping floodplains and alluvial fans along and between streams that cross from mountains of the Sierra to reach the San Joaquin River. The subsection elevation range is from 20 to about 180 feet. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly in Typic subgroups of Xeropsamments, Xerorthents, Haploxerolls, and Haploxeralfs. Natric Durixeralfs are common in the drier south end of the subsection. Soils on floodplains along the main rivers are mostly Fluventic Haploxerolls and Aquic Xerofluvents. The soils are well drained, except for Aeric Haplaquepts on alluvial fans. Bicarbonate weathering and leaching and accumulation of clay and silica in subsoils are the main pedogenic processes in the alluvial fan soils. Calcium carbonates accumulate in some of the soils. Soil temperature regimes are thermic. Soil moisture regimes are mostly xeric, with some aquic.
The predominant natural plant communities are Needlegrass grasslands and Valley oak series. Fremont cottonwood series occurs along streams.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Purple needlegrass series.
Shrublands: Button bush series.
Forests and woodlands: Fremont cottonwood series, Mixed willow series.
The mean annual precipitation is about 10 to 14 inches. It is practically all rain. Mean annual temperature is about 59° to 62° F. The mean freeze-free period is about 250 to 275 days.
Streams in this subsection drain to the San Joaquin River. All but the larger streams are generally dry during the summer. The Stanislaus, Tuolumne, and Merced Rivers cross this subsection. There are no lakes.