Lithology and Stratigraphy
This subsection contains Pleistocene and recent alluvium. The alluvium is from granitic, volcanic, sedimentary, and metamorphic rock sources.
This subsection is mainly floodplains and very gently sloping recent alluvial fans. There are some Pleistocene terraces west of the Sacramento River that are in northern part this subsection. The subsection elevation range is from 150 to about 300 feet. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Typic, Aquic, and Mollic Xerofluvents and Cumulic Haploxerolls on floodplains and Argixerolls, Haploxeralfs, and Palexeralfs on late Quaternary alluvial fans. On Pleistocene terraces they are mostly Abruptic Durixeralfs and Typic Palexeralfs. The soils are mostly well drained, but some on floodplains are somewhat poorly drained to moderately well drained soils. Most of them are leached free of carbonates. The soil temperature regimes are thermic. Soil moisture regimes are xeric.
The predominant natural plant communities are Needlegrass grasslands. Fremont cottonwood series occurs along streams, and Valley oak series is common on recent alluvial plains. There are some Northern claypan vernal pools on Pleistocene terraces.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Purple needlegrass series.
Vernal pools: Northern hardpan vernal pools.
Forests and woodlands: Blue oak series, California sycamore series, Fremont cottonwood series, Foothill pine series, Valley oak series.
The mean annual precipitation is about 16 to 24 inches. It is practically all rain. Mean annual temperature is about 59° to 60° F. The mean freeze-free period is about 250 to 275 days.
Streams in this subsection drain to the Sacramento River, which runs through the subsection. All but the larger streams are generally dry during the summer. There are no lakes, but there is temporary ponding in vernal pools on Pleistocene terraces.