Lithology and Stratigraphy
This subsection contains recent alluvium. The alluvium is from granitic, volcanic, sedimentary, and metamorphic rock sources, but it is mostly from sedimentary and metamorphic rock sources along the western edge of the subsection.
This subsection is on a nearly level basin floor, with some alluvial fans along the western edge of the subsection. The subsection elevation range is from about 20 to about 120 feet. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Typic Haploxererts, Xeric and Sodic Endoaquerts, and Cumulic Vertic Endoaquolls. They are mostly moderately well to poorly drained. Soil temperature regimes are thermic, and soil moisture regimes are xeric and aquic.
The predominant natural plant communities is Needlegrass grasslands. Fremont cottonwood series occurs along streams. Emergent aquatic communities are common.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Purple needlegrass series.
Wetlands: Bulrush series, Bulrush - cattail series, Cattail series, Sedge series.
Forests and woodlands: Fremont cottonwood series, Mixed willow series.
The mean annual precipitation is about 14 to 18 inches; it is practically all rain. Mean annual temperature is about 60° to 62° F. The mean freeze-free period is about 250 to 275 days.
The Sacramento River overflowed onto this area when it flooded before being controlled by dams, artificial levees, and diversions. Streams draining eastward from the northern California Coast Ranges are diverted southward in overflow channels that run parallel to the Sacramento River. Most of the streams are generally dry during the summer. There are no lakes, but winter ponding occurs.