Lithology and Stratigraphy
This subsection contains Quaternary sediments covered by organic deposits.
This subsection is a practically level plain, except for the levees of the Sacramento and San Joaquin Rivers. Many artificial levees have been constructed to prevent flooding of land committed to agriculture. The subsection elevation range is from a few feet on levees of the Sacramento and San Joaquin Rivers to sea-level, or lower, on the rest of the plain. Decomposition of the organic deposits and consequential land subsidence is the main geomorphic process. Fluvial erosion and deposition are the main geomorphic processes on and adjacent to levees.
The soils are mostly Typic and Terric Medisaprists and Cumulic Haplaquolls. Fluvaquentic Haploxerolls, Fluvaquents, Haplaquepts, and Medihemists are common. Most of the soils are poorly to very poorly drained. Soil temperature regimes are thermic (nearly mesic). Soil moisture regimes are mostly aquic, but they are xeric on levees.
The predominant natural plant communities are Emergent aquatic communities, such as the Bullrush - cattail series.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series.
Wetlands: Bulrush series, Bulrush - cattail series, Cattail series, Ditch-grass series, Sedge series.
Mean annual precipitation is about 14 to 16 inches. It is practically all rain. Mean annual temperature is about 56° to 60° F. Summer temperatures are moderated by marine air. The mean freeze-free period is about 250 to 275 days.
The Sacramento and San Joaquin Rivers follow meanders, that are somewhat modified artificially, across the subsection. There are many overflow channels. Brackish tidal water enters the area when river flow is low during the summer and autumn.