Lithology and Stratigraphy
This subsection contains Pleistocene and recent alluvium from volcanic rock sources.
This subsection is on very gently sloping to sloping terraces and small areas of floodplain and alluvial fans along streams that cross from mountains of the Southern Cascade Ranges to reach the Sacramento River. The subsection elevation range is from 200 to about 600 feet. Fluvial erosion is the main geomorphic process.
The soils are mostly Typic Durixeralfs and Mollic Haploxeralfs. The soils are well drained. Bicarbonate weathering and leaching and accumulation of clay and silica in subsoils are the main pedogenic processes in the terrace soils. The soil temperature regimes are thermic. Soil moisture regimes are xeric.
The predominant natural plant communities are Needlegrass grasslands, and Northern claypan vernal pools are common.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Purple needlegrass series.
Vernal pools: Northern volcanic mudflow vernal pools.
Forests and woodlands: California sycamore series.
The mean annual precipitation is about 18 to 25 inches. It is practically all rain. Mean annual temperature is about 58° to 60° F. The mean freeze-free period is about 250 to 275 days.
Streams in this subsection drain to the Sacramento River. 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.