The Bay flats subsection is on parts of the plain at the southern limit of San Francisco Bay, California, USA that are less than ten feet above mean tide level. The microclimate in this mudflat area is generally hot and subhumid.  MLRA 14d.

Lithology and Stratigraphy

This subsection is on Quaternary bay-fill, mostly silt and clay.

Geomorphology

This is a nearly level delta and estuarine area that is mostly flooded by high tides, or was before artificial barriers were built.  The entire subsection is less than about 10 feet above mean sea-level.  The main geomorphic processes are coastal marine, and, on the inner edges of the subsection, fluvial.

Soils

The soils are mostly Sulfic Fluvaquents and Endoaquepts, Typic Hydraquents and Haplaquepts.  Soil temperature regimes are isomesic.  Soil moisture regimes are aquic.

Vegetation

The predominant natural plant community of the intertidal zone is the Pickleweed series.  Saltgrass series is prevalent around the inland margin of the tidal saltmarsh.  Sedge meadow communities and Emergent aquatic communities are common on the inner edges of the subsection, away from the bay, or where flooding and drainage patterns have been altered artificially.

Characteristic series by lifeform include saltmarsh vegetation: Cordgrass series, Ditchgrass series,  Pickleweed series, Saltgrass series and Sedge series.

Climate

The mean annual precipitation is about 12 to 15 inches. There is virtually no snowfall in this locale.  Mean annual temperature is about 58° to 60° F.  The mean freeze-free period is about 250 to 275 days.

Surface Water

High tides inundate most of the area.  The deltas of Coyote Creek, which drains the Santa Clara Valley, and Alameda Creek, which drains the Livermore - San Ramon Valley, are in this subsection.