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This
subsection is on a coastal plain and terraces between mountains of the
northern California Coast Ranges and the Pacific Ocean. The mouths
of the Mad, Van Duzen, and Eel Rivers, which drain much of the northern
California Coast Ranges, are on this coastal plain. The subsection
climate is temperate and humid. There is substantial oceanic influence
on climate, including summer fog. MLRA 4b.
Lithology and Stratigraphy. This subsection is predominantly Quaternary nonmarine and marine terrace deposits and recent alluvium. Also, there are some Plio-Pleistocene nonmarine sediments and, along the ocean shoreline, substantial areas of beach and dune deposits.
Geomorphology.
This subsection is on a low coastal plain and marine terraces adjacent
to the ocean. Beaches line the outer edge of the plain and dunes
of sand spread inland from the beaches. There is a large bay behind
this shoreline that has a permanent outlet to the ocean. The Quaternary
terraces are only slightly dissected, but the Plio-Pleistocene sediments
are highly dissected. They reach elevations up to about 1000 feet.
Ocean currents, waves, and wind are active to forming the beaches and dunes.
Fluvial erosion and deposition are the main geomorphic processes on the
plain and terraces behind the beaches.
Soils.
The soils on terraces are mostly Typic Humitropepts and Typic Tropohumults.
Tropofluvents, Fluvaquents, and Humaquepts are common soils on recent alluvium.
Tropopsamments occur on the dunes. The soil temperature regimes are isomesic
and mesic. Soil moisture regimes are mostly udic, and some aquic.
Vegetation. The predominant natural plant communities on the plain and on marine terraces around Trinidad Head and Patricks Point are Sitka spruce series on the outer edge and Redwood series inland. Much of the area has successional cover of shrubs or Red alder series. The dunes support a succession of plant communities, from bare dune through Native dunegrass series, Sand - verbena - beach bursage series, and Beach pine series or Bishop pine series to Sitka spruce series. Pickleweed series is present, but not extensive, in this subsection.
Surface Water. Water runs off of the terraces rapidly,
but there are large areas of floodplain along the Eel, Van Duzen, and Mad
Rivers. Tidal water enters Humboldt Bay and the mouths of the Mad
and Eel Rivers. Brackish water lagoons are common along the coast.
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