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Subsection M261Bd
Clear Lake Hills and Valleys

This subsection is a relatively low part of the Northern Coast Ranges that is surrounded by mountains.  Most of the Franciscan rocks in this basin have been covered by late Tertiary sedimentary and volcanic rocks.  The volcanics are not included in this subsection.  Clear Lake, the largest natural lake in the Coast Ranges, occupies much of this subsection. The subsection has a hot and subhumid climate.   MLRA 14d.

Lithology and Stratigraphy.  This subsection contains Jurassic and Cretaceous Franciscan rocks of the Central and Eastern Belts, nonmarine Plio-Pleistocene sediments, and Quaternary alluvium.

Geomorphology.  This subsection is in a structural low, or graben, in the northern California Coast Range mountains.  It contains moderately steep hills, highly dissected Plio-Pleistocene sediments, and nearly level to gently sloping Quaternary alluvial fans, terraces, and  basin-fill.  The elevation range is about 1300 feet to 2000 feet.   Fluvial erosion and fluvial and lacustrine deposition in the basin bottom are the main geomorphic processes.  Mass wasting is a minor process, except on ravine sideslopes in the highly dissected Plio-Pleistocene sediments.

Soils.  Soils of Franciscan terrain are mostly Lithic Xerochrepts and Mollic Haploxeralfs.  Those of gentle to moderately steep slopes that predominate in dissected Plio-Pleistocene sedimentary terrain are mostly Mollic Haploxeralfs.  Fluventic and Cumulic Haploxerolls, Aeric Fluvaquents, Fluventic Haplaquolls, and Pelloxererts are common in alluvial fan and basin-fill deposits, and Ultic Palexeralfs on terraces.  The hill and terrace soils are generally leached free of carbonates, but calcium carbonates and salts accumulate in basin-fill.  Soil temperature regimes are thermic.  Soil moisture regimes are xeric, except for some soils with aquic moisture regimes in alluvium around Clear Lake.

Vegetation.   The predominant natural plant community is Blue oak series.  Needlegrass grasslands, Valley oak series, Riparian habitats, and Emergent aquatic communities are common on alluvium and basin-fill around Clear Lake.

Climate.   The mean annual precipitation is about 20 to 40 inches.  Most of the precipitation is rain, but some is snow.   Mean annual temperature is about 50° to 56° F.  The mean freeze-free period is about 150 to 200 days.

Surface Water.  Runoff from hills and the Plio-Pleistocene sedimentary terrain is rapid and all but the larger streams are dry through most of the summer.   Runoff is stored in and around Clear Lake and the lake level rises when runoff from the surrounding hills and mountains is more rapid than drainage from the Lake.  There is some hydrothermal activity and accumulation of minerals from it, as in and around Borax Lake.

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