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Ecological Sections: Colorado Desert

Subsection 322Cd
East Mesa - Sand Hills

This subsection includes alluvial fans, sand dunes, the Cargo Muchacho Mountains northeast of the Imperial Valley and the floodplain along the Colorado River.  It has a very hot, arid climate.  MLRAs 30h and 31g.

Lithology and Stratigraphy.   Quaternary alluvial and eolian deposits predominate in this subsection.   The Cargo Muchacho Mountains are Mesozoic granitic and Pre-Cretaceous metamorphic rocks, with the former predominating.

Geomorphology.   This subsection consists mainly of very gently to moderately sloping alluvial fans, much of them covered by eolian sand, and moderately steep to steep sand dunes. The Cargo Muchacho Mountains and nearby Ogilby Hills are steep to very steep.  There is a little pediment around the mountains.  Some floodplain of the Colorado River is included in this subsection. The elevation range is from about sea-level to 1200 feet on alluvial fans up to 2221 feet in the Cargo Muchacho Mountains.  Fluvial erosion and deposition and eolian deflation and deposition are the main geomorphic processes.

Soils.  The soils are mostly Typic Torrifluvents, Typic Torripsamments, and Typic Torriorthents on younger fans, plus Typic Calciorthids and Typic Haplargids on older fans, and shallow Typic Torriorthents where Pleistocene of Pliocene sediments have been eroded to form “badlands”.  They are Typic Torripsamments in dune sand.  Soils on the rocky Cargo Muchacho Mountains are mostly Lithic Torriorthents.  Those on floodplain of the Colorado River are mostly Typic Torrifluvents.  The soils are well drained.  Soil temperature regimes are hyperthermic, and some are thermic on north-facing slopes at higher elevations.  Soil moisture regimes are aridic.

Vegetation.   The predominant natural plant communities are Creosote bush series and Creosote bush - white bursage series.  Vascular plants are sparse or absent from most of the Algodones Dunes and other sand hills.  Fan palm series occurs in some canyons near the Chocolate mountains.  Mesquite series occurs in some riparian areas, and Tamarisk series has invaded riparian areas along the Colorado River.

Characteristic series by lifeform include:

Grasslands: Big galleta series, Desert sand - verbena series, Indian ricegrass series.

Shrublands: Catclaw acacia series, Creosote bush series, Creosote bush - white bursage series, White bursage series.

Forests and woodlands: Blue palo verde - ironwood - smoke tree series, Mesquite series.
Climate.  The mean annual precipitation is about 3 to 4 inches.  It is all rain.  Mean annual temperature is about 68 to 75 F.  The mean freeze-free period is about 300 to 350 days.

Surface Water.  Runoff is rapid from most of the area.  Drainage is mostly to the Salton Sea, or to the sand hills, where streams disappear, and, from the southeastern edge of the subsection, to the Colorado River.  Streams have no water in some years and are dry most of each year in other years.  There are no streams in the sand hills. 

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