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This
subsection includes the Orocopia and Chocolate Mountains on the southwest
and the Chuckawalla, Little Chuckawalla, Palo Verde, and Mule Mountains
on the northeast, valleys between these two rows of mountains, hills
in these valleys, and Shaver Valley at the northwest end of the subsection.
It is about half alluvial plain and pediment and half mountains and hills.
It has a very hot arid climate. MLRA 30g.
Lithology and Stratigraphy. The mountains, hills, and pediments are mainly Mesozoic granitic, Tertiary volcanic, Eocene marine sedimentary, Oligocene and Miocene nonmarine sedimentary, Pre-Cretaceous metasedimentary, and Precambrian igneous and metamorphic rocks. There is a small area of Quaternary basalt at the southeast end of the Chocolate Mountains. The remainder of the subsection is mostly alluvium, but includes lacustrine deposits in Hayfield Lake.
Geomorphology. This section is on steep mountains, moderately steep hills, very gently to moderately sloping pediments and alluvial fans, and nearly level basin floors. Most of the mountains and larger valleys between the two rows of mountains are aligned northwest-southeast, but the Little Chuckawalla and Mules Mountains trend toward the northeast. Shaver Valley is aligned east-west. The elevation range is from about 200 to 2500 feet on alluvial plains and up to 3815 feet in the Orocopia Mountains, 3047 feet in the Chocolate Mountains, and 4500 feet in the Chuckawalla Mountains. Mass wasting and fluvial erosion and deposition are the main geomorphic processes.
Soils. The soils are mostly Lithic Torriorthents and Lithic Camborthids on mountains and hills, plus Typic Haplargids on volcanic rocks. They are mostly Typic Torrifluvents, Typic Torripsamments, and Typic Torriorthents on younger fans and basin-fill and mostly Typic Calciorthids, Typic Haplargids, and shallow Typic Durorthids on older fans. The soils are well drained. Soil temperature regimes are mostly thermic in the mountains and hyperthermic on pediments and alluvial plains. Soil moisture regimes are aridic.
Vegetation. The predominant natural plant community is Creosote bush - white bursage series. Blue palo verde - ironwood - smoke tree series occurs in washes. Currently Tamarisk series occurs in riparian areas along the Colorado River.
Characteristic series by lifeform include:Climate. The mean annual precipitation is about 4 to 6 inches. It is practically all rain. Mean annual temperature is about 60° to 75° F. The mean freeze-free period is about 250 to 325 days.
Grasslands: Big galleta series, Indian ricegrass series.
Shrublands: Allscale series, All-thorn stands, Bush seepweed series, Brittlebush series, Brittlebush - white bursage series, Catclaw acacia series, Creosote bush series, Creosote bush - white bursage series, Desert-holly series, Fourwing saltbush series, Iodine bush series, Mixed saltbush series, Ocotillo series, Teddy-bear cholla series, White bursage series.
Forests and woodlands: Blue palo verde - ironwood - smoke tree series, Mesquite series.
Surface Water. Runoff is rapid from mountains and alluvial fans and slow from basin-fill. Drainage from the southeast end of the subsection is through Milpitas Wash to the Colorado River, and drainage from the northwest end is through Salton Creek or Box Canyon Wash toward the Salton Sea, or to Hayfield Lake. Streams have no water in some years and are dry most of each year in other years.
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