The Sonoran Desert

The Sonoran Desert as currently defined covers approximately 100,000 square miles (260,000 sq. km.) and includes most of the southern half of Arizona, southeastern California, most of the Baja California peninsula, the islands of the Gulf of California, and much of the state of Sonora, Mexico. It is lush in comparison to most other deserts. Two visually dominant life forms of plants distinguish the Sonoran Desert from the other North American deserts: legume trees and columnar cacti. It also supports many other life forms encompassing a rich spectrum of some 2,000 species of plants.

The amount and seasonality of rainfall are defining characteristics of the Sonoran Desert. Much of the area has a biseasonal rainfall pattern, though even during the rainy seasons most days are sunny. From December to March frontal storms from North Pacific Ocean occasionally bring widespread, gentle rain to the northwestern areas. From July to mid-September, the summer monsoon brings surges of wet tropical air and frequent but localized violent thunderstorms.

The Sonoran Desert prominently differs from the other three North American Deserts in having mild winters; most of the area rarely experiences frost. About half of the biota is tropical in origin, with life cycles attuned to the brief summer rainy season. The winter rains, when ample, produce huge populations of annuals (which comprise half of the species in our flora).

On our flights you will see breath taking views of Lake Pleasant, a 10,000 surface lake with over 100 miles of shoreline.

On real clear days, You can see the San Francisco Peaks located in Flagstaff, AZ 172 miles away. We enjoy conture flying over the mountains just south of the Bradshaw mountains that in the early years of the state history, were know for the large prodution of gold quanities.

Subdivisions of the Sonoran Desert

Forest Shreve

Defined seven vegetative subdivisions in the 1950s. One (the Foothills of Sonora) has since been reclassified as foothills thornscrub, a non-desert biome. The status of two other subdivisions - Arizona Upland and Plains of Sonora - may also be reclassified.

Lower Colorado River Valley

Named for its location surrounding the lower Colorado River in parts of four states, this is the largest, hottest, and driest subdivision. It challenges the Mohave Desert's Death Valley as the hottest and driest place in North America. Summer highs may exceed 120 F (48.5 C), with surface temperatures approaching 180 (82 C). The intense solar radiation from cloudless skies and low humidity (often less than 10%) suck the life-sustaining water from exposed plants, water that cannot be replaced from the parched mineral soil. Annual rainfall in the driest sites averages less than three inches (75 mm), and some localities have gone nearly three years with no rain. Even so, life exists here, abundantly in the rare wet years. The geography is mostly broad, flat valleys with widely-scattered, small mountain ranges of mostly barren rock. There is also a sand sea (the Gran Desierto) and the spectacular Pinacate volcanic field. The valleys are dominated by low shrubs, primarily creosote bush (Larrea divaricata) and white bursage (Ambrosia dumosa). These are the two most drought- tolerant plants in North America, but in driest areas of this subdivision even they are restricted to drainage courses (i.e., they become riparian plants!). Trees are found only in the larger washes. The mountains support a wider variety of shrubs and cacti, but the density is very sparse. Columnar cacti, one of the indicators of the Sonoran Desert, are rare (virtually absent in California) and restricted to drainages. Annual species comprise well over half the flora (90% at the driest sites); they are mostly winter-growing species and appear in numbers only in wet years.

This is the only part of the Sonoran Desert that extends into California, where it is usually called the Colorado Desert. North of a sagging line between Palm Springs and Needles, California, it merges almost imperceptibly with the lower Mohave Desert.

The Northeastern Section

Mostly in south-central Arizona and northern Sonora, is the highest and coldest subdivision of the Sonoran Desert. The terrain contains numerous mountain ranges, and the valleys are narrower than in the Lower Colorado River Valley subdivision. Trees are common on rocky slopes as well as drainages, and saguaros are found everywhere but on the valley floors. This community is also called the saguaro-palo verde forest. It is the only subdivision that experiences frequent hard winter frosts, so many species of the lower elevation and more southerly subdivisions cannot survive here. Nevertheless it is a rich area. The small range that is the Desert Museum's home, the Tucson Mountains, has a flora of more than 630 taxa.

An ever-increasing number of biologists is concluding that the Arizona Upland's climate, vegetation density, and biodiversity resemble thornscrub more than desert. Don't be surprised if this subdivision is reclassified as thornscrub in the future.