Natural History
by Kent Duryee
The Sonoran Desert;

Where is it?

Anza-Borrego Desert State Park lies within the north western Sonoran Desert.  Named after the Mexican state of Sonora, this desert stretches from east of the Gulf of California, through Sonora and southern Arizona.  It reaches north up through the Colorado River Delta area, (here properly called the Colorado Desert), and the Salton Trough/Imperial Valley, finally bordering the Mojave Desert just north of Palm Springs.

mojave desert
The Mojave/High Desert

The boundary between The Sonoran and Mojave Deserts is the subject of some debate, but it is agreed that the Sonoran Desert is a low desert, while the Mojave is a high desert.  In the hills and plateaus above Palm Springs, the Sonoran and Mojave deserts mingle together, but at an elevation of about 4,000 feet, the Joshua Tree is found. This is considered to be a marker species of the Mojave Desert community.  If you see a Joshua Tree, you are no longer in the Sonoran Desert. 

colorado desert
The Colorado/Low Desert
photo by Don Gennero
A good discussion of the subtleties of the borderlands between the Mojave and Sonoran Deserts can be found in David Darlington's book The Mojave, and in The California Deserts, by Edmund C. Jaeger.

sonoran desert
map of the Sonoran Desert

Rain in the Desert

The San Andreas Fault System is one of the prime factors creating the Sonoran Desert Ecosystem in the Anza-Borrego region.  Continued action of the faults that make up the San Andreas system has created a huge trough which contains the Salton Sea at its bottom.  The mountains produced by these faults dictate the weather in the area to a large extent.  In fact, because of the "rain shadow" produced by the mountains to the west, the average annual rainfall in the Anza-Borrego region is only 7 inches.

rain shadow
Rain Shadow
Image by University of Connecticut

The Rain Shadow

Bordering the Salton Trough to the west are the Peninsular or coast ranges, including the Santa Rosa and Laguna Mountains. Storms moving east from the Pacific are blocked by these ranges, and drop most of their moisture over the coastal plains to the west. This phenomenon is known as a rain shadow.  Another famous example of a rain shadow desert in California is Death Valley.  Lying the shadow of the Sierra Nevada, Inyo and Panamint mountain ranges, Death Valley is the driest place in North America. 

Some rain of course does reach Anza-Borrego.  The winter months of January through March bring most of the rainfall for the year.  These storms come in from the North Pacific and the Gulf of Alaska, but are sapped of much of their energy by the effects of the rain shadow. In the summer months, especially August and September, the monsoon action from the Gulf of California is the dominant weather pattern, and thunderstorms regularly ravage the desert and surrounding mountains. There is no interveening mountain range to stop this flow of warm, moist air. This is the time of the flash flood and the hail storm, and caution should be exercised when visiting the desert at this time of year.

What rain does fall on the desert is responsible for the famous wild flower displays of the early spring in Anza-Borrego. A perfect combination of rain in both wet seasons results in an un-rivaled display of flowers from February through March and into April. The last strong wildflower year as of this writing, (June, 2002), was 1998, an El Niņo year. Rumor has it that the coming winter will bring another El Niņo pattern to the Southern California deserts, so hopefully Spring 2003 will be another great year for the annuals.

cactus in Little Blair Valley

Life in the Desert

Like most deserts, the Anza-Borrego is a place of extremes.  Rainfall averages only 7 inches per year.  Temperatures can reach 120 degrees or more in the summer.  It has been known to snow even in the lower reaches of the park in winter.  Trees such as the Washingtonia Palm and the Cottonwood can only grow near springs and ephemeral streams because of the scarcity of water.  Anza-Borrego is known internationally for the spectacular displays of wildflowers that carpet the park in late winter and early spring. 

This land of extremity produces an incredible wealth of life.  Plants found in the region include the ocotillo, creosote, desert lavender, century plant, salt brush, jojoba, acacia, mesquite, Washingtonia palm and the smoke tree, to name only a very few of the more noticeable plants.  Birds known to inhabit and visit the region include the White wing dove, LeConte's Thrasher, Phainopepla, California Roadrunner and Bell's vireo.  Other animals include the magnificent Desert big horn sheep, Mountain lion, Bobcat, Antelope squirrels and of course Rattlesnakes.

Each of these forms of life has developed its own unique means of dealing with the harsh environment of the desert.  These adaptations all have one thing in common, and that is the conservation of water.  Water is the single most important material in the desert.  It is also the most rare and precious material found.  The scarcity of water mutes the vibrant greens of plants, and creates a canopy of growth near ground level, quite different from the lush greenery found in more well-watered regions.  The desert is a place of subtlety and nuance, and must be approached from a completely different frame of mind.  To see the interactions and finer points, you have to dive in.  Southwestern writer Edward Abbey put it this way in his book Desert Solitaire:

"Do not jump into your automobile next June and rush out to the [desert] hoping to see some of that which I have attempted to evoke in these pages.  In the first place you can't see anything from a car; you've got to get out of the goddamned contraption and walk, better yet crawl, on hands and knees, over the sandstone and through the...cactus. When traces of blood begin to mark your trail you'll see something, maybe."

sand verbena
sand verbena