Modern desert communities have been present for only about five percent of th 2.4 million years of the Peistocene, while ice age woodlands in the desert lowlands persisted for about 90% of this period.

Great summer rainfall would suggest, tropical oceans were warmer than they are today.

As far as many large animals, the modern distribution may not accurately reflect their physiological range limits because of human predation in the last 11,000 years.

The delta of the Colorado River was historically a very wet area that supported extensive cottonwood gallery forests with abundant beaver.

Paul Martin of the University of Arizona presented the case that big game hunters caused widespread extinctions within a few hundred years after their entry into North America from Siberia via the Bering Strait. However, the paleobotanic recon give no evidence of climatic changes severe enough to have resulted in the extinction of so many large animals over such a broad, diverse area. At the beginning of the Holocene, the last glacial/interglacial climatic shift, there are essentially no records of speciation or extinction in plants or small animals.

The region likely supported tropical deciduous forest from the Eocene to the early Miocene... Geology mocks our notion of permanence.

Mountain chains appear near coastlines for various geologic reasons, setting up orographic (mountain-induced) cooling of rising moist air masses to form coastal fog deserts and rain shadow deserts on the protected sides, such as coastal Baja California and the hyperdry Mohave Desert, respectively. Upland canyons, peidmonts, and mountaintops create new ecological niches, sites of adaptations and evolutionary change.

As climates and habitats change, plant and animal species either adapt, migrate to more favorable ground, or become extinct.

Ancient life affects later geologic and climatic conditions. Biologically produced gases (oxygen, carbon dioxide, methane, nitrous oxide) maintain a chemically reactive atmosphere that in turn influences rates of rock weathering, the nature of sedimentary deposits, and the content of gases in the atmosphere.

Desert soils, highly variable in their water-holding capability, salinity, and alkalinity determine the kinds of plants that will survive on them.

Topography is an important influence upon the unique climate of the Sonoran Desert, since topographic barriers direct, confine, or block moist air masses.

Formation of Range and Basin

Volcanism and Regional Arching...

Intense heat rising into the crust was hot enough to entirely melt and soften portions of the lower continental crust into viscous fluid...

By about 12 million years ago, the entire substrata of Basin and Range country was involved with the expanded taffy-pull, stretching out some thirty to eighty percent more than its original width, while the brittle crusts above shattered into hundreds of long, thin segments.

Basin and Range country is unique--no other region of similar origin is identified on the planet.

Near the edge of major Basin and Range formation, about six million years ago, a severe sideways ripping action began along the Pacific coastline; this continues in our own time [San Andreas fault].

Earthquakes

...The Sonoran Desert is seismically quiet...

The Pleistocene Climate

The last permanent high-elevation ice masses were rapidly melting about 14,000 years ago, and the regional climate was becoming drier and modern (interglacial) by 12,000 years ago. This last climate change marks the birth of the modern Sonoran Desert ecosystem.

Landforms

Layered volcanic bedrock
crystalline bedrock
alluvial fan - debris flown downhill from mountains by rare, heavy rains that produce torrents of mud, rocks, and vegetation
stream terraces
floodplain
pediment -- buried shoulders of mountain rock
basin and range faults
stream through valley
gravel
sand and mud
schist
insleberg -- isolated small hills of exposed rock masses near mountains
granite bedrock
bajada or piedmont -- coalescence of neighboring alluvial fans
terrace gravels

Since the 1890's, river floods have tended to widen the [valley] channels, so that the floodwaters do not flow out onto the floodplains, except locally.

This post-1890's channel enlargement is part of the regional trend throughout the West called "arroyo cutting," likely caused by a combination of factors, including increased cattle grazing following development of regional railroads in 1882, devegetation of hillsides by the mining industry for mine timbers and coke, and a possible unrecognized, subtle-climate shift.

Haphazard bank stabilization increases channel erosion (bank caving) and floodplain inundation downstream of the protected reaches. This is because cement-lined channel walls prevent infiltration and force more water down the channel.

Desert pavement is a sparsely vegetated desert flatland totally covered with a single layer of desert-varnished rocks. Desert varnish is a black, shiny coating on the exposed surfaces of undisturbed rocks.

Sonoran Desert Region Natural History

Hints of profound relations are everywhere around us: storm tracks funneled by Basin and Range topography, which ultimately define the region's ecological limits; the very different past worlds represented in the fossil record; hard granite rocs torn apart by small lichen colonies; the wide variety of landforms and rock types.  ...the often-neglected third dimension is vital, as miners, soil engineers, and water well drillers know. All these effects of the past are interwoven into a tapestry of cause and effect on a grand and wondrous scale.

Desert Soils
parent materials
alluvial fans
sediments: sand, silt, clay
mountainous: rock types, slopes, and exposures
soil-forming environments
water
physical reactions
chemical reactions
wind-blown dust
life in the soil
soil layers: clay, colors, caliche
caliche: "nearly impenetrable, cemented layers, or petrocalcic horizons"
calcium carbonate accumulation

Atmospheric additions of calcium contained in dust and precipitation are the predominant sources of the calcium contained in calcic horizons of most desert soils.

Clay-rich layers are called argillic horizons.

The weatering (oxidation) and accumulation of iron-bearing minerals contained in the soil produce this [red] color.

Soil and Desert Life, Joseph R. McAulliffe

Animals can profoundly affect characteristics of desert soils. The animals that dig, wiggle, tunnel, and burrow through desert soils range in size from microscopic mite and nematodes to badgers and coyotes. Their activities move soil around and cycle nutrients among the realms of "animal, vegetable, and mineral."

Ecological wealth generates more wealth...

Soil mites play a variety of ecological roles.

In the warm deserts of the American Southwest, termites are perhaps the greatest earth-movers. Desert termites accomplish the same kind of ecological role as do earthworms in the soils of moister regions.

Termites annually brought over 1760 pounds of soil materials per two and one-half acres (one hectare) to the soil surface.

Subtle changes in [soil] permeability and texture affect how much precipitation is either absorbed by the soil or lost to runoff, and how deeply water infiltrates the soil.

Human Ecology in the Sonoran Desert, Thomas E. Sheridan...1922, Aldo Leopold

"For all we could tell, the Delta had lain forgotten since Hernando de Alarcon landed there in 1540. When we camped on the estuary which is said to have harbored his ships, we had not for weeks seen a man or a cow, an axe-cut or a fence. On the map the Delta was bisected by the river, but in fact the river was nowhere and everywhere, for he could not decide which of a hundred green lagoons offered the most pleasant and least speedy path to the Gulf. So he traveled them all and so did we. He divided and rejoined, he twisted and turned, he meandered in awesome jungles, he all but ran in circles, he dallied with lovely groves, he got lost and was glad of it, and so were we. For the last word in procrastination, go travel with a river reluctant to lose his freedom to the sea." Leopold, 1949, p141.

By the time Leopold wrote those words in the 1940's, he knew he was writing an elegy, not a paean. The river mighty enough to support a jungle in the desert had already lost its freedom, not to the Gulf of California into which it emptied, but to California farmers and the City of Los Angeles. Beginning in the 1890's, Anglo-American promoters and government engineers strove to break the Colorado to the new Western order. Their first attempts nearly triggered a geological catastrophe, when floods in 1905 sent the Colorado roiling down a canal with no headgate and turned the Salton Sink into the Salton Sea.

In 1936, however, a white wall of more than three million cubic yards of concrete rising 726 feet against black rock halted the river in its tracks.  Erected to prevent floods and to provide hydroelectric power, Hoover Dam turned the Colorado into a tame ditch for the last 300 miles of its course to the sea. The Colorado and its tributaries, along with the other major rivers that brought water to the Sonoran Desert, such as the Yaqui and the Mayo, became ghosts of the past, victims of the twentieth-century, carcasses of sand whose lifeblood had been diverted into cotton fields, copper mines, and vast, sprawling cities.

"Man always kills the thing he loves, and so we the pioneers have killed our wilderness." Leopold, 1949, p 148.

Human groups have shaped the flora and fauna of the Sonoran Desert, including the Colorado delta, for millennia.

Pre-Columbian America was not a "pristine natural kingdom" where "the native people were transparent in the landscape, living as natural elements of the ecosphere" [as Shetler said, 1991, p226].

The most intensive way pre-Columbian Native Americans transformed their environments was through agriculture. Archaeologists are finding evidence that so-called "Archaic" peoples were growing maize (corn) at least 3000 years ago in well-watered areas like the Tucson Basin. Then came pinto and tepary beans, gourds, squash, cotton, and a host of other plants including amaranth and devil's claw. The Cocopas cultivated panic grass in muddy sloughs along the Colorado delta.

They, like the Quechan, Mojaves, Yoemem (Yaquis), and Yoremem (mayos), practiced flood-plain-recession agriculture, planting their crops as flood waters receded. The Hohokam and their successors, the Akimel O'odham (Upper Pimas), on the other hand, dug canals to diver water from Sonoran Desert rivers onto their fields. Hohokam canal systems along the Salt and Gila rivers snaked across the desert floor for nearly 100 miles (160 km) in the Florence area and for 125 to 315 miles (200-500 km) in the Phoenix Basin. Hohokam farmers did not use all sections of these canal systems at any one time. Nonetheless, they still irrigated between 30,000 and 60, 000 acres (12,000-24,000 hectares) in the Phoenix Basin alone.

Hohokam farmers also constructed ditches and brush weirs along alluvial fans to diver runoff onto their fields after summer rains. This form of agriculture, sometimes called ak-chin among Tonoho O'odham in southern Arizona and temporal among mestizos (people of mixed Hispanic and Indian ancestry) in rural Sonora, is still being practiced today. North of Tucson, however, the Hohokam developed an enormously labor-intensive type of agriculture that did not survive into the historic period.

Archaeologist Paul and Suzanne Fish and their colleagues at the Arizona State Museum discovered more than 42,000 rock piles in association with contour terraces and checkdams on the western slopes of the Tortilla Mountains. They also found huge roasting pits containing charred fragments of agave. The rock piles protected young agave plants form predation by rodents and conserved moisture by reducing evaporation around their bases. During the twelfth and thirteenth centuries, more than 100,000 agaves may have been simultaneously growing in these rock pile fields.

In the winter of 1358-59 AD, a massive flood roared down the Salt and Verde rivers, washing out the canals and washing away fields in the Salt River Valley. The flood was followed by two decades of drought and more floods in the early 1380's. Hohokam communities along the Salt may have never recovered from those climatic changes.

By the time the first Europeans settled in the region in the late 1600's, however, Hohokam civilization had collapsed. Some archaeologists speculate that centuries of highly mineralized irrigation water may have saturated Hohokam fields with salts until they could no longer produce crops. Others argue that increasing political conflict may have caused Hohokam society to implode.

Once the Europeans arrived, however, one of the greatest ecological revolutions in the history of the world transformed both lives and landscapes. Biological historian Alfred Crosby calls this revolution the Columbian Exchange--that flow of genes, microbes, plants, and animals between the "Old World" of Eurasia and Africa and the "New World" of the Americas. The first revolutionary wave may even have preceded the Europeans themselves: Indians infected with Eurasian diseases like measles, influenza, and smallpox may have unsuspectingly unleashed contagions as they traveled up ancient trade routes from central Mexico. Some archaeologists and ethnohistorians even contend that these diseases may have contributed to the demise of Hohokam civilization itself.

What we do know is this: thousands of Yaquis and Mayos were perishing when the first Jesuit missionaries ventured into Cahita territory in the early 1600's.The pattern repeated itself again and again at five- to eight-year intervals as Spanish settlers pushed northward in to the Opatieria and Pimeria Alta. Epidemiologically virgin populations had no genetic resistance or cultural mechanisms to resist the microbial onslaught. Like native peoples all over the Americas, the number of Indians in the Sonoran Desert declined by as much as ninety-five percent over the next two centuries.

For the Indians who endured, however, the Europeans brought new crops, new tools, and new animals that revolutionized their economies and their means of transportation. Winter wheat filled an empty niche in their agricultural cycle because it could be planted in November, when frosts at higher elevations in the Sonoran Desert would have withered corn, beans, or squash. Mules and oxen enabled them to cultivate their fields with wooden or iron plows, intensifying their reliance upon agriculture. Cattle, sheep, and goats allowed them to convert non-edible forbs and grasses into beef, mutton, cheese, milk, leather, and tallow. Horses expanded their ranges and shrank distances. A Euro-American agropastoralist economy--irrigation agriculture along the floodplain, animal husbandry in the uplands--supplemented, complemented, and slowly replaced digging-stick agriculture and wild food gathering.

Stock raising was the most land-extensive Euro-American transformation of the Sonoran Desert. Cattle, horses, goats, and sheep searched for forage from river floodplains to mountain crests. In more settled areas like central Sonora, overgrazing became endemic during the Spanish colonial period. The presido (military garrison) and town of Pitic (modern Hermosillo) alone ran 5000 head of cattle, 3422 sheep, 435 goats, 2138 horses, and 367 mules (Radding 1997, p218).

The discovery of silver in 1683 south of the Rio Mayo led to the development of Alamos, a city of wealth and social stratification based upon the enormous capital investment required to extract and process silver ore. Cieneguilla in northwestern Sonora, on the other hand, was a desert boom town where gambucinos (prospectors) dry-winnowed alluvial deposits for particles of gold. Vein-mining operations like that of Alamos depended upon large, stable labor forces organized into hierarchies of occupations. Placers like Cienguilla attracted restless, mobile congregations of Spaniards, mestizos, Yaquis, Opatas, and Pimas, most of whom worked for themselves. No other economic activity so thoroughly rearranged social relationships on New Spain's and then Mexico's northern frontier.

These social and ecological patterns replicated themselves after the United States wrested away more than half of Mexico's national territory during the Mexican War of 1845-48. Gold and silver lured Anglo-Americas and Europeans to the northern Sonoran Desert. Precious metals were the only commodity worth the enormous transportation costs on such an isolated and dangerous frontier.

Because Apaches, Yavapais, and River Yumans resisted European and Euro-American conquest so successfully for two centuries, Euro-American impact upon the desert environment was intermittent rather than sustained.

That all changed in the 1880's, when the Era of Extraction began. In 1877, the Southern Pacific Railroad reached Fort Yuma on the Colorado River. Three years later, after its largely Chinese crews had laid tracks across some of the hottest, driest terrain in North America, the railroad steamed into Tucson. At a gala celebration on March 20, 1880, Mexican intellectual Carlos Velasco raised a toast to the "irresistible torrent of civilization and prosperity" that would follow the steel rails.

Six years later, when Geronimo surrendered to General Nelson Miles for the final time, the frontier came to an end. Suddenly, Arizona and Sonora were safe for global capital, which poured in from eastern United States, California, and the British Isles, as the Southern Pacific and other railroads extended their arteries of commerce across deserts and mountains. Both Arizona and Sonora became extractive colonies of the industrial world, their natural resources ripped from the ground and shipped somewhere else for finishing, processing, and consuming. In Arizona, this was the era of the "Three C's," when cattle, copper, and cotton dominated the economy.

The first major extractive industry to explode across the landscape was stock raising. In 1870, there were perhaps 38,000 head of cattle in the Arizona territory. By the early 1890's, there were about 1,500,000 head of cattle and more than a million sheep.

The second major extractive industry--copper mining--depended even more heavily upon the railroads. Unlike gold and silver, copper was an industrial rather than a precious metal. The evolution of the industry therefore became the triumph of technological innovation over declining grades of ore. Staggering amounts of earth had to be moved from mine to smelter in railroad cars, not in freight wagons or on the backs of mules. In copper districts like Bisbee, where Phelps Dodge Corporation's Copper Queen reigned supreme, thousands of miles of shafts and tunnels burrowed underground. And while most of these early districts were in the uplands fringing the Sonoran Deseret, Phelps Dodge and other giants eventually chewed into the desert as well, particularly after open-pit mining became feasible. At Ajo, Mammoth, Twin Buttes, and Silverbell, giant holes begat gargantuan slag heaps, which rose above the desert floor like pyramids erected in honor of the Electrical Age.

Water Control and the Transformation of the Desert

The third major extractive industry--agriculture--led to the ultimate transformation of the Sonoran Desert. For 3000 years, farmers had cultivated their crops along those few stretches of the desert where surface water flowed near arable land. In Sonora, the major agricultural areas were located in the river valleys of the zona serrana, the mountainous central and easter portions of the state. ...  During the Spanish colonial period, the serrana attracted Spaniards and their mestizo decedents as well, who raised wheat, fruits, vegetables, and sugar cane along its cottonwood-shaded floodplains.

After Mexico won its independence from Spain in 1821, however, Guaymas became Sonora's most important port of entry, and a strong commercial axis between Guyamas and Hermosillo (formerly Pitic)--the gateway to the serrana--developed. Merchants and military officials cast covetous eyes on the rich coastal floodplains of western and southern Sonora, particularly the Yaqui and Mayo river valleys.

Speculators soon descended upon the Yaqui Valley with grand plans to irrigate the coastal plains. In 1890, the Mexican government granted Carlos Conant Maldonadl 300,000 hectares (one hectare, or ha = 2.47 acres) [741,000 acres, or 1160 square miles] along the Rio Yaqui, 100,000 ha along the Rio Mayo, and 100,000 ha [383 sq mi] along the Rio Fuerte in northern Sinaloa in return for surveying the area and building canals along each river [for a total of 2000 square miles]. ... The Richardson Construction Company bought Conant's grant in 1906. In exchange for selling 400-ha blocks of land and supplying irrigation water to the colonialists, ... Richardson received exclusive right to sixty-five percent of the Rio Yaqui's flow for 99 years.

The Richardson brothers had big plans to build storage dams on the Yaqui to generate electric power and furnish water to a network of canals capable of irrigating 300,000 hectares. The Mexican Revolution and World War I destroyed their enterprise but the dream of transforming the Yaqui Valley into a vast grid of irrigated agribusiness bore fruit in 1952, when the Mexican government completed Alvaro Obregon Dam at Oviachi forty miles away. Along with dams upriver, the Oviachi reservoirs controlled flow along the lower Rio Yaqui and eventually channeled its water into three major canals that irrigate nearly 600,000 acres in the Yaqui Valley.

Cuidad Obregon, a city of more than 500,000 people arose to service the largest irrigation district in Sonora. Recognizing the Yaqui Valley's importance, the Rockefeller Foundation established a wheat-breeding station on the outskirts of Obregon. This station became one of the hearths of the Green Revolution, that controversial program that dramatically increased wheat production--all of it dependent upon high inputs of chemical fertilizers and pesticides--around the world.

North of the Yaqui Valley, advances in pump technology after World War II allowed other coastal irrigation districts to bulldoze desert plains and convert them into wheat and cotton fields. The largest was the Costa de Hermosillo where, at its height, 887 pump-powered wells regurgitated water onto more than 100,000 hectares. But discharge exceeded recharge by 250 percent. As water tables plummeted and salt water intruded from the Gulf of California, the Mexican government finally stepped in and halved the amount of water that could be pumped. Other farmers switched from relatively low-value crops like cotton to high-value, high-risk crops like brandy grapes, citrus, garbanzo beans, and vegetables destined for US markets.

Because of these developments, Sonora's demographic, political, and economic center of gravity shifted from the serrana to the coast during the twentieth century. Dam-building and groundwater pumping enable capital-intensive agricultural districts to plow under the great mesquite bosques (forests) west of Hermosillo and desert ironwood plains around Caborca. Those twin pillars of modern water control also allowed older cities like Hermosillo... to expand and entirely new cities ... to spring up like an industrial flower south of the dry channel of the Rio Yaqui. ... Hermosillo already experiences severe water shortages during dry seasons and dry years. Groundwater districts like Caborca and the Costa de Hermosillo are contracting painfully as aquifers drop and pumping costs escalate. Even the Yaqui Valley with its huge reservoirs faces an uncertain future as the North American Free Trade Agreement (NAFTA) reshapes Mexican agriculture.

In Arizona, the future of agriculture is held hostage to urban growth. During the early twentieth century, the newly created Reclamation Service (precursor of the US Bureau of Reclamation) erected the Roosevelt Dam in 1903 on the Salt River east of Phoenix and turned the Salt River Valley into one of the largest agricultural centers in the Southwest. And when a British embargo on long-staple industrial cotton during World War I triggered Arizona's cotton boom, commercial agriculture spread across the saltbrush and cresosote bush flats between Phoenix and Tucson. Arizona became one of the leading cotton producers in the World.

But World War II and the postwar boom thrust Arizona from the Era of Extraction into the Era of Transformation, turning an overwhelmingly rural state into an overwhelmingly urban one. Thousands of acres of citrus and cotton sprouted subdivisions and malls as Phoenix and its satellites sprawled in to a metropolis of more than 2,500,000 people by 1995. Metro Tucson approached 750,000. By the time the Central Arizona Project (CAP)--a farmers dream since the 1920's--reached Maricopa, Pinal, and Pima counties, many farmers could not afford its water. The CAP became one more bargaining chip in the water game, that escalating contest that pitted relentlessly expanding cities against farmers, miners, and Indian nations.

Visions and Nightmares

Water has always been the ultimate limiting factor on human society in the Sonoran Desert. Until the late nineteenth century, people largely relied upon surface flow, adapting to rivers rather than making the rivers adapt to them. This century, however, dams have domesticated all major rivers in the region while pumps have mined groundwater aquifers far beyond recharge.

Areas like metropolitan Phoenix will spread unchecked as long as they can wrest more water away from farmers and Indian nations. For much of its history as part of the United States, Arizona has suffered from a bad case of "California Envy," battling its more powerful western neighbor for Colorado River water, yet emulating its explosive growth. Unless it consciously decides to restrict growth, however, Arizona will become the southern California of the twenty-first century.

Aldo Leopold may have underestimated past people's manipulation of the Southwestern landscape. He would have shuddered at how utterly we have transformed the Sonoran Desert since he and his brother drifted along the green lagoons.