NATIONAL PARK SERVICE Sonoran Desert National Park, Arizona: A Proposal

The Study Area

As shown on the accompanying map, the study area encompassed by this report consists of the Organ Pipe Cactus National Monument in southern Arizona, the Pinacate volcanic field in Mexico, and all of the Cabeza Prieta Game Range except a small portion lying east of the Growler Mountains. This latter area was excluded because of military radar use of Childs Mountain, possible mineralization, and because it contains part of the only section of the Game Range on which there is an existing grazing allotment. The study area also includes a 80,000-acre extension west of the Game Range which contains the significantly important Tinajas Altas Mountains.

The 330,874-acre Organ Pipe Cactus National Monument was created in 1937 by Presidential Proclamation to protect and preserve especially fine examples of Sonoran Desert vegetation, wildlife, and scenery. It is, in fact, a veritable natural desert botanical garden and wildlife habitat, and exhibits a biological cross-section of the entire region. In addition, the Monument includes two species of columnar cactus — the organ pipe and senita — which are fairly common in Mexico, but extremely rare north of the border.

Adjoining the National Monument on the west and north and, like the Monument, lying along the Mexican border, the Cabeza Prieta Game Range is an area larger than the State of Rhode Island without a single permanent inhabitant. Its size is more than 1,340 square miles, or about 860,000 acres. No roads passable with standard cars traverse its wide-sweeping valleys and rugged mountains, and the few faint traces of man's past activities are swallowed up in the immensity of the stark desert wilderness. Furthermore, at the present time all trespass is forbidden, and visitors must obtain special permits to enter.

Administered by the Bureau of Sport Fisheries and Wildlife, the Game Range was established in 1939 primarily for the protection of desert bighorn sheep. It also provides an undisturbed habitat for a remnant population of pronghorn, commonly known as antelope; the collared peccary, a wild pig locally called javelina; gambels quail; and white-winged dove. Travel in the area is severely restricted, as it is within a military aerial target range. Although there is no air-to-ground firing, occasional stray shells and falling targets are considered to be potential dangers.

Together the National Monument and Game Range occupy only about one-third of a vast, sparsely settled and mostly undeveloped arid region north of the border, which is continued southward by an equally large area in Mexico — altogether some 11,000 to 12,000 square miles.

Adjacent to the study area on the east is the Papago Indian Reservation, and to the north stretch roadless, almost uninhabited public domain lands to transcontinental Interstate Highway 8. West are the "Medanos," or sand dunes of the Yuma Desert, which spread in golden waves almost to the Colorado River, while to the south, in the Mexican State of Sonora, lies the famed Pinnacate region, one of the roughest and most remarkable volcanic fields on the continent.

There are only two communities of any consequence in the vicinity of the study area. These are the Arizona copper-mining town of Ajo, with a population of 7,049, a few miles north of the National Monument, and Sonoyta, a Mexican border settlement with 1,275 inhabitants, located just south of the Monument. But the area is easily accessible by car. A paved highway reaches Ajo from Tucson, to the east, and another leaves Interstate 8 at Gila Bend and goes south to Ajo and through Organ Pipe Cactus National Monument to the popular Mexican sport-fishing resort of Punta Penasco, on the Gulf of California. Also, following the border westward from Sonoyta is paved Mexico Federal Highway 2, which connects with through routes to Yuma and California points. South of Sonoyta the same road continues to Mexico's west coast Federal Highway.

The Sonoran Desert

The Cabeza Prieta Game Range and Organ Pipe Cactus National Monument, which make up the major portion of the study area, are particularly important because they represent the last sizeable expanse of relatively unspoiled Sonoran Desert remaining in the United States.

Except for the extensive Great Basin sector, the so-called Great American Desert is divided into the Chihuahuan Desert inn the Mexican State of Chihuahua, west Texas, and part of New Mexico; the Mohave Desert in Southern California and Nevada; and the Sonoran Desert in the Mexican States of Sonora and Baja California and in southern Arizona and California. Each is distinguished by botanical, zoological, and climatic differences.

The Mohave Desert contains principally vast open stands of low shrubs, tall yucca at higher elevations and, in contrast to the Sonoran, but few trees. The Chihuahuan Desert is characterized by low shrubs, low-growing succulents such as pricklypear, cholla, and small barrel cactus and yuccas; small trees are confined to drainageways. The Sonoran Desert, lying between these provinces, is by far the richest in number and variety of life forms and in diversity of biotic communities. It contains large numbers of both evergreen and deciduous shrubs, many species of small trees, and a great variety of succulents — notably cactus. It is known for its unusual variety of mammals, reptiles, and birds.

Desert Areas of North America (click on image for a PDF version)

Of these three deserts, the Sonoran is without doubt the most unusual and distinctive. One of the great biotic provinces of North America, it stretches northward from the vicinity of Guaymas, Sonora, 500 miles to beyond Phoenix, Arizona, and is more than 300 miles across at its widest point. Extremely dry, the region throughout is searing hot in summer, but enjoys one of the world's pleasantest climates from November to April. Annual precipitation averages from 2 to 10 inches and a brilliant sun shines the year round 80 percent of all daylight hours. Although there are barren waste lands of sand and naked rock, in general the country supports a surprising wealth of varied vegetation and wildlife. In fact, it demonstrates a complex ecological balance that has produced an individual environment different from any other on earth.

Organ Pipe Cactus National Monument and the Cabeza Prieta Game Range lie wholly within the Sonoran Desert. However, the western part of the Range begins to display botanical and zoological indicators that represent a transition between the Sonoran and Mohave Deserts. Ecologists describe this mingling in the west half of the Game Range as "an interesting interdigitating mosaic pattern of zones influenced by various physical and climatic factors." Examples are the changes from east to west in species of creosotebush and bursage, which represent the transition from one biotic province to another.

Proposed Sonoran Desert National Park Vicinity Map (click on image for a PDF version)

The study area is also at the northern limits of several predominantly Mexican plants and animals, such as the organ pipe and senita cactuses, the elephant tree, and the peccary. Within the Monument and Game Range are more than 400 botanical species, representing some 75 families; and the fauna includes 34 species of mammals, over 128 species of birds, 16 of reptiles, and 1 rare fish. The last is the inch-long, shiny blue Percy minnow or pupfish, found only in the pond at Quitobaquito Springs in the southern part of the Monument, and one other place in Arizona.

Geographically and geologically Organ Pipe Cactus National Monument and Cabeza Prieta Game Range are a part of the Basin and Range province, which stretches from southern Oregon to west Texas, and includes the entire intermontane region between the Wasatch Mountains in Utah and California's Sierra Nevada. The province as a whole is characterized by abrupt, long but relatively narrow, north-and-south-trending mountain ranges separated by broad valleys and plains. Due to the general aridity, much of the drainage water never reaches the Pacific, but collects and evaporates in shallow intermittent lakes, called playas. Most or them have no outlets and at the present time are usually dry expanses of hard-packed sand, silt, or alkali, although during the Pleistocene pluvial epoch they contained permanent water and many drained to the ocean.

The study area is an almost perfect type-example of basin-and-range topography. From the eastern boundary of the Monument to just beyond the western end of Cabeza Prieta, seven mountain ranges roughly parallel each other in a northwest-to-southeast direction. Between them are six valleys, in places so wide as to resemble nearly level plains. There are also groups of low hills and isolated buttes and mesas scattered here and there. Except in parts of the extreme south, the valleys slope gently northward to the Gila River.

The mountain ranges are the most prominent aspect of every view, in spite of the fact that they occupy a relatively small percentage of the total area. This is not evident from the ground, but on maps and from the air these ranges appear as secondary features in a vast expanse of flat desert. Their length varies from 15 to over 40 miles, but they are seldom more than 2 miles wide, and rise from valleys on both sides to single linear crests of peaks and notches which vary little in altitude for considerable distances. Steep, plunging canyons gouge the mountains' rugged flanks, while cliffs and knife-edge ridges are comnon. In fact, these desert ranges seem like the skeletons of mountains whose naked ribs stand out in barren savagery, unrelieved by the softening effect of vegetation. Color alone gives them interest and variety, and they glow with white, grey, yellow, sepia, brown, and chocolate in the brilliant southwestern sunshine.

Moreover, the mountains are surprisingly impressive considering their moderate altitudes. Highest is Ajo Peak, 4,770 feet, in the Ajo Range, which forms the eastern boundary of Organ Pipe Cactus National Monument. Westward, elevations are lower, the Growler Mountains having the only summits in the Game Range topping 3,000 feet. But as the valley floors vary in elevation from 600 to 1,000 feet, precipitous escarpments in excess of 2,000 feet are numerous. Without trees or noticeable vegetation to give the mountains scale, the impression is that they are twice as high as is actually the case.

The Cabeza Prieta Mountains in the western part of the Game Range differ from all the others. Consisting of a maze of canyons, peaks, and ridges, 10 miles long and 5 miles across, they contain some of the roughest country in the entire area. The refuge gets its name from lava-capped Cabeza Prieta Peak, 2,650 feet, near the south end. The Spanish meaning is "dark head." The dappled, pinto-like combination of brown volcanic rock superimposed on gleaming white granite is a spectacular feature of these mountains.

The Game Range receives considerably more precipitation than does California's Death Valley. But it rivals that world-famed dry spot in aridity, and probably has less water resources. There are no permanent streams, reliable springs are non-existent, and the playas in the lowest parts of the valleys have not held water in many years. Wells have been dug or bored in a dozen places and these supply limited amounts of water, mostly by means of windmills.

However, since prehistoric times both men and animals have taken advantage of the so-called "tinajas," translated into English as "tanks." These are natural rock basins in the mountain canyons, scooped out by centuries of violent but infrequent cloudbursts. Most of them retain some water all year. There are many in the Cabeza Prieta Game Range, but the best known are in the Tinajas Altas Mountains just outside the western boundary. The tanks were first used by the Indians and have been life savers to white travelers for nearly 400 years. The Bureau of Sport Fisheries and Wildlife has deepened several mountain tanks by constructing dams at their lower ends, and has excavated artificial tanks in the vicinity of valley washes, which provide drinking water for wildlife and resting places for aquatic birds.

Almost every characteristic of the Sonoran Desert is represented in the Cabeza Prieta-Organ Pipe Cactus study area. Here one has an unmatched opportunity to observe the animals, birds, plants, scenery, and the results of geological forces on a grand scale in a widespread land little altered by man.

Climate

In the Southwest, temperatures drop sharply with increasing altitude, and precipitation increases. The climatic factors which produce the ecological environments typical of the Sonoran Desert extend upward in southern Arizona's highlands and mountains with diminishing effect to an elevation of 4,000 feet. Above, desert conditions are replaced by semiarid grasslands alternating with open groves and woodlands of oak, juniper, and pinyon. Elevations in the study area are too low for this transition to take place, with the exception of the Ajo Range in Organ Pipe Cactus National Monument. There, perched on the highest crests, is a small, isolated island of grassland, scattered with rare Ajo oaks and one-seed junipers.

In the lower elevational limits of the Cabeza Prieta Game Range mean annual temperatures probably do not vary by more than 6 to 8 degrees Fahrenheit from the valley floors to the mountain summits, and there is apparently little increase in precipitation. So the area is an excellent sample of unbroken Sonoran Desert having sufficient size to demonstrate its climatic characteristics without conflicting influences.

There are no official weather stations in the Game Range, although the Assistant Manager has established automatic thermometers and rain gauges at several points. However, the year-round climate may be inferred from the National Park Service station at the Monument Headquarters and those of the United States Weather Bureau in the nearest towns. The extreme maximum and minimum temperatures ever recorded at Ajo, elevation 1,800 feet, 4 miles from the Game Range east boundary, are 115° F. and 17° F., respectively, but at Mohawk, 1,200 feet lower elevation in the Gila River valley, to the north, the figures are 126° F. and 16° F. It is probable that in the lower sun-heated canyons of the Game Range summer daytime temperatures may reach 130° F. As day and night differ by about 40° F., the minimums accompanying such highs would be in the low 90's or high 80's. Winter daytime temperatures range from 60° F. to 70° F.

Annual precipitation decreases rapidly from east to west. At Ajo the average is 9.14 inches, but the western portion of the Game Range is within the belt of least moisture in the United States, and yearly precipitation does not average more than 3.5 to 4 inches. On the other hand, evaporation rates are probably greater than 120 inches a year. Snow whitens the highest peaks of the Monument and Game Range two or three times each winter, but disappears in a few hours, and a snowfall below 2,000 feet is a very rare event.

Summer rains account for 60 to 70 percent of the total precipitation. They fall as convection showers, usually accompanied by thunder and lightning. Often reaching cloudburst proportions, these storms are mostly small in extent, moving across the country and often producing a flash flood in one creek bed and skipping the adjacent watershed entirely. However, torrential rains of short duration may be expected anywhere at any time from late June to mid-September, and they add their bit to the hazards of summer travel in the area. The transition from no water to too much water can be sudden and sometimes dangerous.

More gentle are the winter rains. They are the remnants of west coast storms which have had a large part of their moisture wrung out of them by California's mountain ramparts. If the Pacific storm track is farther south than usual, desert rains are generous. But if the annual cold-season parade of low-pressure areas passes well to the north, little moisture results. Then the winter may be almost rainless, as is normally the case in spring and fall.

This is a country of contrasts: between night and day; winter and summer; moisture and dryness. But it is above all a great natural climatic laboratory where every experiment can be clearly seen and their combined results easily studied.