Land of Organ Pipe:

Lower Colorado River Valley


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As the Colorado River nears the end of its course, it passes through some of the hottest and driest land in the southwest. This is the Lower Colorado River Valley, an area of small, rugged mountain ranges, broad alluvial plains, and temperatures frequently exceeding 100 degrees Fahrenheit from June to October. Rainfall averages only two inches a year near Yuma to about eight inches a year near Ajo. This is also the site of Organ Pipe National Monument and the Cabeza Prieta National Wildlife Refuge, the third largest wildlife refuge in the lower 48 states. The southern border of the refuge is shared with Sonora, Mexico, and marks the historic route known as El Camino del Diablo-- the Devil's Highway.

El Camino del Diablo was used by Indians, the Spanish conquistadors, and later, Mexican and American ranchers and miners seeking a shortcut to the north or to the west coast. Travelers braved thirst and windstorms to avoid marauding Apaches and save 150 miles over the route from Tucson following the Gila River. Travelers also remarked on the dozens of graves along the way testifying of those who didn't make it.

Phil and I braved the heat and thirst in our air conditoned car on a weekend in March. The day was warm enough to warrant a stop at the Dairy Queen in Ajo, where we studied the wide expanse of desert devoid of saguaros. "Why aren't there saguaros here? Where are the organ pipe?" I asked Phil.

At the visitor center at Organ Pipe National Monument, my question was answered by a map dividing this part the Sonoran Desert into two dominant subdivisions: the Lower Colorado River Valley and the Sonoran Upland.

Where we stood looking at creosote bush and triangle leaf bursage, the topography, soil and climate demand vegetation that can withstand intense heat and drought. Columnar cacti such as saguaros and organ pipe grow better in the upland, or bajadas surrounding the low mountains, where the volcanic, gravelly soils hold water. The saguaros and organ pipe need the moisture that comes mainly in the summer "monsoons", intense rainstorms that cause brief local flooding. The cacti's shallow roots soak up water quickly before it disappears as runoff, and the expandable accordion- pleated trunk stores water for use in drought. Organ pipe cannot survive much farther north than the Monument because their thinner branches freeze more quickly than the saguaro's thick main trunk. If either plant undergoes a hard freeze, the ice bursts the cells of the plant, causing them to rot and die.


Where the organ pipe and saguaros cope with drought through succulence, other plants cope with the climate through drought tolerance or drought avoidance. Creosote leaves are tiny and covered with a waxy coating, enabling the plant to minimize water loss and avoid burning. Triangle leaf bursage and brittlebush have fuzzy, small leaves, also reducing heat buildup and water loss, and drop their leaves in drought.


Creosote bush Brittlebush Ocotillo Triangle Leaf Bursage


The Ocotillo plants had leaves and red flowers on them, a sign that there had been rain recently and it was spring. The branches of the ocotillo flower in March whether there has been rain or not. Hummingbirds and other pollinators depend on the ocotillo's sense of timing to provide food for them as they migrate back north. The leaves of the ocotillo are much more sensitive to weather; if it rains, leaves sprout within 48 hours. If no more rain follows, the leaves drop to reduce water loss. Ocotillos can grow on limestone, a rock made of calcium carbonate. Personally, I find calcium carbonate a nuisance. Phil had to use a jackhammer to punch through caliche (calcium carbonate) in our yard to let the tree roots grow, but here the ocotillos seem to find a way to live with calcium carbonate and other salts in the soil. One of the discoverers of Kartchner Caverns, Randy Tufts, said that he looked for a stand of ocotillos as an indicator of limestone bedrock, which can harbor unexplored caves.


Where saguaros and organ pipe cope with the desert climate through succulence, and creosote, triangle leaf bursage and ocotillo cope through drought tolerance, wildflowers are perfect examples of drought avoidance. They simply live through the hot part of the year as seeds. When the winter rains come, along with cooler weather, they sprout. Heavy winter rains will bring on a spring show of intense wildflower color; lighter rains will result in a more reserved show, but there will still be color along hillsides and the sides of the highway, where rain runoff provides more water. The flowers grow quickly, produce millions of seeds which are dispersed widely, and then die as the summer heat approaches.




Desert Marigold Desert Chicory Globemallow Purple Mat, or Nama



Just north of Organ Pipe National Monument is the visitor center for the Cabeza Prieta Wildlife Refuge. The Refuge was established in 1939 for the conservation of natural wildlife resources. Here, U.S. Fish and Wildlife believes the population of desert bighorn sheep is the same as originally found in the mountains of Cabeza Prieta. Elsewhere in Arizona, human activities such as ranching and hunting have contributed to the decline of bighorn herds through destruction of habitat, introduction of disease by domestic livestock, and competition for water.

The desert bighorn is well adapted to the hot and dry mountains of southern Arizona. Their body temperature can safely fluctuate several degrees, and in times of drought they can find adequate water from food and rainwater collected in small temporary rock pools. Even after losing up to 30% of its body weight, a bighorn can recover quickly from dehydration after drinking water. Bighorn have padded hooves that enable them to scramble up steep, rocky terrain quickly to avoid predation by mountain lions, coyotes and bobcats. At the Desert Museum in Tucson, I recall seeing a newborn bighorn sheep get up and follow its mother up an incredibly steep slope only a few hours after being born. The bighorn's most impressive feature is the set of horns that grow on the heads of both sexes. The head of a fully grown ram, with a set of magnificent curling horns, can weigh more than thirty pounds. Both rams and ewes use their horns to break open cactus, which they eat, and for fighting.


The animals at the Cabeza Prieta Wildlife Refuge visitor center weren't going anywhere. They were mounted on walls and in dioramas. Phil and I have seen few live animals on our trips, and those we have seen refuse to stop and pose for photos. Here we snapped pictures of the animals in their frozen poses and read that in the desert, many animals come out at night, in the early morning, or late evening, after we have put away our cameras.

The Sonoran Pronghorn is an endangered species that is neither goat nor antelope, but an ancient species found only in North America. Its family dates back 20 million years. Today there are few small bands remaining, and those are mostly in Mexico. Unlike the bighorn sheep, pronghorn run over open terrain. They can attain speeds up to 70 miles per hour for short bursts and maintain a speed of 45 miles per hour. Most of the time they can run from their predators, but if the terrain or presence of young forces them to fight, they can drive off even a coyote with their hooves. Pronghorn browse on sagebrush, rabbitbrush, grasses and even cacti, and seem to need little water. They have hollow outer hairs that insulate against cold or can ruffle up to help disperse heat during the day.
The ringtail is an active mouser by night, pouncing on and devouring its prey. Miners placed them mines to keep the rodent population down; in addition to hunting mice, the ringtail eats rats, rabbits, grasshoppers, crickets, small birds, spiders, scorpions, snakes, lizards, toads, frogs and fruit. They live in dry, rocky terrain, under boulders or in canyons, trees, mine shafts, caves, or even in buildings. These are versatile animals well suited to the desert and undisturbed by human encroachment on their territory.


The Desert Tortoise hardly ever burrows in the Sonoran desert, where the soil is often too hard, dry, and rocky to dig. Farther north, in the alluvial fans or sandy soils in the Mojave desert, it will dig a half-moon shaped burrow. Here it finds refuge in rocky crevices or sometimes mammal burrows. It escapes the worst of the temperature extremes by going dormant (called estivation in summer, hibernation in winter). It gets almost all of its water from the plants it eats (flowers, cactus fruits, and grasses). It has a large bladder that can store water and nitrogenous wastes, and when water is available after a rain, it will drink a lot and excrete the wastes.

Reptiles, including 24 species of snakes, live in the Cabeza Prieta Wildlife Refuge. They depend on the desert warmth for their own body heat, and are not even active until temperatures reach 65 degrees. Six kinds of rattlesnakes live here. The western diamondback is the one I have seen in the wild. There is also the sidewinder, which moves in its distinctive sideways motion. My son, who is an active member of Southern Arizona Search and Rescue, says the rattlesnake with the worst venom is the Mojave. Its venom contains both a neurotoxin and a hemotoxin, and victims require prompt and sophisticated medical treatment to minimize permanent damage to muscle and nerves. The rattlesnake doesn't particularly want to bite humans; it doesn't want to waste its venom on something too big to eat. Rattlesnakes usually reserve their venom for rodents, birds, lizards, toads and rabbits.
Although Phil and I would not survive long in the desert without our air-conditioned car, ice-cold drinks, well-marked trails and maps, the Cabeza Prieta Wildlife Refuge is a sanctuary for 300 kinds of wildlife and 391 plant species that do just fine. Plants and animals that thrive here might not do well in a cooler, wetter climate. They are adapted to the heat and drought and depend on each other to maintain a balanced ecosystem.

Over 90 percent of the refuge was declared as wilderness by the 1990 Arizona Wilderness Act. Visitors must take nothing but photographs and leave nothing but footprints -- and those must be on the designated paths.

American Kestrel Gambel's quail badger Giant Hairy Scorpion



Quitobaquito Spring, a rare water source hawk Horned Lizard screech owl


Ocotillo Blossom Desert Flood summer monsoon Arizona poppies

All pictures were taken at Organ Pipe National Monument, Cabeza Prieta Wildlife Refuge, and in their visitor centers.
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