Coral Reefs of the Desert

CORAL REEFS OF THE DESERT ARROYOS

Text by Tom Dollar Photographs by Jack W. Dykinga Southern Arizona maps depict them as spidery lines webbing through basin and range terrain shaded blue where they descend mountain slopes, pale green in the valleys where they run dry. We call them washes, dry washes, or arroyos. The ones that used to carry water part of the year we still call creeks or rillitos. Desert biologists call these habitats xeroriparian, combining Greek and Latin words meaning dry bank or shore.

Where local roadways cross a wash, signs read DIP or warn DO NOT ENTER WHEN FLOODED. It is wise to heed such warnings, for through these channels a rare summer monsoon thunderstorm can drive a flash-flood torrent powerful enough to capsize a 7,000-pound fourwheel-drive pickup-camper rig and tumble it downstream like a pebble. Despite this, an arroyo is one of the least noticed features of the desert landscape. No other ecosystem, however, is as important to sustaining life in the desert as a dry wash.

Indeed, in the driest places arroyos are as essential to the survival of desert plants and animals as are coral reefs to the survival of marine life.

Picture a dry wash as a biologically topsy-turvy version of a coral reef. On reefs, many of the animals are immobile. Corals, sponges, sea anemones some of them looking very much like plants are fixed in place and depend on currents of warm water to steer nourishment in the form of microscopic plankton within reach. Without these nurturing currents, the animals would die, reef-building itself would cease, and a unique marine habitat would vanish.

A dry wash works the other way around. Now familiar plants are stationary, relying on intermittent currents of nutrientrich runoff water for sustenance. Without water the plants would die, and with them a desert riparian habitat that supports freeroaming insects and spiders, reptiles and amphibians, birds and mammals all drawn to washes as fishes and invertebrates are to reefs, by the promise of food, protective cover, and breeding sites.

The term xeroriparian seems a contradiction to us, so locked are we into the notion that riparian means flowing rivers and streams. Arid-land scientists, however, make no distinction between the terms wetland and riparian. "Regardless of how...one defines the term, riparian areas are functionally wetlands," say University of Arizona biologists Roy Johnson and Charles H. Lowe. "They are supported by inflowing water, be it perennial, intermittent, or ephemeral."

It is the presence of water, however scant, that makes a dry wash a biological reef. Everything that happens in a wash is a function of water. Because soils deposited in wash channels by running water are more porous than upland ground, seeds germinate there more easily. Water also helps to disperse seeds, transporting them downstream, mixing and grinding them over sand, rock, and gravel to nick and gouge their tough seed coats and make them sprout.

ARROYOS

Paloverde, mesquite, acacia, and ironwood trees, the so-called “big four” of dry washes all legumes with small pea-like flowers and seed pods have adapted remarkably to desert conditions. One adaptation is a fast-growing taproot that begins to thrust muscularly to the water table beneath the parched surface as soon as the seedling sprouts. By the time a paloverde seedling is a foot tall, its taproot already may have penetrated to a depth of a yard, three times its height. At maturity the velvet mesquite may have plunged its taproot 75 feet into the earth. In terrific heat and drought, when temperatures stick at 110° F. for days on end and whole seasons pass without rain, these trees continue photosynthesis, even without leaves. The blue paloverde, for example, converts sunlight to energy through its blue-green bark, growing leaves only in spring, or sometimes after a rain, but dropping them as soon as drought returns.

Paloverde, mesquite, acacia, and ironwood are the master links on the chain of life in a dry wash ecosystem. Although all the dry wash big four can grow on the uplands, in bone-dry desert regions they grow only in wash beds, as starfish, sea fans, barnacles, and sea urchins prefer reefs to the open ocean.

In the Yuma Desert or the Cabeza Prieta National Wildlife Refuge in the southwest corner of Arizona, where annual rainfall averages scarcely three inches, the giant columnar cactus called the saguaro seems also to “prefer” life in washes over the uplands, although it can grow in both environments. A whole community of birds depends on the saguaro: red-tailed hawks, great horned owls, and roadrunners nest in crotches formed by saguaro arms; Gila woodpeckers and flickers drill nest holes in the trunks and arms; and American kestrels, Western screech owls, elf owls, ferruginous owls, ash-throated flycatchers, and others move into the abandoned holes. By necessity all of these birds have become wash dwellers.

Wherever climate is extremely dry and hot, birds and mammals live almost exclusively in dry washes. A bird-count by biologists Roy Johnson and Lois Haight discovered that arroyos support bird populations five to ten times greater than in surrounding deserts. Another study found that nearly all of the mule deer spotted in the King Valley northeast of Yuma were in washes, while the creosote-bursage flats were almost empty of deer. Bees, cactus wrens, javelina, antelopejackrabbits, desert tortoises, coyotes, banner-tailed kangaroo rats, banded gecko lizards, and other creatures are drawn to desert washes much as marine animals move onto coral reefs for forage or shelter. Deer and other large mammals browse on twigs and pods; butterflies and bees harvest nectar; rodents and birds search for seeds. At other points along the food chain, carnivores like rattlesnakes, redtailed hawks, and tarantula wasps zero in on washes because that's where their prey congregates.

Early people, too, clustered their encampments around wash sites, preferring places where depressions worn in bedrock, called tinajas, captured water. They set up their community gristmills there, grinding grains and mesquite seeds in metates. Evolving in harmony with dry places, early humans learned how to forage and hunt these inland reefs, how to find plant materials to make weapons, utensils, baskets, and medicines, and how to redirect the meager flow of water from wash channels onto their cultivated fields. Until the coming of the groundwater pump in the 1930s, in fact, the Tohono O'odham had for centuries depended entirely on infrequent rains to bring water to wash channels where they redirected it with brush weirs onto nearby crops.

Our lives don't depend on washes anymore. Or so we think. Unless we remindourselves of what we have lost to freeways and strip malls. Remind ourselves of the parkways and greenbelts we could have developed, for instance, along a network of wash channels as they flowed out of the mountains into our cities. Places supporting not only the occasional flow of water but also the flow of life, so that in our backyards we might keep company with deer, javelina, and coyote as they moved among us through halls of green. Places providing cover for birds and small mammals; places for wildflowers and trees, shade and quiet physical and psychological buffers against steel and bricks, pavement and exhaust fumes, noise and haste.

Instead we interrupt our wash channels or line them with concrete trapezoids. We alter watershed dynamics by paving more roads and parking lots and building more houses with pitched roofs, forcing a greater volume of water into fewer channels, overloading them. The result is that the water supply squirts away from us without recharging our aquifers. We need to hold on to our washes, or desert reefs. They may be our lifeline.