Life lines in the sand: after winter rain, arroyos become the desert's green arteries

Natural History, March, 2002 by Peter J. Marchand

The people of the Sonoran Desert have a name for winter rains. Las equipatas, they call them. Unlike the violent thunderstorms of summer, which squander water in widely scattered torrents, these gentler rains come in a procession of "little packages" from December to March and soak into the land, swelling every desert pore with liquid life. In parts of the southern desert, las equipatas may drop only an inch or two of rain all winter, but in a wonderful collaboration with arroyos--the normally dry streambeds, or washes, that concentrate runoff as they fan across the desert--temporary relief is brought to the parched land.

Now, in early spring, the winter rains are abating. As ground moisture is pulled back into the dry atmosphere, ephemeral wildflowers slowly fade from the upland slopes, signaling harder times to come. Vibrant strands of green, however, continue to mark the course of the dry washes through the desert. Occasionally the thin lines of foliage broaden into oases of denser vegetation where a depression in the underlying bedrock traps water a little longer or where a rare groundwater seep moistens the sand beneath the surface. As I gaze across southern California's Colorado Desert (named after the Colorado River), one of the most arid regions on the continent, I am struck by how big a difference even a few extra days' worth of moisture makes along the arroyos.

Arroyos are estimated to occupy less than 5 percent of the desert landscape in the lower Colorado watershed of Arizona and California, but this figure belies the ecological significance of the usually dry stream channels. Plants such as desert willow, chuparosa, blue palo verde, and smoke trees are found almost exclusively along these washes. The smoke trees--mere wisps of gray-green that look as if they could dissipate into the air at any moment--are so well adapted to this special habitat that they are rarely found beyond the gravelly arroyo bottoms. Only by tumbling downstream during occasional flooding is the hard seed coat of this species abraded sufficiently to allow its germination.

Wherever there are trees in the desert, there is bound to be a concentration of animal activity. Ninety percent of the region's birdlife is found in this linear habitat, including the tiny verdin, with its distinctive spherical nest, and the stately phainopepla, a flycatcher that thrives on the berries of parasitic mistletoe growing on ironweed and mesquite trees. Dimples and scratches in the sand also reveal the presence of numerous small mammals, including kangaroo rats, ground squirrels, pocket mice, and wood rats. Tracks of kit foxes and coyotes commonly crisscross the wash bottoms; the hoof marks of collared peccaries are often stamped over everything. (Collared peccaries, or javelinas, may be the agents most responsible for moving leguminous tree seeds back upstream. In one study, these omnivores were found to spend nearly half their time in the vicinity of arroyos where, during the summer months, almost a quarter of their diet consisted of pods from palo verde, mesquite, and other trees.)

In my desert explorations I have sat stone still in the early morning as a bobcat stalked past me down the center of a wash, alternately melting into and materializing out of the dappled shade of palo verde trees. I have encountered Gila monsters working the shoulders of the arroyos in spring, searching in their lumbering way for quail eggs or the nests of burrowing mammals. These predatory lizards take advantage of other animals' nesting time to store up fat in their robust tails. The energy will see the Gila monsters through the rest of the year, during which time they are relatively inactive. Judging by the thickness of their tails, some of the lizards find the arroyo banks fertile ground. Twice I have stepped over western diamondback rattlesnakes that were waiting under the cover of overhanging rocks to ambush small prey. One night not long ago, I intercepted a mountain lion in a dry wash. I had been surveying bats, and in the gleam of my light I spotted the big cat's green eyeshine moving away from me.

Arroyos have long played a role in both the natural and cultural history of the desert Southwest. Most arroyos in the lower Colorado Desert carve their way south into Mexico, but they are not simply one-way conduits sending runoff now and then across the border. In the months, or sometimes even years, between flows, a silent countercurrent moves in the opposite direction. Along the washes of Organ Pipe Cactus National Monument, as well as in other arroyos, Sapium biloculare, the Mexican jumping bean, fingers its way northward into the United States, accompanied by a handful of other species, including the tiger rattlesnake. And plants and animals are not all.

One night I camped on the edge of an arroyo near Arizona's southern border. The moon was past full, but still bright in the hours after midnight. Restless, I turned over in my sleeping bag at one point and opened my eyes just in time to see the shadow of a man moving past me, headed north. All along the dry streambeds of this desert, ancient petroglyphs chipped into the rocks tell of the arroyos' frequent use in the past by humans on the move, and little has changed. In an uncharted desert, arroyos are the roads--critical dispersal corridors for plants, animals, and humans.