As monsoon season approaches, my mind gravitates toward its waterways, the vast majority of which actually lack water for most of the year, and specifically our arroyos. The monsoons hold the alluring promise of precipitation, sometimes buckets of it. All of that water has to go somewhere. Some soaks into the ground, soils greedily sopping up a portion of the precipitation. More substantial storms overwhelm even the most absorbent soils, creating runoff. Given the paucity of perennial streams and rivers in our region – think Sonoita and Aravaipa creeks, as well as the Santa Cruz and San Pedro rivers – the vast majority of storm runoff funnels through our seemingly countless arroyos. Even though most of them only briefly harbor water, they hold great value to local wildlife.
The extra soil moisture afforded plants by ephemeral flows along arroyos often leads to dense thickets along their sides. These sometimes impregnable habitats in turn support a wide range of wildlife species. Birds often selectively nest in the protective embrace of the vegetation. They may do most of their foraging there as well, given that insects, other invertebrates and fruit- or seed-bearing plants are all crammed together along arroyos – nature’s strip malls. Nocturnal mammals, including kangaroo rats, desert cottontails and mountain lions, seek sustenance and cover along their stretches. Additionally, the compact arroyo habitat often has plant species not found away from its banks. These “arroyo-phile” plants in turn help support the heightened levels of biodiversity found along these normally dry waterways.
The wettest arroyos – those that may have flowing water for days after a flush of monsoon flooding – qualify as riparian habitat if they host on their banks species such as velvet ash, Arizona walnut, seepwillow and Arizona sycamore. Due to human-related water drawdowns, these types of arroyos are increasingly rare.
Some of them, in fact, are the remnants of perennial streams that flowed sometime within the last few centuries or even decades – a stark reminder to all of us to safeguard our precious water.
Most arroyos, however, are fringed by more arid-adapted plants that grow more densely on their edges than in the surrounding habitats. In the foothills of our taller mountains, these xerophile plants may be joined by Emory and Mexican blue oaks, as well as several species of juniper. In the lowest elevations of the Sky Islands, such as in Saguaro National Park, desert ironwood, velvet mesquite, a few palo verde species, catclaw acacia and a variety of dense shrubs such as wolfberries can predominate in this linear habitat.
Here at Raven’s Nest Nature Sanctuary – situated at 4,000 feet elevation and hence in between the higher oak-juniper arroyos and the lower desert ones – we are fortunate to have a diverse array of arroyos that are true cornerstones of biodiversity. Our main arroyo averages about 50 feet wide. Winding its way through the preserve, its seasonal waters head west towards the Santa Cruz River. Its banks host mature velvet mesquites and catclaw acacias, as well as the occasional netleaf hackberry, all forming a sometimes dense overstory. Beneath them they shelter spiny graythorn shrubs and catclaw mimosa bushes. Summer witnesses a flourishing of plants, with twinevine, morning glories and wild cucumber all trellising up the woody vegetation. The result creates a seasonal “jungle” look along the arroyos. At ground level a wide array of native grasses and wildflowers spring up throughout the year.
This riot of vegetation in turn supports a myriad of wildlife. Vermilion flycatchers, Lucy’s warblers, broad-billed hummingbirds and other birds nest in the arroyo woodland zone. When rarities such as elegant trogon, thick-billed kingbird and yellow-billed cuckoo show up here, it’s generally along an arroyo. Mammals such as Merriam’s kangaroo rats, four species of skunks, and bobcats not only forage in this zone, but they also often preferentially employ the open arroyo itself as an energy-efficient travel corridor. Why thrash through the brush when you can easily ambulate along the sandy expanse of the arroyo? When summer monsoons kick in, Sonoran Desert toads ponderously lumber in and near the arroyo, searching for feeding opportunities and ephemeral breeding pools.
Select reptile species also gravitate towards the more austere, open habitat of the arroyo itself. Both zebra-tailed and greater earless lizards are quicksilver speedsters, daring predators to catch them in the loose sand. Any greater roadrunner or coachwhip snake that takes them up on the offer may well see their potential meal mysteriously vanish. These lizards are supremely adept at pulling a slick disappearing act into the loose substrate, which they also use as a thermoregulatory tool – deeper sand is cooler on a scorching day.
Perhaps the epitome of the bio-drama hosted by arroyos is embodied in what I witnessed several years ago at Raven’s Nest. An October storm dumped about 2 inches of rain in our area, bringing the arroyos to life. They ran with a vengeance for hours. Surveying a smaller arroyo the next day, I came along a truly otherworldly scene. I discovered a substantial ice dam that had diverted the arroyo’s flow. Copious hail had somehow accrued in a pinch point, forming a frozen blockage about 8 feet wide and 3 feet deep! Nearby lay the corpses of a kangaroo rat, a spadefoot toad and a large diamondback rattlesnake – all overwhelmed by the flow.
The normally still countenance of arroyos – always givers of life and sometimes purveyors of death – belies their role as cornerstones of our biodiversity. This is why I implore people never to drive in arroyos. Some may look like natural roads, but they are decidedly not. They need to be left to their own devices. Try walking in one instead, discovering their many treasures along the way.
Vincent Pinto and his wife, Claudia, run RAVENS-WAY WILD JOURNEYS, a nature adventure and conservation organization devoted to protecting and promoting the unique biodiversity of the Sky Islands region. RWWJ offers a wide variety of private, custom-made courses, birding & biodiversity tours. Visit ravensnatureschool.org.