Rain in the Sonoran desert feels different depending on the season. Summer monsoon rains are violent events bringing thunder and downpours. Winter rains are more peaceful events, bringing cool air, low clouds, and steady drizzle. And for the rest of the year, there is no rain at all. Seasons have a very different rhythm here.
When a large winter storm passed through Tucson a few weeks ago, it felt like the world was changed. Dark grey skies, the smell of water and creosote on the air, and the sound of raindrops on tin roofs and hard rocks are some of the joys particular to this part of the world.
When there is rain in the mountains, the runoff flows into our canyons and washes. Because many of these surfaces are relatively impermeable, often with bedrock bottoms, the result is a series of flash floods – one of my favorite parts of desert life.
I went with a friend on an exploration of Bear Canyon in the front range of the Catalina Mountains. Normally this is a quiet canyon with seasonal flows, usually smooth and clear, no deeper than a few inches. Here you can see the canyon in spring – part of a Sierra Club ICO trip.
Here is what the canyon looked like after that winter rain. We made several crossings of the canyon in its safer and wider sections, but the water was swift and more than hip deep. Bouldery sections created long whitewater wave trains, and larger bedrock sections become fast-flowing flumes and waterfalls.
Further down the valley, where Bear Canyon meets Sabino Canyon, the scene was similar. Waters were high above riparian mesquite and sycamore trees, flooding a wide expanse of normally sandy and quiet terrain.
The United States Geological Survey monitors many streams, including this one. Today, as I write this, the flow rate is only 2.4 cfs (cubic feet of water per second). The day I took the previous photos, the flow was 1290 cfs – almost a factor of 600 higher. This flood wasn’t even a very large one – it was produced by only approximately two inches of rain – fairly average for a winter storm.
Rarer much larger storms can raise flow rates by factors of 100,000 – these floods are the ones that transport immense quantities of sediment, scour and downcut further the canyons, and disperse animals and plants to entirely new habitats. One of these days I hope to see one of these landscape-transforming flows.
The day after this storm, sun returned to the desert; two days later, flow rates were nearly back to baseline; by midweek, the rivers were dry again. Just another day in the desert.
The Sierra Ancha Wilderness is a rugged landscape of deep canyons, and seemingly a difficult place to live. Yet for two centuries this area was inhabited by the Salado people, a group potentially related to the northern Pueblo culture that populated the wider area. Floodplain agriculture and mesa hunting provided resources, while populations were concentrated in stone pueblos and cliff dwellings. These people and their culture began to fragment in the early 1300s and disappeared completely by 1450, leaving behind traces of their life only in archaeological evidence such as ceramics and charcoal and the occasional stone structure.
Last weekend I went in search of some of these abandoned cliff dwellings. Some are well-protected and easily visited, but others keep their secrets close. Our long and dusty journey into the mountains down a dirt road brought us to an uncrossable stream, and we set off on foot to explore some of the canyons you see in the background of this photograph.
Scrambling up these steep canyons, I began to wonder at the logistics inherent in a cliff lifestyle. Unreliable water sources, cliff-side traverses and loose slopes, and multi-mile walks to agricultural floodplains. Why live in such an inhospitable and difficult-to-access place?
We climbed up through bedrock and cliff ledges. The cliff dwellings remained hidden, well protected by the canyon’s sharp turns and sheer sides.
And then one appeared. On the north side of the canyon, a set of stone and mud structures appeared, perched underneath a small indentation in the rock where two stratigraphic layers came together.
Crossing the canyon directly was impossible – a deep chasm with sheer cliffs separated us. We instead traversed the ledge further up the canyon, behind a waterfall, and then made a final scramble up.
The buildings was of stone and mud, with large wooden beams used as roof and floor supports. Here you can see a 700-year old handprint preserved in the mud, indicating the work-intensive method of construction.
Another nearby canyon, much narrower, held an even more impressive surprise. The approach was through the bedrock of a stream and up slippery manzanita-choked slopes. A final bend in the canyon revealed a fortress-like structure, balanced carefully on a narrow ledge.
The logistics of construction seemed nearly impossible. Long journeys would have been needed to carry rock and mud from the floodplain below, and the long pine timbers used as cross-beams and floors for these multi-level structures would have had to been carried from elevations thousands of feet higher on the upland mesas.
Defending these places would have been a simple task. The dwellings had a wide view of the canyon and no access routes except along a single narrow ledge. Nearby seeps could have provided water, and the cliff overhang prevented access from above.
The beams used in construction date somewhere between 1280 and 1350 A.D., established via tree-ring methods led by the archaeologist Emil Haury at the University of Arizona. His 1934 investigation into this region (The Canyon Creek Ruin and the cliff dwellings of the Sierra Ancha) remains one of the most in-depth studies of the region, involving long field expeditions under conditions far more challenging than we experienced. This Arizona State Museum publication records more recent information.
So what happened to the Salado people? No one is completely sure, but it seems likely that a series of extreme climate events (several long and prolonged droughts) in the early 1300s made their lifestyle inviable (e.g. Waters et al., Graves et al.). Perhaps the cost of acquiring resources to support a complex civilization outweighed the costs, as Joseph Tainter had earlier suggested in The Collapse of Complex Societies. In the end, all that remain to use are ruins and mysteries.
A few miles brought us back to our car, and an easy gasoline-powered trip out of the desert and back home. On the trip back out, I began thinking about what these silent places meant to me. And I think they felt like a warning to us – and to our resource-intensive lifestyle in the contemporary southwest.
In the first photograph of this post (actually taken on the way home), you may have noticed a piece of paper left on the car windshield. Someone else was apparently thinking the same thing about the ease of travel and life in the desert.
Only in Arizona.
Most of the country is experiencing a snowy winter, but it is hard to imagine that cold here in the Sonoran desert. Our latitude and placement relative to the Pacific Ocean give us the climate of a typical midlatitude desert. Life thrives in the winter, especially in the canyons where the winter precipitation concentrates as runoff. Here are a few quick shots from the Aravaipa Canyon Wilderness.
A javelina (Pecari tajacu) crossing the water.
Cottonwood trees (Populus fremontii) caught in late afternoon light – leafless, but very much alive.
Saguaros (Carnegiea gigantea) and sycamore (Platanus wrightii) in a canyon bottom.
Insects (unknown species) mating in a late afternoon sunbeam.
The last light of the day catches the canyon walls and is reflected on the water rushing through this wilderness. There is no snow, and that is just fine.
The landscape is a harsh volcanic one, dominated primarily by lava flows and ash deposition. The rocks are mostly basalts, tuffs, and rhyolites.
Narrow canyons are incised into these planes. The canyons are where the mountains’ secrets are kept.
Descending through these broken landscapes across boulders and woody debris, the bedrock of the canyon bottom can be hard to find.
But occasionally bedrock sections do appear, and in these impermeable places water can be reliably found. At least, when there has been rain. When there has not been rain, the water slowly evaporates away. And the organisms that have purposed it as habitat and home begin to die. The landscape becomes alien.
This was once a floating mat of green algae, a sort of floating plant. As the water began to disappear the algae began to fold itself over the rocks, and soon enough was bleached by the sun. The only remnant of earlier life is this ghost.
The dead algae had a fibrous texture, and would rip and tear just like a rough paper. Green algae produce a range of polysaccharides to form their cell walls, including cellulose – a key component of modern papers. Interestingly, most paper is produced from wood, but wood comes only from vascular plants (which evolved from green algae) and contains lignin, a biopolymer that is typically removed chemically in high-quality paper to prevent the degradation that occurs for example in news-paper. No surprise, then, that this algal mat had such fine texture to it. The gold of the Superstitions will remain lost, but I was glad enough to find this ghostlike secret here.
In red rock country, the bottom of a canyon can be a difficult place for a plant to live. Why?
Here you can see the depths of Boynton Canyon, where ponderosa pine (Pinus ponderosa) and blue spruce (Picea pungens) must grow tall and straight to survive. Growing deep in a canyon restricts the hours of direct sunlight to a scarce few – perhaps too few to support enough photosynthesis to offset respiration. On the other hand, canyons rarely endure major wildfires, preserving large populations of majestic trees that have grown up high enough to reach the sun.
And here, in Oak Creek Canyon, you can see the constant risk of drought. Canyons are often wet places, serving as the lowest points of large watersheds, but the vagaries of the canyon’s history of downcutting mean that many portions are exposed and dry. Here you can see a bigtooth maple (Acer grandidentatum) blindly seeking for water, with blind roots pushing through hidden fissures in the canyon walls.
Canyon life has other downsides, as well. Flash floods can erase centuries of growth as they scour canyon bottoms, and the narrow confines of the walls can prevent fertilization or dispersal of seeds and so reduce fitness. They are beautiful but harsh places, and I am glad to not be a plant as I walk through their hidden worlds.
My trip to México took me to the small town of Zapotitlán Salinas. The town and nearby hills were once home to Popoloca people, and served as an important trading center. Then the main resource was salt, but today the town exists primarily for the onyx and marble mining industries. It is also surrounded now by the Tehuacán-Cuicatlán Biosphere Reserve. I learned that the reserve provides some protection for the landscape, but that low wages for local workers and the dominance of mining interests mean that the landscape is far from being sustainably managed.
Even with these concerns, it is a beautiful place to see columnar cacti. Coming from Arizona, I am accustomed to landscapes dominated only by saguaros. Here the desert was shared by several species, which seemed very saguaro-like from far away, but not on a closer inspection.
This first species is a tetetzo (Neobuxbaumia tetetzo), and was most common in lower areas. It grew tall and straight, and had a pale green color that harshly reflected the sunlight.
This second species is a cardón blanco (Cephalocereus columna-trajani), and was more common on higher-elevation slopes. It seemed more curved and soft, and would sway softly in the wind. The heads were covered in long white spines that seemed almost like a soft mat of hair.
This third species is a quiotilla (Escontria chiotilla), and was most common at lower elevation near dry riverbeds. It would grow into massive multi-armed structures, with a beautiful dark green skin.
And this fourth species is a trick – it is a saguaro (Carnegiea gigantea) growing hundreds of miles north, in Tucson!
Cacti have converged many times on the stem-succulent, columnar growth form. It seems to be a very effective way to live in dry desert-like conditions. But not all cacti do it – some of the most basal species in the family (Pereskia spp.) are small leafy plants inhabiting very different environments. You can learn more in this Edwards & Donoghue (2006) paper. Or by simply walking among the beautiful and heavily-exploited landscapes of Tehuacán…
The first thing I noticed on my visit to Puebla was the dust. The dust comes from many sources. The fallow agricultural fields. The desert. The active Popocatepétl volcano. The two million inhabitants of the city and their automobiles. The air is windy and dry in the winter, so the valley sits constantly under a yellow-gray layer of air.
My friend Alicia, a Mexican ecologist, invited me on an adventure far above the dust. We were aiming to climb Volcán La Malinche (Matlalcuéyetl), which rises nearly 4,500 meters above sea level. (The mountain is named for a Nahua woman who became the interpreter of Hernán Cortés). The peak is an island in the sky, and an oasis of clean air and alpine biodiversity.
One of the most marvelous things about mountains is how rapidly their ecology changes as you ascend. This mountain switched from an oak/alder (Quercus / Alnus) dominated forest rapidly to one dominated by the beautiful oyamel (Abies religiosa).
As we reached 4,000 meters elevation, the firs dropped out and were replaced by pines (Pinus hartwegii) and the occasional juniper (Juniperus monticola). For someone recently-arrived from sea-level, this was a difficult climb, but one full of ecological magic.
Climbing up higher still, the woody plants were gone entirely, replaced by a grassland growing on the volcanic ash, dominated by Festuca and a few forbs.
Ice was common here, probably due to condensation from the clouds formed by orographic lift. Shadows trapped the ice where the sun did not reach.
Higher up yet, no plants remained at all. We found nothing more than a barren landscape of ash, rock, and lichen.
At the top, we found ourselves high above the clouds on a beautiful clear morning. In places like this, the concept of a sky island is most evident. We talk about mountains as islands, in the sense that they trap species on their ridges that cannot pass through the valleys below. Here you can see the summit of La Malinche, with its collapsed caldera in the foreground, and the stratovolcano Pico de Orizaba (5636 m) some eighty kilometers away. As you can imagine, the animals and plants and microbes living on each peak show strong genetic differentiation because of their geographic isolation.
I had never before seen such a clear illustration of sky islands, and would have stayed all day to enjoy the view.
But the clouds began to rise, and safety lay below, so we descended.
Back to the dust and the chaos of the city.