Although it has eroded and aggraded many times over the last few million years, the Kanab Creek arroyo (Utah) began its latest manifestation on a July afternoon in 1883.
From the Hard-To-Believe File: a little over one month ago I wrote about preparing my 1960s travel trailer for evacuation in Utah’s extreme heat. In the last week, I received many National Weather Service Flash Flood Emergency Alerts via cell phone. Jarring disaster tones screeching from my pocket are new; summer flash floods are not. On July 29, 1883, Kanab residents watched a flood rip, in one afternoon, the meandering, crop-level Kanab Creek a new home: the 90-foot-deep, cliff-walled arroyo we know today. This caused, ahem, a few problems for those who ate only what they could grow. Not only was most of their year’s crop (and its field) on course for Grand Canyon, but also there would henceforth be a bit of a problem irrigating whatever was left, given the crops were now perched 90-feet above the stream. Thus ensued hundreds of years of erecting dams, digging ditches, and cursing, well, whatever was and is cursed.
Tsegi Canyon (Navajo National Monument)
This cycle of arroyo cutting and filling has been repeated every few millennia depending on rain and whether sediments are washed away or build steadily over time. Rain is most likely tied, as in the 1883 storm, to El Nino events. There was another Southwestern erosional cycle ca. 1200-1300, which may have contributed in the same way to Puebloan abandonment. And if it doesn’t stop raining soon this year, I may be moving to the DRY Tortugas.
Last week a tropical storm’s swirling clouds uncoiled overhead like a galaxy’s spiral, the only evidence of their passing successive waves of heavy rain. This was a September rain, one that lingered and turned days to jackets and umbrellas, not the usual August afternoon monsoon of suddenly refreshed hikers. A winter storm when great storms hove off the heavy, grey-waved Pacific, desert skies mirror roiling oceans, and mountain ponderosa warm desert adobes aswirl in sea-cast winds.
But today the breeze blows warm, and rain falls from monsoons bubbling off overheated southern deserts. Mr. Harris, taken by the cancer long-ago, used to say that’s where the big storms came from, the big flash floods. Not the west, from out across the Great Basin where the Pine Valley Mountains’ drain a storm’s last waters, but from the south, “Down off The Sand,” he’d say, “Down off The Sand up to Pipe Spring, following the Mail Trail over from Kanab.” He said southern storms were big because they got caught by the canyon, circled around and came in the wrong way. Angry. The biggest floods he’d seen.
Calling its battalions
When I moved to Zion Canyon in the 1980s, afternoon monsoons and summer flash floods were clockwork reliable. Every day about 3 or 4 p.m. the day’s wadded-tissue clouds, inflated by desert-heated thermals and dead-headed by the Troposphere, burst with their accumulated moisture. Monsoons and floods were common in Mr. Harris’ day too, and in the late 1800s, but not so much in the 1990s – 2000s, although this summer has certainly been wet! Whyizat?
Zion sits precisely at the monsoon’s western extent, and when that boundary shifts, summer rains never come. Our monsoon-edge location also means more of our annual precipitation arrives in winter than summer. Canyonlands and Arches, with similar elevations and rainfall, are more centered in monsoon boundary and thus receive their annual precipitation about equally between summer and winter—a small difference with great effect.
Winter’s slow rains soak more deeply into the soil, benefiting deeply rooted plants such as shrubs and trees. Summer rains flash across the soil, aiding shallow-rooted species, forbs, for instance. Thus winter rains create different plant (and thus animal) communities than summer-shifted precipitation. Further, different plant species use water at different times. That way, one species or another will survive no matter when it rains. Ingenious.
But some plants use water only during certain seasons; if rains arrive at the wrong time, these plants ignore it. That’s one reason environments can differ: the seemingly inconsequential difference of when rain falls.
Now for the kicker: scientists anticipate that global climate change will not only shift (is shifting, has shifted) the monsoon boundary, but also has/is/will affect seasonal precipitation. The U.S. Geological Survey states a switch to a drier climate, particularly reduced winter rainfall, will reduce groundwater recharge, lessen perennial stream water, increase strong winds and dust storms, weaken biological soil crusts, reduce plant cover and change species composition, remobilize sand from stable dunes, and increase forest and range fires.
What will this mean for us humans besides having to dust more often? In Zion, reduced winter rainfall will shift vegetation boundaries; different species will invade and replace the Colorado Plateau species we’ve come to know and love. Economic plants such as ponderosa pine (lumber) will survive only in the smaller acreages available at higher elevations; piñon and juniper will die off or move to higher elevations, leaving lower elevations to heat-adapted species, creosote for example. Grasslands will fill not with native grasses, but with such invasive exotics as unpalatable cheatgrass (there goes the wildlife neighborhood). The lifestyle and economy we’ve based on the present regime will change—drastically.
With luck, there will be time for us to change with it—for ranchers to learn new skills, for farmers to switch to crops adapted to hotter, dryer summers. Less water might mean mass human movement away, just as it did for Anasazi and pioneer alike. The species Zion hoped to protect will be gone. It’s something managers have never confronted before: their park up and moving away. If we hope to protect a certain species, we may have to set aside lands now where we think that species might end up later. Bizarre to be sure, but also necessary if we hope to plan for the future. This is not idle speculation. Scientists who study butterflies are already predicting that with climate change, the monarch’s Mexican refuge will be too cold and wet for the species to survive the winter. Can you imagine a summer without monarchs? I don’t want to.
All these dire predictions remind me of pre–Hurricane Katrina warnings of what New Orleans would experience after being hit by a massive hurricane whipped to a frenzy by global warming: broken levees, flooded wards, stranded populations, decimated wetlands. The preceding forecast is the Southwestern version of Hurricane Katrina—a list of things that will change so slowly we won’t notice until they hit us unprepared and head-on.
A recent climate study predicts that at our current rate, atmospheric carbon dioxide concentrations will double from preindustrial levels by 2070, triple by 2120, and quadruple by 2160. The study predicts “profound transformations; some potentially beneficial, but many disruptive. Climate zones will shift hundreds of miles north.”
It’s true that the Zion we see today is just one data point on an evolving continuum. Zion has always changed; that’s the one constant. But now, we’re the agent and we’re moving too fast. At one time a large forest covered Zion and extended to Grand Canyon’s lowest level; as ancient climate changed, sotoo did the forest. The difference was that the transition happened slowly, giving plants and animals time to adapt. But when climate change happens quickly, well, ask the dinosaurs how well that worked.
It’s easy to feel powerless against such overwhelming forces. But the real take-home message is act now. The longer we wait, the worse the consequences.
The Occasional Chair 