A line in the Sand: Introduction
A four part series on the plants, animals, land, and water of Arizona’s San Pedro River.

Introduction | The Animals | The Water | The Plants | The Land

Introduction
The San Pedro River in Southeastern Arizona is something of an experimental desert petri dish: combine water, warmth, and copious amounts of sunlight and see what happens. Among the results are 400 species of birds (fully half of all birds migrating between North and South America), 84 species of mammals, 41 species of reptiles and amphibians, and even a handful of fish. In short, one of the greatest profusions of desert life in North America.

But for all its life today, the river and surrounding ecosystem used to be far richer – a small yet emblematic part of a network of Southwest riparian habitats that has been reduced by almost 80 percent in the last century. While the San Pedro is still one of the only desert rivers to flow completely free of dams, and is the only river in Arizona to cross the U.S./Mexico border, its flow has been reduced and in some placed altogether stopped by a growing and very thirsty human population. With the water went many species of plants and animals. But for every glaring change there are a handful of subtle and arguably more insidious changes that are rising slowly like a tide—not of water but of invasive species, new floral regimes, and an international border; tying these many issues together is the largely invisible overtaxing of groundwater.

In 1988 the Bureau of Land Management (BLM) established the 56,000-acre San Pedro Riparian National Conservation Area (SPRNCA along 45 miles of the upper San Pedro. That designation has helped, yet much of the damage was done long before the river was protected. While local Native groups had some impact with their limited hunting and gathering, burning, and farming as far back as 10,000 years, the impacts did not start to add up until the Spanish arrived with livestock in the 16th century. Cattle increased along the river through the centuries, but the human population did not catch up until the late 1890s, when commercial mining and farming came to the valley. The human population grew through the 20th century, particularly after the 1950s as Fort Huachuca, a U.S. Military intelligence base, ballooned with personnel and their families in nearby Sierra Vista.

I spent this last summer attempting to understand the river on a personal level and to separate doom and gloom speculation from the reality of the ecosystem. I hiked the river in the heat and dark, in thunderstorms and under blue skies. I trudged through coarse, blistering sand littered with cow dung and discarded water bottles, and through slowly eddying emerald pools bristling with reeds and rushes and swarming with a phantasmagoria of life. I found the ecosystem to be fantastically beautiful and eminently fragile; the worst kinds of destruction were taking place out of plain view in the replacement of specialized native species with generalized newcomers and in the very complex dynamics of the river’s water systems. In the end I found that by simply walking the river every day, not effecting any real change beyond my footprints in the sand, I was slowly weaving myself into the fabric of the land. It changing me, I know, and I hope I have not much changed it.

Part One – The Animals
It was July in Southeastern Arizona and the heat was truly staggering. Waves of the stuff rippled off the gravel as my wife and I made our way down a well-trodden trail toward the San Pedro River. A dried-up lizard moved across the trail, borne by a mass of ants, while a bedraggled dove gaped from the desiccated branch of a lone mesquite tree. Cryptic grasshoppers leapt from under our feet and hurled themselves headlong into the dry, neck-high grass. There was no question that we were in the desert, yet just in front of us was a verdant wall of greenery that exhaled a cool breeze and drew us in with its promise of water and life.

We were looking for the fabled architect of the San Pedro River, an accomplished builder and unlikely steward of riparian areas who, incidentally, is also the world’s second largest rodent. The beaver is second only to man in its ability to alter the environment, but while no man-made dam to date has helped an ecosystem, beaver dams actually create ecosystems. Industrious beaver built the river ecosystem over the millennia; then the usual suspect of Southwest riparian degradation – people and their cattle – managed to tear much it down in just a few short centuries. We saw evidence of this as we hiked along eroded riverbanks and in the closely cropped grasses just outside SPRNCA land. We had seen the river as its been shaped by people and livestock, but we were eager to catch a glimpse of the river as it used to be, when it was still managed by beavers rather than by people.

The beaver is as unconscious of its beneficial habits as people are of their harmful habits. Beaver fell trees with their sharp teeth to get at the nutritious cambium layer bellow the bark and use the trunk and branches for building their homes and dams. By damming a river, beaver create the safe aquatic environment they need to stay clear of predators and also stimulate the growth of the plants they feed on. Unbeknownst to the beaver, the broader riparian ecosystem benefits enormously. When a river is dammed in this way, the water table rises and the soils, sometimes for miles around, are saturated, allowing countless plants and the immense network of animals that rely on them, to thrive. Beaver dams also slow the flow of a river, tempering the wrath of floods, keeping water in the system longer, and allowing more water to seep into underground aquifers. This groundwater is what is responsible for keeping the San Pedro and other rivers on the surface. When beaver are eliminated, the water table drops and rivers go underground. The additional pressure of local human populations extracting water from underground aquifers brings the future health of deserts rivers like the San Pedro into serious question.

The fate of the beaver, and of that riparian ecosystem, were all but sealed when in the mid-1600s Spaniards began to pour into the Southwest with goats, sheep, and cattle. At that time the San Pedro flowed more or less continuously 150 miles from Mexico north to its confluence with the Gila River. Early explorers and settlers reported that the river was blocked every mile or so by beaver dams, creating a stepped river of vast and lush ponds and marshes. At that time the giant Fremont cottonwood, so emblematic of the river today, was comparatively rare, unable to thrive in the waterlogged soil. Instead, great bodies of standing water, or cienegas, supported grasses, sedges, willows, and rushes and attracted and sustained a huge amount of wildlife. Another sight that stunned early explorers was the lush green grass thriving on a pittance of precious topsoil, painstakingly created over the centuries. These grasslands, only barely interrupted by mesquite trees, carpeted the ground from river to mountain creating habitat for deer and pronghorn antelope as well as the predators that depended on them, such as jaguar, bobcat, mountain lion, and bear. At the center of this plethora of life was the beaver, so prevalent in 1825 that James Ohio Pattie, a trapper from Kansas, dubbed the San Pedro “Beaver River.”

As it turns out, the Spaniards were some of the last people to see those valleys of green, for within a generation cattle had grazed the grass to nubs and the thin skin of topsoil had been baked, trampled, and had blown away. The number of cattle around the river exploded from 68,000 in 1883 to 156,000 three years later. The dry, hard, grassless soil was compacted by hooves and effectively became a rain slicker for the earth, shedding the lifeblood of the desert – the summer monsoon rains. Rainwater rushed off the land and into the San Pedro, eroding the landscape, washing out beaver dams and flooding the river. Simultaneously trappers hunted beaver and settlers destroyed dams with dynamite in an effort to eliminate the breeding ground for malarial mosquitoes. By 1900 all but one cienega were gone, the San Pedro River channel had been eroded to 40 feet deep in some areas, the river was dry in more places and for longer, and the San Pedro beaver was extinct. With the beaver and their dams went many of the amphibians, reptiles, insects, birds and, in particular, native fish. By 1950 all but two of as many as 13 native fish species were extinct in the San Pedro. Only the longfin dace and desert sucker persist today in isolated populations, fighting for space and food amidst 14 decidedly more tenacious exotic species.

Almost 100 years after the San Pedro beaver was extirpated, 15 were introduced by BLM between 1999 and 2003 to help rebuild the riparian ecosystem. Today there are more than 60 living in several family groups within the conservation area. A recent study by the University of Arizona has confirmed what ecologist have long suspected, that the presence of beaver is unequivocally linked to increased species richness in riparian ecosystems. For this reason beaver are what scientists call a “keystone” species, that is, the health of the broader ecosystem, to a large extent, hinges on them.

My wife and I finally made it to the wall of trees and were delivered from the desert through a leafy gate. The temperature dropped and we took our sunglasses off, admiring the sturdy trunks, artfully overlapping roots, and green grass along the path. Though it had rained for an hour the night before the river beside us was not more than 2 inches deep and 6 inches wide. One hundred and fifty years ago we would perhaps have been standing knee-deep in a cienega, listening to the sound of beaver territorially slapping their tales on the water. We easily stepped over the stream and started upriver, taking our sunglasses off, then putting them back on in the dappled light. As the miles passed, we wondered if the beaver was really doing as well as the BLM ranger had led us to believe. The Gooding willow and Fremont cottonwood gallery forest that had filled in around the river about the time the beaver was extirpated would certainly provide copious food and building materials, but a dam and food with no water means nothing to an aquatic rodent.

As we rounded the next bend our questions (and prayers) were answered in a small but exquisitely crafted beaver dam. It stretched from one bank to the other in a delicate arch of mud and sticks. We approached the monument to riparian health with an almost religious reverence as the air cooled and filed with the sounds of birds, frogs, and insects. Grey/green water cloaked in aquatic plants was backed up as far as we could see and barkless cottonwood trunks crisscrossed the pool at regular intervals. A rare Chiricahua leopard frog hummed underwater; overhead impossibly red vermillion flycatchers took turns snatching insects from a smoky band of sunlight. I slipped into the knee-deep water and waded from one riparian miracle to another. A mass of frog eggs clung to some reeds, the oblong wriggling black dots within even more amorphous than the tadpoles they would become. A spider almost as large as my hand rested on the surface of the water, eying a marauding pack of water striders. Minnows flitted about in the shallows, while lethargic turtles soaked up the sun and a hawk kept an eye on things from above. The dam and the life within was our holy grail, and we explored it with the reverence due a spectacle that few had seen in the past 100 years.

The river’s recovery does indeed need to be engineered. With the beaver replacing man as the foreman of that project, success seems much more likely. Though the resident beaver we desperately wanted to see were nowhere in sight, we were pleased to note that neither were cattle or other people.

Part Two – The Water
On my tattered map of Cochise County, torn from a book of Arizona maps, the San Pedro River appears as a crisp blue line making its way nonchalantly across the U.S./Mexico border. The line unwaveringly crosses tears and creases and divides the Mule and Huachuca Mountains, gratefully accepting a handful of incipient streams before merging its waters with those of the Gila and Colorado rivers; then it pours into the Sea of Cortez. As much as I wanted that authoritative blue line to be a reflection of reality, I knew it was not. The Colorado River did not gush into the Sea of Cortez at all, but feebly petered out somewhere in the desert from overuse. Likewise the San Pedro was more of a dotted line these days, diving underground for long stretches before tentatively reemerging, often as little more than a trickle.

Who or what had driven the San Pedro underground, and where had all that water gone? With this simple but loaded question rattling around in my head, I set out to learn a little more about rivers and water.

The first order of business was to have a look at the upper San Pedro, a place where water and international politics cross paths, and the nearest to the river’s headwaters I could get without a passport. After navigating a labyrinth of right-angle roads bisecting cattle land and modern ranchetes, I came to an abrupt stop at a formidable wall of NO TRESPASSING signs. Behind these was a much smaller sign identifying a narrow corridor of land managed by SPRNCA; a less-than-welcoming gap in the barbed-wire fence allowed access. Following game and ant trails through the grass and mesquite, I approached the river and the wall of trees that envelop it. After my eyes adjusted to the dim light of the cottonwood and willow gallery forest I looked around for the river. Dry wash after dry wash split the towering trees, but not even a hint of moisture could be seen in any of them. Overhead a few somber birds called and I counted four black butterflies silently plying the understory. I was a paltry 25 miles from the river’s source and the San Pedro was little more than a river of sand. In studying water one expects to have some contact with the stuff, but perhaps that was the first lesson: that the study of modern desert rivers is often the study of empty riverbeds.

Back at home I sat down at my desk with a phone, a stack of books, and an internet connection, determined to understand the river I had expected to see earlier that day. First, taking a philosophical approach, I tried to imagine water in all its simplicity. The decidedly anthropogenic vision of glasses and buckets of it popped into my mind but ignored the reality of the earth’s natural water systems, which were after all what I needed to understand. But understanding these systems is challenging since water exists in nature in more places than between the bottom of clouds and the surface of the earth, and in more forms that just liquid. Plants expel water into the atmosphere as vapor, the ground stores water hundreds and sometimes thousands of feet bellow the earth’s surface, and rivers transport water from one place to another, usually in plain sight. This delicate balance of water inflow and outflow is how the San Pedro managed to stay above ground before people settled the valley, and it is how the river came to support one of the richest riparian ecosystems in the Southwest.

To break down something as sacred as a desert river to a pie chart seems a little sacrilegious, but this tool of the business world is in fact the best way to understand the exceedingly complex natural water systems at work in the San Pedro watershed. At its most basic, a water system consists of water coming in and water going out – two full pies each neatly accounting for nearly every drop of inflow and outflow. This balance equals a healthy watershed and means that the life that depends on it can do just that: depend on it. But people, like most living things, adore water and very often find themselves near it, especially in the desert (my wife and I had planted our roots there and were as thirsty as anyone). Like other Southwest desert rivers the San Pedro has attracted people since the end of the last ice age, but especially in the last few centuries, starting with the Spaniards and their cattle up through today with Sierra Vista and its 43,000 people. New people mean more than extra mouths to water; they also mean industry and mines to hydrate, sewage to flush away, crops to water. All these needs must be served in addition to the system’s natural means of expelling water such as river outflow, evaporation and the process that bridges the gap between natural and artificial, a simple but powerful process I will get to later called evapotranspiration. These extra losses mean that the inflow pie has not changed, while the outflow pie has changed considerably—putting the system into the red.

The first layer of water system complexity is what I have come to call the savings account of a watershed, also know as the aquifer. It was the conspicuous absence of a healthy aquifer that I had seen manifest that day on the dry upper San Pedro. Virtually all the water coming into the San Pedro comes from rain during the summer monsoons, but instead of all this water leaving immediately on the currents of the river and through evaporation, the ground stores much of it in aquifers, then releases it slowly over the course of the entire year. This is how a river can flow in the desert months after the last rain of the year. These watershed savings accounts protect the system from drought and allow the plants and animals that have evolved to exist in the constant presence of water to thrive. The water is stored underground as deep as 1,300 feet, or just bellow the soil’s surface. When aquifers drop, the rivers that are literally riding their surface go underground, and that’s precisely what’s happened with the San Pedro.

Before large numbers of people arrived, the San Pedro valley had found an equilibrium with an average of 16 million gallons of water entering and leaving the system every day, or just under 5 billion gallons per year; that’s about 18 Sears towers full of water every year. But since the early 1900s human water use has gradually increased, topping out today at over 11 trillion gallons per year. That’s almost 2 Lake Powells sucked up every year. Of the total human use, 3.65 trillion gallons are used directly by people, with the remainder going to various types of industry. Because rainfall has changed very little over those same 100 years, the San Pedro valley water system has fallen into a deficit.

“As long as the system is in overdraft” said Donald Pool of the U.S. Geological Survey (USGS), “there is going to be negative affects on the river.” Simple math reveals that human use, primarily through tapping of the aquifer, has seriously overtaxed the system. The reason the deficit is not more serious is because some but not all of the water used returns to the system in the form of agricultural runoff, mining recharge, and effluent injection. The Center for Biological Diversity projects that the portion not returning to the system will reach 4.2 billion gallons annually by 2020.

The watershed’s supply first began to suffer at the hand of Spanish cattle ranchers and their beasts in the 18th and 19th centuries. It didn’t seriously escalate until the late 19th century, when the significant factors of population growth, mining, agriculture, industry, and Fort Huachuca combined to seriously tax the system. In the late 1880s large-scale mining in Bisbee and Tombstone began to extract ground water in entirely unprecedented volumes. Between 1 and 2 trillion gallons of groundwater were used each year between 1905 and 1987 in Bisbee’s Copper Queen Mine alone. That’s a total of 90 to 180 trillion gallons in all, about the volume of Lake Erie, the planet’s 12th-largest lake. Further taxing the watershed was Fort Huachuca, established outside present-day Sierra Vista in 1877.

Despite the enormous amount of water drawn from the ground for mining, though, the river remained reasonably healthy through the 1950s. Betty Foster Escapule remembers in her book The Five Fosters that the river only dried up once at the San Pedro House near Sierra Vista in the 20-plus years she lived there.

“In those days the river was wide and had lots of water in it,” she writes, “there were lots of frogs in the river and all summer the river was full of tadpoles.” Today the river where she once lived is rarely larger than a trickle, with tadpoles and fish relegated to pools predominantly created by the newly re-introduced beaver.

Through the 20th century Fort Huachuca used a great deal of water on-fort, but was also indirectly responsible for a great increase in use by fueling growth in nearby Sierra Vista. At the behest of conservation groups, the fort has recently cut on-fort water use dramatically to just a few billion gallons per year, with advanced recharge systems putting most of that water back into the ground.

“Fort Huachuca has become much more efficient and is now somewhat of a leader in the area of water conservation” said Pool of USGS. Randy Serraglio of the Arizona conservation group, Center for Biological Diversity (CBD), agrees.

“Fort Huachuca reduced its consumption by 50 percent,” he said. “It’s funny when long-haired environmentalists like me point to Fort Huachuca and say ‘I wish we had military discipline like that!’”

But for all the fort’s efforts, its regional spending has increased by almost 90 percent recently, further fueling growth and water consumption outside the base, where local government bureaucracy rather than military discipline holds sway. Benefitting from such spending are industrial users such as the defense contractors ManTech and Northrop Grumman; together, they and others use 7.35 trillion gallons of water annually in their manufacturing processes—a significant part of the 11 trillion total gallons used by people. Rick Renzi, a member of the family that owns ManTech and a Republican Arizona representative, was charged in 2008 with 35 counts of money-laundering, extortion, and conspiracy for his 2003 Fort Huachuca Preservation amendment (known as the Renzi Rider) and other abuses of power. The bill, supported by Arizona Senator John McCain, ensured that Fort Huachuca could not be closed and would instead grow. That benefitted ManTech, which held more than $1.5 billion in current and future contracts with the fort. “It’s not a matter of whether [the San Pedro] will dry up, it is when it will dry up,” McCain later told the Sierra Vista Herald, with little sense of irony and without mention of his support for the Renzi Rider.

Standing in the dry riverbed that afternoon, I looked through a spindly barbed wire fence into Mexico and toward the river’s headwaters. I could not see them, but envisioned clear springs gushing from solid rock, then flowing toward where I stood, life springing up as it went. But I was standing in a dry riverbed with shoes full of sand, and guessed that this was probably not the case. In Mexico, as in the rest of the watershed, water usage has increased in the last century and is far from sustainable. An increasing population and the Cananea Mine, which extracts copper from an open pit, are to blame. The mine’s water use spiked in the late 1980s to more than 4 trillion gallons per year, but has leveled off closer to 3 trillion gallons per year today, with a further 2 trillion gallons per year going to residents. If mining were stopped in Mexico, the San Pedro would have an estimated 20 percent more water on average—almost another Lake Powell.

“It’s a loosing battle,” said Alex Howe of BLM. “It’s very disheartening sometimes.”

But not all water losses are the result of human activity; before people arrived, natural losses were crucial to balance the system. When the beaver was extirpated in 1900 the watershed began to change in many unexpected ways. Unable to grow in the marshy conditions created by the beaver, the Fremont cottonwood proliferated in the dam builder’s absence. The mesquite trees of the surrounding grasslands also flourished as fires were suppressed and cows mowed down the tall native grasses, their primary competitor for light and nutrients.

While more trees may seem a boon for an arid and sun-baked part of the world, the greater number of trees actually exasperated the water shortage through a seemingly innocuous process called evapotranspiration. That’s the process whereby a tree begrudgingly releases a little water into the air as it “inhales” carbon dioxide through tiny pores on its leaves, called stomata. The deep roots of both cottonwood and mesquite allow them to draw huge amounts of water from the water table and expel it into the air. While a breathing tree may not seem like a significant source of water loss, consider that a single full-sized tree can transpire 40,000 gallons of water into the air every year—or 20,000 flushes of the toilet.

“Each tree is like a little water pump,” says Donald Pool. “Look at a big cottonwood: there are lots of leaves there, all evaporating water.” Pool estimates that evapotranspiration along the San Pedro River has increased from just under 7 million gallons to more than 12 million gallons per day, due to a 150 percent increase in both cottonwood and mesquite trees. That’s an extra 7 Olympic-sized swimming pools lost every day, in addition to natural losses and an ever-increasing human-caused portion of the outflow pie.

Historic photos show just a few cottonwood trees along the river, yet there I was standing in the cool embrace of an entire forest, trying to decide if I would swap it for a little water. After an hour of hiking in dry sand crisscrossed with logs from the last flood and littered with empty water bottles from the most recent wave of illegal aliens, I decided I would. As if responding to my pleas for a little water, a storm rumbled in the distance, and I emerged from the trees just in time to see three distinct and fast-moving storms merge over Mexico. Their water-laden bulk filled brilliantly with red and white light before each clap of thunder. As I returned the way I had come, blunt grey fingers of rain reached out of the undulating mass, as if to grab me, while the first tendrils of wind moved the treetops. Moments later a wall of wind slammed against my back and I stumbled forward through flailing grass. As I closed the car door, the first raindrops hit the windshield and ran downward through the powdery dust in steaks of red mud. I closed my eyes and prayed for a hard and long rain.

Part Three – The Plants
Imagine walking along Southeastern Arizona’s San Pedro River in 1400. You’re Native American and the new world has not yet been “discovered.” You pass through a few small villages each day that haven’t changed much in the past thousand years. The villagers hunt and gather and tend small irrigated fields along the river. Lone cottonwood and ash trees erupt from the bank every few miles, casting welcome patches of shade. Where a beaver dam or sharp bend in the river has slowed the current, lush green cienegas occupy acres of land. The higher bits of land support thorny and dense mesquite bosques. From the river to the mountains, sacaton and other native grasses continue in an unbroken expanse except where fires have recently burned; the grass in those areas is much richer.

Now imagine it’s 1700 and you’re a Spaniard running cattle along the river. Your herd is a small part of the 100,000 head grazing the valley. The small Native American farms are gone, their villages abandoned for missions. Much of the grass has been grazed off and the wind sends dust devils through the naked landscape. But the river is alive and doing reasonably well, trampled around the banks but running wide and deep in most places with abundant beaver, fish and aquatic plants.

It’s now 1900 and you’re a miner working in one of Tombstone’s 50 silver mines. Cottonwoods have multiplied along the river at the edge of town, making its path visible from afar for the first time. The rough old oaks that once surrounded both Tombstone and nearby Bisbee are gone, replaced by sun-blackened stumps. In the last 10 years the Tombstone mine used 50,000 cords of wood, enough to stretch 151 miles stacked 4 feet high. Until the drought of 1891-1893, a handful of ranches ran 1.5 million head of cattle along the river. More than three quarters are dead, but their bones still litter the landscape. The river flows quickly and is deeply channeled now that the beaver is extinct. Mesquite grow in patchy forests throughout the valley and the cowboys complain that they can’t ride through them. The rich green cienegas and deep pools are almost completely gone, but on the upside, there is no more malaria.

These days I don’t have to use historic photos and written accounts to understand what the San Pedro is doing; I just strap on my boots and go for a hike. And that’s just what I did this last Saturday under a burning blue mid-day Arizona sky. Leaving from the Fairbank bridge, I trudged up the dry riverbed, unable to rid my mind of a bound collection of black and white images of Southeast Arizona I had been pouring over for the past week called The Changing Mile. Even as the lush greenery rustled in the wind and the smell of unseen water met my nostrils in waves, I found myself focusing more on the lost landscape than on the one before me—the new one. A wall of cottonwood leaves where I would have once seen sky. Mesquites eroding off a cut bank 30 feet overhead where there used to be a cienega. Yet another arroyo where there used to be sacaton grass.

These changes are highly visible because they are affecting the plant community, which is the skin of the ecosystem – the surface which people see and judge. But the changing flora, almost as much as the shrinking river, is just the most obvious symptom of a far-reaching and mostly unseen ecological shift taking place along the San Pedro—a change that has been unconsciously engineered by people since before the Spaniards arrived in the 1500s, but has rapidly increased its pace since. The river has entered a new phase of evolution in which a select portion of the floral community, having been favored by the new prevailing conditions, has supplanted much of the river’s former diversity.

While the Native inhabitants of Southeastern Arizona burned grasslands as a hunting tool, consumed large amounts of cactus and changed the riparian ecosystem with small-scale farming, the landscape was minimally altered by their presence. But as the Spaniards and their livestock flowed over the land in the 16th, 17th, and 18th centuries, the floral community began to change radically. One of the most significant human-caused plant invasions was that of the mesquite into grasslands. The reasons for the spread of mesquite are complex but tied to factors as diverse as a greater amount of atmospheric C02, fewer fires, a decrease in rodents, and the presence of cattle. The complexity of this transformation is emblematic of many ecosystem shifts, all of which make it hard to understand what kinds of landscapes are natural and which have been engineered by humans.

Before cattle grazed the San Pedro valley, mesquite were found alone in grasslands or in small bosques along the river. But when cattle were turned loose on the landscape conditions changed; instead of being ideal for grasses they favored the mesquite. A cow’s insatiable apatite for both grass and mesquite beans simultaneously dispersed the tree’s seeds and gave them light. The average cow can eat about 167 pounds of grass per day and as many as 1,617 mesquite seeds have been found in a single cow pie. As a result, the mesquite gained a significant advantage. In addition, the cow’s digestive tract is ideally suited to scarify the seed’s shell, which allows them to sprout. As the seeds germinated and grew a reduced number of seed eaters (for example, the black-footed prairie dog was eradicated by the U.S. government in 1900) eased the transition from seed to tree.

An additional boon for the growing tree was the increased levels of atmospheric C02 (up 36 percent since pre-industrial times according to the National Oceanic and Atmospheric Administration), primarily caused by the burning of fossil fuels worldwide. While most grasses are actually harmed by increased levels of C02, woody plants such as mesquite and cottonwood benefit from it, gaining an increased ability to fix nitrogen and make use of available water. Furthermore, the fires that once raged across the valley, cleaning the mesquite out, were slowed by denuded grasslands and later all but stopped by people protecting their homes. As grasses were replaced by mesquite, cows ironically suffered from a lack of food. But so too did the many rodent species that, prior to cows, were the grasslands’ primary grazer, consuming as much as 40 percent or more of the total grass biomass annually.

I saw evidence of this invasion everywhere cattle were still permitted to graze. Only inside SPRNCA land, where some areas have been cleared of mesquite, have the grasslands returned; they are prospering as well as they can in the absence of fire. As I hiked along the riverbed, looking up at the amassed armies of mesquite along the high eroding bank, I thought about the satellite images I had been poring over of this particular stretch of river. While the river did look lush from space, the striations of greenery were reminiscent of layers of cholesterol coating a struggling artery.

“At St. David the mesquite were so thick that the cows would get in there and make tunnels that horse and rider couldn’t fit through,” said Betty Foster Escapule when I talked to her by phone. “They wouldn’t come out, so they’d send my brother in after them.” Escapule lived on the river through the 1930s, 40s and 50s and witnessed countless changes from a changing floral community to the dramatic influx of people and roads. While cottonwoods filled in the river’s edges and mesquite took over the grasslands, other weeds filled in the gaps. “When farmers came in and plowed the flats along the river the vegetation changed,” she said. “Cockleburs and other weeds took over the disturbed land. When we saw them we got off our horses and pulled them.”

Another recently prolific species is the Fremont cottonwood, a tree now ubiquitous along most of the San Pedro River and for many casual observers iconic of the river and synonymous with its health. “The Fremont cottonwood and Gooding willow gallery forest is one of the rarest types in the U.S,” I was recently told by a BLM ranger. But I wonder if we are talking about the same forest. In any form that might be considered a forest, the cottonwood did not exist along the San Pedro until the early 1900s. Today the river is crowded by a thick corridor of trunks sometimes 200 yards deep and 50 feet tall stretching from the river’s headwaters 150 miles to the confluence with the Gila River. These seemingly majestic forests came about not exclusively through natural processes, but by the removal of cienegas, the channelization of the river and through increased levels of atmospheric C02. Thanks to these anthropogenic factors, cottonwoods have increased along the San Pedro by more than 150 percent, according to Donald Pool of the U.S. Geological Survey.

“These trees aren’t very old,” said Betty Escapule. “People say the big tree at the Wolf Place is hundreds of years old,” referring to a gnarled but beautiful and ancient-looking cottonwood at the San Pedro House. “But when I was a kid my mother could easily reach around it. It’s probably no more than 80 years old.”

Tamarisk is another problem species marching through the San Pedro. The bush is similar to cottonwood and mesquite in its invasive nature, but it is non-native and even more successful in its quest for valuable riparian habitat. While tamarisk has not yet filled in the banks of the middle and upper San Pedro, researchers find its front advanced along the lower potions of the river each season. But tamarisk is more than an invasive species killing off natives right and left. To species like the endangered southwestern willow flycatcher, which has disappeared in over 95 percent of its range, the plant has actually provided acceptable habitat; according to Chris Kirkpatrick of the University of Arizona the bird might actually prefer it to native plants. Some song sparrows and the yellow-breasted chat also thrive in its presence.

While these new forests of cottonwood, mesquite and tamarisk have been beneficial to some species, many others that evolved into a mostly tree-free riparian habitat have been driven out. “Anytime you have foliage changes there are winners and losers,” said Juliet Stromberg, an expert on the San Pedro at Arizona State University, adding that grassland birds have declined around the river.

“There is a huge affect on birds when you go from a cienega to a gallery forest,” added Kirkpatrick. The unexpected interspecies alliances, combined with the rapid change in foliage, have added an unwelcomed layer of complexity to an already exceedingly complex issue. “Do we manage this habitat in its current state or as it was before people?” lamented Kirkpatrick in this most philosophical and fundamental of conservation questions. “I just can’t say.”

Back in the dry riverbed upstream of Fairbank, I remembered a friend’s comment that people who spend time in the desert often develop an obsession with water and an almost sixth sense for finding it. But I felt more dehydrated and dizzy than focused, and obsessed less by the water itself than by the remaining native plant communities I hoped I would find near it. I suppose I wanted to find the real San Pedro: the pre-human San Pedro, if there is such a thing.

After an hour of hiking the meandering and entirely dry riverbed, the parched super-heated air had shrink-wrapped my contact lenses to my eyeballs and scorched my lungs. The river of old––that perennially flowing stream cloaked in greenery and alive with the sounds of animals––probably once occupied this stretch but certainly not any longer, or at least not today. Instead the same row of cottonwoods followed me, rising from the riverbed like sentinels while mesquite perched on the eroding banks with their naked roots hanging in the air. But the monotony was broken, if just for a second, by a sensation more than a smell; it made me think water. I spotted a dark patch in a field of dried mud trapezoids and ran to it.

A lone pool holding out against the sun and porous ground? A bit of water momentarily forced to the surface by bedrock? Nothing so spectacular; a cow had simply urinated, then stupidly slipped in the mud. I followed its tracks across the wash through a mesquite thicket to a broken fence at the edge of SPRNCA land. They weren’t supposed to be there, but there they were, just as they had been for 500 years.

In the wake of such excitement and disappointment I sat down under a cottonwood—thankful for its shade, resentful of its success—and contemplated the issues at hand. There is a branch to fit every bird’s foot. A leaf or blade or needle for every specialized herbivore. A flower colored and shaped for every insect and hummingbird. But here there is a discrepancy between what the flora provides and what the rest of the biotic community needs. While a select few animals have profited, most have been marginalized and squeezed out, not by habitat destruction but by a habitat that has outpaced evolution.

Part Four – The Land
The land of the San Pedro watershed seems like an easy enough thing to define. It’s what we hike on and get under our fingernails when we garden. It’s the sand at the bottom of the river and the riverbed when the water had dried up. It’s what’s inside or outside fences, and it’s covered in cows, farms, houses or native grasses. It’s state, federal and private; it’s Mexico and America. It’s theirs or ours, yours or mine.

That’s what books, maps, and conventional wisdom say, anyway. But after a long summer spent hiking along the river and studying every part of the ecosystem in isolation I discovered that the land of the San Pedro River is more than the sum of its parts. This understanding came to me not as a sudden epiphany or late-night breakthrough at a pile of books, but slowly, through talking to people who live on the land and through personal experience with sunburns, soaked shoes, scratched flesh, biting flies and the accumulation of a thousand individual observations of beauty. The land is not just soil and rock, but also ecosystems, water, and the lives of people.

Before leaving Southeast Arizona and my beloved San Pedro River for the summer, my wife and I set out on one final adventure as the clouds gathered over Mexico and sent their thunderous warnings across the border. After the many phone calls, interviews, books, and long hours staring at Google Earth, I wanted to spend some time on the river without a mission, without anything in particular to look for or even think about. I wanted to experience the land not as a collection of organisms, minerals, and water but as a whole entity, what James Lovelock called “Gaia.” With this vague mission in mind, we set out from Bisbee in our Subaru one hot but nicely cooling Arizona Sunday afternoon.

As always, the line between cultures blurred as we approached the border on highway 92, the radio stations merging into a sort of scratchy “Spanglish” indecipherable to Mexicans and Americans alike. We rounded the Mule Mountains and the metallic glint of the border fence caught my eye as it rose to crest a low hill like the fin of some prehistoric lizard. While the battle for the airwaves was still being decided, the line between Mexico and America had clearly been established. In my quest to understand the land, this icon of division, specifically where it crossed the free-flowing San Pedro, served as a constant reminder that land can mean “us and them” or “mine” just easily as it can represent the unity of all living and inert things. But before the day was over we had watched the rivulets of water from a storm cascading over Mexico defiantly flow across the border, erasing the tracks of emigrants, animals and Border Patrol Vehicles. As the storm passed, gusts of wind blew earth through the fence, proving that people, animals and even land can move as easily as water across something so seemingly permanent as a border fence. Land is land no matter how divided.

The day before this most recent trip I called Betty Foster Escapule at her home in Tombstone seeking some perspective on this problem of what is the land? Lacking an advanced education of any kind, and more fond of salty sayings than calculated scientific postulation, she was still the most knowledgeable person I had met while learning about the San Pedro. Having spent her entire life on the river as a rancher’s daughter and tomboy, she had just the kind of incredibly specific knowledge of the land I was looking for. As the phone rang I studied the portrait on the back of her book The Five Fosters. Her white hair and dated glasses said little about the uniqueness of her life; the deep lines of her face looked not unlike others who have aged under the Arizona sun. But in the subtly upturned corners of her mouth and the wide, eminently awake eyes there was a distinct rural tenacity and a good helping of the kind of knowledge not found in books.

She answered with a forceful “Hallo,” a greeting not many 71-year-olds can muster. With little prompting, she delved into a series of stories, her voice wavering now and then with age but mostly crisp and precise.

“We grew up isolated,” she told me, then added “and we loved it,” perhaps suspecting that such a statement might not find too many understanding ears in the somewhat urban Southeast Arizona of today. “There were just a few other families along the river, and a handful more in Fry [Present day Sierra Vista, now home to 43,000 people].” While a newcomer like me sees the private property signs, the barbed-wire SPRNCA land perimeter and the international border all as things set in stone and timeless in their authority, Betty’s concept of the land was clearly quite different. She had seen the patterns of weather change, then change again, and the tide of emigrants from Mexico wax and wane. She had seen animals move through and increase and decrease in their abundance. She had seen plant communities change and watched roads turn from gravel to cement and sometimes back into gravel. She had seen Fort Huachuca bring new people, and she had seen people flee when mines closed. To her, the ebb and flow of people before her time and long after is the best way to understand the land.

Speaking to her that afternoon I had the sense of watching a time-lapse movie of the valley: the river adding a new bend here and there but remaining relatively constant while the hectic goings-on of plants, animals and people moved in hyper-speed. The common sentiment that things were better at some time in the past or will be better some time in the future is a naive sentiment, according to Betty. The land just is, and people pass over it like water. When the flood is done, the land is not worse, just different. But still, like so many people of the rural west who have seen their homes overrun by suburbs, she misses the land she grew up in. “I went to school where the Chili’s is now,” she said, laughing. “I like Wal-Mart, and Chili’s is okay, but it’s sad, you know? The country has settled up.”

Hiking the river that Sunday afternoon with my wife, I thought about Betty walking five miles to see her future husband, Charles Escapule, in 1954. Our story mirrored theirs, though less dramatically and 55 years later. Without even considering the countless other lives at play in the valley then, now, and in the future, I was already starting to get a clearer idea of Betty’s understanding of the land. As thunderheads grumbled cantankerously overhead, I found myself a living part of the land, not because of anything specific I had done or knew, but because I was simply there, and conscious, and respectful of it.

That day even a dry riverbed was a sublime thing. Concentric rings of stones, organized according to size, radiated downriver, while artful heaps of sticks, some shaped like people, were stacked against lone rocks and mid-river trees, grotesque and beautiful like Giacometti sculptures. Abandoned clothing and empty water bottles, barkless sticks with beaver-chewed ends, the myriad tracks from that morning’s animal wanderings—all were harmless and interesting artifacts of the land’s use, life and identity. And bisecting all this were our tracks, temporarily attesting to yet another human story interwoven with the braids of the river to form the fabric of the land.