The point where southern Arizona's San Pedro River crosses Interstate 10 is not a memorable sight for passers-by zipping merrily along to Los Angeles or El Paso.

At the crossing, marked by an almost ironic sign, the riverbed is little but a thin band of tan dirt and stream cobbles, occasionally graced by a trickle of water that has caused more than one chuckle among travelers used to greener and wetter pastures.

Sorry as it may seem, though, the San Pedro is an important river, important far out of proportion to its size and carrying capacity. The distance from its source in the Sierra Mariquita near the mining town of Cananea, in the Mexican state of Sonora, to its confluence with the Gila River in the heart of what was, before last year's floods, Winkelman, Arizona, is barely 140 miles. Its water flow, too, is significantly less than that of rivers like the Colorado or the Gila, against which the San Pedro seems little more than a small stream.

But the San Pedro is unique, a river flowing in a nearly straight line from south to north, acting as a natural beacon for migrating bird and animal species, ultimately connecting the ecologies of the tropics and the Sierra Madre with those of the far north.

In its pristine form, the river was less a free-flowing watercourse than a series of marshes, or cinagas. The San Pedro has, therefore, always acted as a sort of magnet for birds winging northward from their Latin American wintering spots. Now that other riparian flyways like the Colorado River and the Rio Grande have undergone major ecological changes, the San Pedro is an increasingly critical habitat for birds and other creatures. Biologists estimate that 400 bird species, 83 mammal species and 47 amphibian and reptile species inhabit the San Pedro, which enjoys the distinction of being the longest undammed river in Arizona.

The avian life of the river is so rich, in fact, that Birders Digest recently ranked the area near Cascabel as this nation's premier bird-watching site. And the San Pedro basin encompasses nearly all plant communities and life zones in the greater Southwest, making it an ecological treasure vault of uncommon significance.

People have known the river to be a good thing ever since they arrived in this portion of the Americas. They have thought it so good, they have nearly killed it. Cattle, mining and other activities in its watershed have turned the San Pedro from a near marsh to a deep river channel that is alternately almost dry or flooded. Riparian habitats have disappeared, and with their demise came a Homeric catalogue of near extinctions: A full tenth of the 530-odd species now listed under the Endangered Species Act can be found along the course of the San Pedro.

To many people living along the upper river, however, Latin species names and statistical litanies of wildlife death mean little. Cochise County, encompassing nearly half of the river's flow, is booming, and dollars are pouring in. Car dealerships, fast-food joints and apartment complexes are fast replacing dusty desert. With so much growth, municipal water use and irrigated agriculture now take nearly 22,000 acre-feet (about eight billion gallons) of water from the San Pedro per year--a good portion of the river's total flow, which explains why most people crossing the river on I-10 see nothing but a dry bed most of the time.

Environmental and governmental concerns are pushing to restore the river to some approximation of its former state. Other interests want to further control, develop, even dam the river. Caught in the squabble of contending rights to its use---in Sierra Vista alone, there are 14 private and municipal water companies---the San Pedro awaits final judgment.

In the next few years, Arizonans will decide whether it lives free or dies.
@body:In the American West, most attempts to restore destroyed wildlife habitats have been met with vocal, and often successful, opposition from those who profit by use of the land. And the San Pedro watershed has a long history of human use and abuse.

Near the San Pedro lie two significant woolly mammoth kill sites that date to 12,000 years ago; later, in historical times, several thousand Sobaipuri Indians, distant cousins of the Hohokam, dwelled along the river's length in small settlements such as Gayibanipitea and, for a brief time, in the Spanish presidio named Terrenate, near the present town of Tombstone.

The Spanish introduction in the mid-1600s of several thousand head of cattle began to change the face of the river in small ways. It took the arrival of mining companies two centuries later to cause bigger mutations; the mine smelters required thousands of cords of wood daily, and soon the hills lining the San Pedro were shorn of their oak and mesquite trees.

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Gregory McNamee