“The riparian ecosystem along the unregulated San Pedro River is one of the most valuable in the Southwest, particularly for birds.” – Robert Webb, et.al 2007 — The Ribbon of Green
Over the last few years, I’ve spent a lot of time along the San Pedro River in Southeastern Arizona. As a nature photographer and biologist, the river has taught me the value of its presence as a key connection in Nature. It has also given me many beautiful photographic images along its banks. I’m fascinated as to why the river is here, what sustains it, and why it is so important. The answers to these queries are revealed as one begins to understand the dynamic connections that this river has with other patterns in Nature. Indeed, by looking at Nature’s connections, one begins to understand Nature’s patterns. In a recent post, From Raindrops To Rivers , I characterized a river as a natural connecting mechanism “that transports water to all forms of life and directly affects the shape and composition of inanimate forms of Nature”.
The San Pedro River offers a living demonstration of the importance of connections within Nature. To give you a little background, the San Pedro River is a south to north flowing river that begins within the mountains near Cananea, Sonora, Mexico and flows 140 miles into the Gila River near Winkleman, Arizona. It is the last large undammed river in the Southwest. The river’s source of water is mountain springs and rain runoff from mountain ranges that lie on either side of its course.
According to Wikipedia, the river “is of major ecological importance as it hosts two-thirds of the avian diversity in the United States, including 100 species of breeding birds and 300 species of migrating birds”.
The Center for Biological Diversity says: “For tens of thousands of years, they have traveled along the few north-south river corridors for shelter, food and water during their transit. In the past, the Rio Grande, San Pedro, Santa Cruz and Colorado formed these migratory corridors. Today only the San Pedro survives….Nearly 45 percent of the 900 total species of birds in North America use the San Pedro River at some point in their lives”
A major website devoted to the river, states that “the entire valley’s exceptional species richness has complex causes, but for birds it provides a well-watered, verdant corridor running from southern tropics toward northern tundra and back, with stacks of diverse and accessible vegetation resources … all the way to its confluence with the Gila River.”
In addition to the birds, more than 200 species of butterflies and 20 species of bats use this corridor as they migrate between, South, Central, and North America. The Nature Conservancy states that the river “is home to 84 species of mammals, 14 species of fish, and 41 species of reptiles and amphibians. Species such as the jaguar and black bear stalk the region’s forested mountains while the Mexican gray wolf and black-tailed prairie dog reside in the expansive grasslands.”
The San Pedro River valley basin is bordered by mountain ranges to both the west and the east. It is filled with sediment layers and erosion (alluvial fans) from the nearby mountains. Water from rains, mountain runoff, and springs collects in the basin creating a perennial water table just below the sediment surface. Heavy rains have created gullies that reach below the water table and form the actual river. Depending on the time of the year and the geology of a particular section, the river may be flowing above ground or may be flowing below the sediment surface.
The lesson from studying ecology is that all of Nature’s systems and her parts are connected. The enormous “draw” that this riparian area has on species from other geographic areas is a significant connection in Nature beyond the immediate region. The water on and below the sediment surface is the keystone connecting pattern in Nature that drives the rest of the ecology that I’ve mentioned. It is the lifeblood supporting the waterside plants. The cottonwood trees and other plants need stream water to move seeds into the sediment and germinate these seeds. The trees and other plants are the habitat for the birds and other creatures. Without the habitat supported by the water, wildlife could not survive here. If the water were to disappear, the abundance of life would disappear.
Unfortunately, the San Pedro River basin is a threatened hydrological system. The availability of the key connection in Nature, water, is in doubt. The culprit is we human beings. Excessive pumping of groundwater from the valley for agricultural, municipal, and industrial uses has lowered water tables and diminished the water supply necessary to maintain the critical river habitats. According to the Center for Biological Diversity :
“…the San Pedro River is drying up. Unsustainable pumping of the groundwater that supports it has caused base flows to decline by 67 percent since the 1940s. The current population of more than 50,000 people in the upper basin is pumping more out of the aquifer each year than are recharged by rainwater. The burgeoning water deficit is caused by unsustainable population growth and a lack of effective water-conservation planning.”
The population of the town of Sierra Vista has exploded in recent years, driven largely by nearby Fort Huachuca, the U.S. Army post that is the largest single water user in the valley. Fort Huachuca is driving human population growth and excessive, uncontrolled pumping of groundwater both on the military facility and in the surrounding community. Unless things change soon, the San Pedro will resemble the lower reaches of the Santa Cruz, Gila, Salt and other Arizona rivers: dry, treeless and devoid of the diversity of life that once graced its waters and shores.
If humans refrain from removing water, the San Pedro River will continue its role as a dynamic sustainer of life. Fortunately, a number of organizations have recognized that the water in the San Pedro River is a keystone connection in Nature. Through legal action and habitat restoration activities, there seems to be a gradual improvement in the hydrological condition of the river.
I encourage you to visit this ecological treasure and engage with this living demonstration of a dynamic connection in Nature.
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My name is Bill Graham. As a Marine Biologist who has worked in the US and Mexico for 30 years, I am a student of Nature, a teacher, a researcher, and a nature photographer. Through my work, I have acquired an ever growing passion for how everything in Nature is connected. Today, I travel extensively contemplating about, writing about, and photographing Nature’s connections. I also work with conservation projects in the USA and Mexico and mentor talented youth.