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Broadcast on October 9, 2008
By Cynthia McAllister
Texas west of the Pecos is considered part of the Chihuahuan Desert Eco-region. But the Chihuahuan Desert extends far beyond West Texas, and includes wetlands in Mexico that are home to over 150 species of animals and plants that are found nowhere else on Earth. What are the boundaries of the Chihuahuan Desert? Where does the water that forms the Mexican wetlands come from, and what are the threats to this unique aquatic eco-system?
The Chihuahuan Desert, largest of North America’s four deserts, spreads from western Texas across southern New Mexico to the southeastern tip of Arizona. But the majority of the desert’s nearly 200,000 square miles are in Mexico, where it extends south almost to Mexico City and lies between the two “great mother” mountain ranges: Sierra Madre Occidental on the west, and Sierra Madre Oriental on the east.
Near the eastern edge of the Desert, in the Mexican state of Coahuila, is the valley named “Cuatro Cienegas” by early settlers. In this small 580 square mile valley is a desert wetland with over 500 clear, blue, freshwater pools, locally known as pozas. The combination of water and isolation has lead to great biodiversity here. Many of the species found here are endemic, meaning they occur nowhere else. For example, 10 out of the 17 fish species, 13 out of the 26 species of snail and 26 species of plants found in Cuatro Cienegas grow nowhere else on Earth.
For nearly fifty years, scientists have recognized the area as ecologically important. In 1994 it was declared a National Protected area by the government of Mexico, and it’s a UNESCO World Biosphere Reserve.
The region is surrounded and criss-crossed by mountains where deep beds of Cretaceous limestone are uplifted, faulted, and folded into nearly vertical beds. The Sierra San Marcos y Pinos mountain range is the primary rain catchment area for wetlands of the valley of Cuatro Cienegas. Rainwater falls on the mountains, seeps into the limestone and flows along down-dipping beds and fault planes deep into the earth where it is geo-thermally heated. Along the way, the natural acidity of the water dissolves the rock into a complex system of caves and underground streams. Finally, the hot water percolates up through the 200 feet of gravel and sediment filling the valley floor into a network of warm, mineral-rich springs, streams and lakes. This system, isolated from outside water sources, is what makes the valley of Cuatro Cienegas so unique.
Poza Azul, a large, especially pretty pool, is the “mascot” of the town of Cuatro Cienegas, appearing on signs and labels in many places. At this pool, hot water enters through a chasm at the higher end and is sucked back down into the sediment through another chasm at the lower end. This water, now cooler, re-surfaces in pools further downstream.
This system provides the warm, sulfate-rich environment in which thrive stromatolites: a primitive life form found in Pre-Cambrian seas, but rare today. These rounded structures are built up slowly from minute layers of alternating algal mat and sediment.
Whole ecosystems in the pozas are linked to the stromatolites. One tiny snail that lives in the pools and eats the algae, has the hardest shell of any snail in the world. It should be safe from predators, but here also lives a small, endemic fish with teeth strong enough to crunch the snail.
Out on the valley floor is Los Hundidos (oon-dee-dose), Spanish for “the land of subsidence”, where a falling water table has caused the ground to collapse in many places. The compacted sandy soil is pocked with sinkholes, and the earth covering underground streams is eroded away.
The most impressive sinkhole is called La Maroma, Spanish for “the somersault.” This one was named by the rancher who incautiously approached its edge and somersaulted into the deep pool some forty feet below. Fortunately, the far side of the 30 foot-wide hole was eroded down into a shallow marsh, and he was able to get out.
On the other side of the Sierra San Marcos water tells another tale. The Churince system (chur-n-say, accent on n) also starts in a warm, blue pool. The water that disappears underground at the end of the pool, feeds into a large playa lake. The lake has always grown and shrunk, depending on the season, but in recent years it is mostly shrinking. The area between the pool and the playa lake is littered with sinkholes, a classic consequence of a dropping water table.
Just down the road is another area that was once a playa lake. Twenty years ago a student wrote a doctoral thesis on the abundant stromotolites in this lake. Today the stony stromotalites sit high and dry, covered with vegetation. The student, revisiting the place, did not recognize it. There is no water here today; it is being channeled off for big agriculture.
During the 19th century the local economy was based on ranching and agriculture. Cuatro Cienegas was known throughout Mexico for its vineyards and orchards. The Rio Canyon flowed out of the mountains, supplied two public swimming pools and provided water for municipal use and local small agriculture.
Today, agribusiness is increasing outside the valley and water is being diverted from Cuatro Cienegas. Within the last year the Rio Canyon has completely stopped flowing. The swimming pools are filled with grass and shrubs, and dusty, dry orchards of dead trees and dying vines suggest a dismal future for the valley. Still, there are several conservation groups, from both Mexico and the United States, working to heighten the public’s awareness of the area’s unique and fragile nature.
The results of taking water from the Cuatro Cienegas region should be a warning to the rest of the people who live in the Chihuahuan Desert region on both sides of the border. Water in the desert is a precious resource, and should be treated as such.
Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on October 9, 2008.
References & Resources for Educators
- NPR Radio Expeditions
- “Living Laboratory: Studying Life’s Origins in Cuatro Ciénegas” by Deborah J. Ausman in Rice University’s Sallyport (Summer 2005)