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Natural World

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The approximate boundaries of the Lower Pecos archeological region are show in white on this shaded relief map. Its southern extent is unknown but believed to lie in the Burro Mountains of northern Coahuila. Base map from the Perry-Castañeda Library Map Collection.
The approximate boundaries of the Lower Pecos archeological region are shown in white on this shaded relief map. Its southern extent is unknown but believed to lie in the Burro Mountains of northern Coahuila. Base map from the Perry-Castañeda Library Map Collection.

The rockshelters provide only temporary refuge from the relentless cycles of the canyon world: night cool and day heat, drought and flood, bare rock and green plant.

The Pecos River is shallow and mineral-rich. Its source lies hundreds of miles to the north in New Mexico. Photo from ANRA-NPS Archives at TARL.
The Pecos River is shallow and mineral-rich. Its source lies in northern New Mexico, some 685 miles above its confluence with the Rio Grande. Photo from ANRA-NPS Archives at TARL.
Mile Canyon, one of many short, narrow side canyons that empty into the Rio Grande. The canyon wall in the far background is across the Rio Grande in Mexico. Photo from ANRA-NPS Archives at TARL.
Mile Canyon, one of many short, narrow side canyons that empty into the Rio Grande. The canyon wall in the far background is across the Rio Grande in Mexico. Photo from ANRA-NPS Archives at TARL.
The twisting canyon tunnels form a closed-in world framed by white-rock walls and the blue sky high above. Sound, water, and critters travel thorough the canyon tunnels, seeking each other and the hidden green pools of water—tinajas. Photo by Steve Black.
The twisting canyon tunnels form a closed-in world framed by white-rock walls and the blue sky high above. Sound, water, and critters travel through the canyon tunnels, seeking each other and the hidden green pools of water—tinajas. Photo by Steve Black.
Seminole Canyon. In the background the canyon floor has been scoured by flood, in the foreground stacked up boulders trap soil and moisture and provide shelter for trees and shrubs. Photo by Steve Black.
Seminole Canyon. In the background the canyon floor has been scoured by flood, in the foreground stacked up boulders trap soil and moisture and provide shelter for trees and shrubs. Photo by Steve Black.
Greasewood flat. Creosote bush or greasewood, is typical of the Chihuahuan Desert. Photo from ANRA-NPS Archives at TARL.
Greasewood flat. Creosote bush or greasewood, is typical of the Chihuahuan Desert. Photo from ANRA-NPS Archives at TARL.
Small seep springs like this one can be found in many of the canyons in the Lower Pecos. Photo from ANRA-NPS Archives at TARL.
Small seep springs like this one can be found in many of the canyons. Photo from ANRA-NPS Archives at TARL.

Although the Lower Pecos Canyonlands occupies only a small part of Texas, its natural world is far from uniform. It is, in fact, a place marked by contrast: green and brown, canyon and upland, one river and the next, yesterday and today. This section introduces the natural world through pictures and words.

The Lay of the Land

The Lower Pecos Canyonlands lies at the southwestern edge of the Edwards Plateau—the Texas Hill Country as most know it—which in turn lies at the southern edge of the Great Plains. It also is within the northeastern edge of the Chihuahuan Desert. The area is located along the middle part of the Rio Grande drainage basin just below the S-shaped "Big Bend" where the river turns southward toward the Gulf of Mexico. It is there that the Rio Grande is joined by the last two major streams that feed its middle and lower stretches—the Pecos River, and the Devils River. And it is the juncture of these three rivers that forms the core of the Lower Pecos Canyonlands.

Within the region, the Rio Grande embankment is dissected by precipitous canyons and arroyos that enter the river on both sides. North of the Rio Grande, the land is level to hilly and is cut by numerous canyons. South of the Rio Grande lies an extensive plain broken by intermittent streams that originate in the Serranias de los Burros, a mountain range rising to almost 5,000 feet (1,524 meters). These mountains parallel the Sierra del Carmen of Mexico and the southern escarpment of the Edwards/Stockton Plateau. The southern part of the Burros are dominated by the 4,700-foot-tall Cerro Oso Blanco (White Bear Peak), which is located about 70 kilometers (43 miles) southwest of Del Rio.

The deep canyons and porous limestone geological formations of the region give rise to numerous permanent or temporary streams and springs that stand in contrast to the xeric (desert-like) nature of the plains and plateau country that they dissect. The three permanent streams—the Rio Grande, the Pecos River, and the Devils River—are the life blood of the region, providing water and refuge to plants, animals, and people. The Pecos River enters the Rio Grande just 26 miles (42 kilometers) northwest of the confluence of the Rio Grande and the Devils River. All three major streams are joined by numerous small intermittent streams that form narrow side canyons and break up the surrounding topography into ridges, mesas, and fingers overlooking the deep green winding worlds of the rivers.

The uplands and canyons really are two worlds that make one. Upon the uplands the horizon stretches forever, a jagged purple-blue line that slowly fades to black at dusk. Up on top, the wind sculpts squinting faces, sets ceniza and ocotillo branches to dancing, and fuels the rusty silver flash of a well-oiled windmill. There is no lasting natural reprieve from the relentless sun; the trees are small and dark clouds all too infrequent. In the best of times, the upland flats are grassy plains that once lured herds of buffalo down from the north. In the worst of times, there is nothing but thorn, rock, and dust.

And down below, the twisting canyon tunnels form a closed-in world framed by white-rock walls and a blue sky high above. Sound, water, and critters travel thorough the canyon tunnels, seeking each other and the hidden green pools of water. The tenacious roots of sheltered trees try to hold the canyon floor together. But in time, each scene changes as canyons become half-choked with washed-in soil and the plant life that gladly follows, only to be scoured clean by a once-in-three-lifetimes deluge. The rockshelters provide only temporary refuge from the relentless cycles of the canyon world: night cool and day heat, drought and flood, bare rock and green plant. They, too, crumble with time.

Many large springs are located within and adjacent to the Lower Pecos Canyonlands due to its location at the southwestern edge of the Edwards Plateau. These springs occur mainly along the Balcones fault zone where underground rivers (aquifers) come in contact with impervious geological formations, forcing the water to escape to the surface under artesian pressure. Just east of the Lower Pecos Canyonlands, San Felipe Springs are the fourth largest springs in Texas and still serve as the sole source of water for the city of Del Rio. Goodenough Springs, also one of the largest springs in Texas, is located beneath the waters of Amistad Reservoir in a side canyon of the Rio Grande. Although surface water was confined to narrow canyons, numerous springs also existed on the plains to the south of the Rio Grande. The abundance of permanent streams and springs means that water was seldom a limiting factor to travel.

The Modern Climate

The southern and western edges of the Lower Pecos region receive an annual average of as little as 10 inches of rainfall, while the northeastern edge averages almost 19 inches per year, figures which place the area within a semi-arid climatic zone. Most precipitation occurs in two peaks, one in spring (April-May) and one in early fall (September-October), both fueled by thunderstorms at frontal boundaries fed by moisture from the Gulf of Mexico and sometimes the Pacific. The heaviest, drought-busting rains occur when tropical storms stall over the area, sometimes dumping devastating, canyon-scouring quantities of rain in a matter of hours or days. The driest months of the year occur in winter from November to March, and in summer from June to August. The frost-free period averages 300 days between February 13 and December 8. The average annual temperature is 70ºF, ranging from a low of 51ºF in January to a high of 86ºF in July.

Although these averages give a general impression of climate, the truly interesting aspect of southwestern Texas climate is its variability. The Lower Pecos Canyonlands region is positioned at two great climatic crossroads of the North American continent—along the sharp line between the humid east and the arid west, as well as the more ambiguous divide between the markedly seasonal regimes to the north and the winterless tropical climes to the south. Consequently, the region encompassing southern Texas and northeastern Mexico has a semiarid, subtropical climate with dry winters and hot summers. More critically, the climatic region containing the Lower Pecos has a greater degree of year-to-year rainfall variation than any other semiarid region in the world, except for northeastern Brazil. Rainfall is highly unpredictable and drier-than-normal years are much more common than extremely wet years.

In short, the Lower Pecos Canyonlands lies within a climatically transitional region that can shift from a relatively well-watered green and grassy place to a dusty brown desert in a matter of years or even months. It is a marginal region for intensive land use—plant and animal populations are quickly affected by fluctuations in precipitation. Throughout its history, humans have had to cope with climatic uncertainty, savoring the good months and years and suffering the many bad ones.

Vegetation

Mirroring the terrain and climate, the Lower Pecos lies within a major transition zone where three great biotic provinces converge. To the southeast is the mesquite and thorn brush savannah of the South Texas Plains. To the north, the vegetation grades into the juniper-oak savannah associated with the Edwards Plateau, while Chihuahuan Desert Scrub covers the core area of the lower canyonlands of the Devils River, the Pecos River, and the Rio Grande and farther south.

Overall, the Lower Pecos area is a savannah with a mix of grasses, brush, and trees. Depending on the local conditions, the area may favor shrubs or grasslands. The southeastern edge of the Lower Pecos Canyonlands is part of the mesquite-acacia savannah of southern Texas. To the west, vegetation rapidly grades into sotol-lechuguilla-creosote bush vegetation typical of the Chihuahuan Desert. Despite the fact that the region is technically a savannah, woody plants dominate the vegetation in most upland areas within the boundaries of the reservoir today. These include mesquite (Prosopis glandulosa), several species of acacia (Acacia spp.), whitebrush (Aloysia gratissima), Texas persimmon (Diospyros texana), blue sage (Salvia ballotiflora), lotebush (Ziziphus obtusifolia), various buckthorns (Condalia spp.), and spiny hackberry (Celtis pallida). Creosote (Larrea tridentata) and ceniza (Leucophyllum frutescens) are prominent in many areas. Along the upper reaches of canyons, succulents and rosette-stemmed evergreens are also common, including prickly pear and tasajillo (Opuntia spp.), several yuccas (Yucca spp.), and Agave lechuguilla. Small trees are confined to narrow canyons or creek terraces. Littleleaf walnut (Juglans microcarpa), several species of oak (Quercus spp.), Mexican ash (Fraxinus greggii), and Texas persimmon (Diospyros texana) are a few of the more prominent tree resources located in the canyons. Large stands of Huisache (Acacia farnesiana) and mesquite (Prosopis glandulosa) occur on certain of the broad terraces in the eastern part of the region.

Paleoenvironment

Despite the remarkable detail of the prehistoric vegetation record contained in numerous, well-preserved rockshelter deposits, only the general outline of the environmental history of the Lower Pecos is known well. It appears that the Chihuahuan Desert has expanded and contracted throughout the late Pleistocene (prior to 9,000 B.C.) and Holocene (or Recent) periods, leaving widespread relict populations of certain plants in favorable microenvironments such as the protected and relatively well-watered canyons. This is critical for two reasons. First, prehistoric peoples often relied on these relict or stranded populations of plants that are otherwise not commonly available in the area. Second, the mix of local and relict plant communities makes it difficult to reconstruct past environments because all but the most drastic vegetation changes are masked in the archeological deposits.

The general climatic trends can be seen in the pollen record. As allergy sufferers know only too well, during certain times of the year certain plants release large amounts of pollen. Some of this pollen rain survives, especially in permanently wet places like swamps and in very dry places like the rockshelters of the Lower Pecos Canyonlands. By studying samples from deposits of known ages, pollen experts have reconstructed the paleoenvironment of the region at least in broad terms. Decreasing frequencies of pine pollen indicate that the region was slowly drying throughout the Holocene, with a brief wet interval occurring around 2,500 years ago marked by an increase in pine and grass pollen frequencies.

While pollen provides a very general view of regional vegetation, most of what is known about Lower Pecos Canyonlands vegetation has been gleaned from the analysis of plant remains found in archeological deposits in the dry caves and rockshelters. The best known shelter in the Lower Pecos area is Hinds Cave which was excavated by archeologists from Texas A&M in the 1970s. The Hinds Cave work showed that plants common in the Lower Pecos Canyonlands today, including lechuguilla (Agave lechuguilla), yuccas (Yucca torreyi and Y. rostrata), sotol (Dasylirion texanum), acacias (Acacia greggii and A. rigidula), prickly pear (Opuntia phaeacantha), shin oak (Quercus pungens var. vaseyana), mesquite (Prosopis glandulosa,) and juniper (Juniperus spp.), were well established in the Lower Pecos Canyonlands by 7000 B.C.


In its lower reaches, the Pecos River carved a deep canyon entered by many side canyons. This photo was taken during the 1966 excavations at Arenosa Shelter, which is visible in the center of the picture. Today this area is choked by massive silt deposits that have built up since Amistad Reservoir was formed. Photo from ANRA-NPS Archives at TARL.
In its lower reaches, the Pecos River carved a deep canyon entered by many side canyons. This photo was taken during the 1966 excavations at Arenosa Shelter, which is visible in the center of the picture. Today this area is choked by massive silt deposits that have built up since Amistad Reservoir was formed. Photo from ANRA-NPS Archives at TARL.

Click images to enlarge  

The lower Devils River prior to the construction of Amistad Reservoir. Although much shorter than the Pecos and the Rio Grande, the Devils River is spring fed and its waters are clear and "sweet" compared to the often muddy and salty waters of the other two. Photo from ANRA-NPS Archives at TARL.
The lower Devils River prior to the construction of Amistad Reservoir. Although much shorter than the Pecos and the Rio Grande, the Devils River is spring fed and its waters are clear and "sweet" compared to the often muddy and salty waters of the other two. Photo from ANRA-NPS Archives at TARL.
Numerous springs issuing out of the Edwards Aquifer feed the Devils River. Photo from ANRA-NPS Archives at TARL.
Numerous springs issuing out of the Edwards Aquifer feed the Devils River. Photo from ANRA-NPS Archives at TARL.
Spring-fed tributary of the Devils River. Photo from ANRA-NPS Archives at TARL.
Spring-fed tributary of the Devils River. Photo from ANRA-NPS Archives at TARL.
Innumerable canyons like this one provide shelter for plants, animals, and humans. Photo from ANRA-NPS Archives at TARL.
Innumerable canyons like this one provide shelter for plants, animals, and humans. Photo from ANRA-NPS Archives at TARL.

Throughout Lower Pecos history, humans have had to cope with climatic uncertainty, savoring the good months and years and suffering the many bad ones.

Many of the deeply incised canyons are lined by cliffs, their walls so steep that they are all but invisible until you are almost standing on the canyon edge. Viewed from the air, the canyons form tree-like patterns with the rivers as trunks and the smaller streams and side canyons forming limbs and branches twisting and turning across the land. Photo by Steve Black.
Many of the deeply incised canyons are lined by cliffs, their walls so steep that they are all but invisible until you are almost standing on the canyon edge. Viewed from the air, the canyons form tree-like patterns with the rivers as trunks and the smaller streams and side canyons forming limbs and branches twisting and turning across the land. Photo by Steve Black.
An archeological survey team gathers on this upland area. Standing in the middle of one of the flat upland areas between the canyons, it looks as though vast unbroken plains stretch in every direction. Yet less than a hundred yards away is a deep canyon. Photo by Steve Black.
An archeological survey team gathers on this upland area. Standing in the middle of one of the flat upland areas between the canyons, it looks as though vast unbroken plains stretch in every direction. Yet less than a hundred yards away is a deep canyon. Photo by Steve Black.
Prior to the introduction of sheep, goats, and cattle, the upland areas of the Lower Pecos had extensive grasslands like this restored area. During wet climatic intervals, the grasslands flourished and herds of buffalo migrated into the area. During one such interval about 2800 years ago, hundreds of buffalo were driven off the cliff above Bonfire Shelter. Photo by Phil Dering.
Prior to the introduction of sheep, goats, and cattle, the upland areas of the Lower Pecos Canyonlands had extensive grasslands like this restored area. During wet climatic intervals, the grasslands flourished and herds of buffalo migrated into the area. During one such interval about 2800 years ago, hundreds of buffalo were driven off the cliff above Bonfire Shelter. Photo by Phil Dering.
Flower-painted uplands after a wet winter. Photo by Steve Black.
Flower-painted uplands after a wet winter. Photo by Steve Black.
The towering flower stalks of the sotol plant are a common sight in the Lower Pecos. Sotol prefers thin rocky soils and steep terrain and often grows in great abundance in such areas. Prehistoric peoples harvested sotol hearts or "cabezas" (heads) in quantity and baked them in earth ovens. Photo by Phil Dering.
The towering flower stalks of the sotol plant are a common sight in the Lower Pecos Canyonlands. Sotol prefers thin rocky soils and steep terrain and often grows in great abundance in such areas. Prehistoric peoples harvested sotol hearts or "cabezas" (heads) in quantity and baked them in earth ovens. Photo by Phil Dering.