Rio Grande

• Category: Inland Aquatic Biomes.
• Geographic Location: North America.
• Summary: This river supports both humans and a diversity of biota from its headwaters in the southern Colorado Rockies to its mouth at the Gulf of Mexico, along the border of Texas and Mexico.

Originating as snowmelt east of the Continental Divide in the San Juan Mountains at Stony Pass, Colorado, the Rio Grande River travels approximately 1,900 miles (3,057 kilometers) to the Gulf of Mexico. The riparian environment of the Rio Grande biome encompasses close to 356,000 square miles (922,000 square kilometers), and this transnational basin covers portions of Colorado, New Mexico, and Texas in the United States; and Chihuahua, Coahuila, Nuevo Leon, and Tamaulipas in Mexico. The Rio Grande itself is also called Rio Bravo, mainly within Mexico.

Eight notable tributaries contribute to the flows of the Rio Grande: the Conejos River, Red River, Rio Chama, Jemez River, Rio Puerco, Rio Conchos, Pecos River, and the Rio de San Juan. Only the Colorado River exceeds the Rio Grande in size in the American southwest. However, much of the once-navigable, long river of alpine, desert, and coastal plains landscapes has been over-allocated for human purposes.

Geology, Geography, and Ecology

Within the high peaks of the San Juan Mountains in southwestern Colorado, the Rio Grande begins as meltwater near 12,500 feet (3,810 meters) in elevation. The primarily glacial topography here is marked by moraines, U-shaped valleys, and rugged montane spires. The subalpine to alpine vegetation shifts from aspen, pine, and willow trees to a mix of fir, pine, and spruce; and above the timberline, only tundra grasses pervade.

The majority of the Rio Grande High Country near the river’s source falls under the supervision of the U.S. Forest Service within the Rio Grande National Forest—a territory equivalent to the size of Rhode Island. Rainbow and brown trout flourish in the crystal headwaters of the river.

As the river flows east by southeast, it drains off of the uplift dome of the San Juan Mountains into the intermountain depression of the San Luis Valley—between the San Juan range and the Sangre de Cristo Mountains—at an altitude of 9,000 feet (2,743 meters). From central Colorado to northern Mexico, tectonic plate extension, or divergent force, resulted in the Rio Grande rift. The deepest part of the northern Rio Grande rift is located in the San Luis Valley. Both western yellow and piñon pine are prominent across the valley here. Shifting course from southeast to south, the Rio Grande approaches the Taos Plateau north of the Colorado-New Mexico border. Millennia of fluvial erosion formed the Rio Grande Gorge in the basalt-rich landscape of the Taos Plateau region.

Flanked by the Colorado Plateau to the west, and to the east a series of mountain chains—from the southern Sangre de Cristo Mountains to the Franklin Mountains—the river transitions from the Taos Plateau to the Bolson Section. Beginning north of Santa Fe in the Española Valley, the Bolson Section extends south into the Trans-Pecos region of Texas. Arroyos, sand dunes, short grasses, and scrub vegetation such as sagebrush, piñon, juniper, and cedar signal the aridity associated with this stretch of Rio Grande environs that reaches the northern Chihuahuan Desert. Cottonwood galleries line the banks of the river and compete with invasive salt cedars.

Ponderosa pine, aspen, spruce, and fir trees intermingle on forested mountain slopes of the northern midstream basin. The Bolson Section provides sanctuary for migratory waterfowl such as geese, ducks, and sandhill cranes. Additionally, the endangered silvery minnow resides in the waters of the Middle Rio Grande basin.

Extending from the Quitman Mountains to Redford, Texas, the Presidio Section of the Rio Grande is dominated by the very harsh conditions of the Chihuahuan Desert. The Canyon Section stretches from Redford to Del Rio, Texas, and likewise exists within the Chihuahuan Desert. Big Bend National Park is located in the Canyon Section, a series of three canyons—the Santa Elena, Mariscal, and Boquillas Canyons—that are carved by the great river. Persistent heat and aridity limit vegetation to desert varieties, except for the mesquite and salt cedars (tamarisk) that blanket the floodplain. Common desert flora includes: sotol, ocotillo, Spanish dagger (yucca), candelilla, ceniza (silverleaf), chino grass, cacti, and lechuguilla (agave).

At higher plain elevations, Mexican buckeye, Mexican walnut, desert willow, Fresno (ash), and Mexican persimmon thrive. In the Chisos Mountains, piñon pine, juniper, mountain mahogany, evergreen sumac, and bluestem and sideoats grama grasses coexist with desert flora.

The climate shifts from arid to semiarid near Del Rio, then transitions to subtropical as the river approaches the Gulf of Mexico within the Coastal Plains Section of the Rio Grande. Texas live oaks rise above mesquite, chaparral, prickly pear cactus, and wildflowers along the river in this stretch. Swamp cypress trees emerge in the marshlands. In the delta region of the Coastal Plains, native ebony trees accompany prickly pear.

The Lower Rio Grande Valley is home to the only wild Muscovy duck population in the United States. Local fauna also include the white-tailed deer and the ocelot, but the jaguarundi, which once populated the area and can be found in Mexico, is exinct in the United States.

Human Interaction

Humans have long been inter-related with Rio Grande ecosystems. In Sandia Cave, near Albuquerque, New Mexico, within the Cibola National Forest, archaeologists from the University of New Mexico’s department of anthropology discovered evidence of one of the earliest human settlements on the North American continent. Prior to the Sandia site, the Folsom culture of approximately 10,000 years ago was generally accepted as the earliest known civilization in this part of the world. However, geological evidence dates the Sandia remnants as slightly older than the Folsom artifacts. The faunal remains in the cave include sloth, wolf, horse, bison, camel, mastodon, and mammoth, and illustrate the diversity of the archaic Rio Grande biota.

The river has played a central role in geopolitical shifts in this region of North America. After more than two centuries under the colonial authority of New Spain, the Rio Grande basin transferred to Mexican control following the 1821 Mexican Revolution. However, the basin quickly became a contested zone and, in 1836, the Texas Republic claimed the Rio Grande as incorporated territory. Subsequent to the annexation of Texas to the United States and the U.S.-Mexican War, the Rio Grande—from El Paso to the Gulf of Mexico—was negotiated as the international boundary between the two countries as a condition of the 1948 Treaty of Guadalupe Hidalgo.

Agriculture and domestic livestock ranching involving cattle, sheep, goats, horses, hogs, and chickens command the greatest amount of Rio Grande basin resources, and have prompted significant ecological shifts. Considered a Mesoamerican transplant, corn has been cultivated for many centuries by Native Americans settled along the Great River, or Rio Grande. Likewise, for centuries humans have diverted water from the river to irrigate crops. Although the origin of acequias, or ditch irrigation networks, is debated by indigenous and Spanish descendants, the gravity-fed, traditional water-harvesting method supports the cultivation of native agricultural products and foreign cultigens including chiles, onions, potatoes, wheat, barley, oats, lettuce, watermelon, fruit trees, pecans, tomatoes, cotton, and alfalfa.

Environmental Issues

Since the mid-19th century, increased settlement in Rio Grande valleys has exhausted the natural flows of the river, and the competition among jurisdictions on both sides of the international border has exacerbated the limited availability of Rio Grande water in a predominately arid region. Interstate and international pressure for an equitable distribution of the river’s volume resulted in the Mexican Treaty of 1906 and the Rio Grande Compact of 1938. The 1906 negotiation guaranteed Mexico 60,000 acre-feet (74 million cubic meters) of annual water to its canal near El Paso, and the 1938 agreement apportioned water among the states.

The construction of Elephant Butte Dam north of Truth or Consequences, New Mexico, was intended to facilitate the agreements and provide for continued development in the agricultural valleys above and below the dam. The U.S. government lifted a ban on new Rio Grande dams in 1907, which resulted in the construction of the Rio Grande and Abiquiu reservoirs and the Cochiti, Caballo, Amistad, and Falcon Dams. As a result, the river intermittently stops flowing at various points between El Paso and the delta, and the flora and fauna of the Rio Grande biome are constantly stressed.

Twentieth- and 21st-century environmental activism has alleviated some of the ecological impacts caused by human modification of Rio Grande ecosystems. Near the river’s source, the Rio Grande Headwaters Restoration Project devised a plan to mitigate the impacts of stream-bank erosion, flooding, endangered species, and invasive plants. Elk have returned to riparian areas where grasses and willows have rebounded. Midstream improvements include flow meters to promote water conservation, reintroduction of native flora to compete with invasive salt cedars, nurturing endangered species such as the silvery minnow back to healthy numbers, and controlled over-bank flooding to support native flora and fauna.

The U.S. Fish and Wildlife Service funds many of the local restoration projects. In the Lower Rio Grande Valley, the National Park Service hires workers to eliminate salt cedars in Big Bend National Park; the U.S. Department of Agriculture, in cooperation with the World Wildlife Fund, also provides resources to eradicate the destructive salt cedars in the Presidio Section of the river and to hem in the giant cane (Arundo donax) downstream from Big Bend. The University of Texas at El Paso developed and supervised a 372-acre (150-hectare) recovery of the Rio Grande bosque, or riparian gallery forest, a wetlands preserve for native vegetation, Mexican free-tailed bats, and 221 bird species.

Rising temperatures due to climate change have impacted the river, causing some parts of it to dry up. A portion of the Rio Grand near Albuquerque went dry in 2022 due to climate change and has since struggled to regain previous water levels, despite conservation efforts and reserves. The hot, dry summer weather has caused the decline in water level in the 100-mile (161-kilometer) stretch of the river. Water was rationed in the area, and federal and local agencies rescued large numbers of minnows stranded in the remaining puddles of water. Scientists fear that if the effects of climate change are not mitigated, in the future the entire Rio Grand may be gone, taking with it the many plants and animals that live within and nearby it while negative affecting farming in the area.

Bibliography

Armo, Tisi. “Tamarix Ramosissima - Salt Cedar.” Invasive Database, 25 July 2023, www.texasinvasives.org/plant‗database/detail.php?symbol=TARA. Accessed 23 Jan. 2025.

Baxter, David. Big River, Rio Grande. University of Texas Press, 2009.

Belcher, Robert C. The Geomorphic Evolution of the Rio Grande. Baylor University Press, 1975.

“Climate Change Connections: New Mexico (Rio Grande).” Environmental Protection Agency, 16 Jan. 2025, www.epa.gov/climateimpacts/climate-change-connections-new-mexico-rio-grande. Accessed 23 Jan. 2025.

Finch, Deborah M., and Joseph A. Tainter. Ecology, Diversity, and Sustainability of the Middle Rio Grande Basin. U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, 1995.

Fischer, Fallon. “Is the Rio Grande Expected to Go Dry in Albuquerque This Year?” KRQE, 12 Sept. 2024, www.krqe.com/news/albuquerque-metro/rio-grande-could-soon-go-dry-in-albuquerque-water-managers-say/. Accessed 23 Jan. 2025.

Gilpin, Laura. The Rio Grande: River of Destiny. Duell, Sloan, and Pearce, 1949.

Kornfield, Meryl. "Rio Grande Runs Dry in Albuquerque for the First Time in 40 Years." The Washington Post, 22 July 2022, www.washingtonpost.com/climate-environment/2022/07/22/rio-grande-drought/. Accessed 23 Jan. 2025.

Maxwell, Ross A. The Big Bend of the Rio Grande: A Guide to the Rocks, Landscape, Geologic History, and Settlers of the Area of Big Bend National Park. University of Texas, 1968.

Robbins, Jim. "A Warming Climate Takes a Toll on the Vanishing Rio Grande." Wired, 25 June 2022, www.wired.com/story/a-warming-climate-takes-a-toll-on-the-vanishing-rio-grande/. Accessed 23 Jan. 2025.