During the past 35 million years the Rio Grande rift (Figure 8), caused by tectonic plates in Earth’s lithosphere, moved apart and represents the eastern most example of “widespread stretching” in western United States. The rift consists of three large basins and many smaller basins [6]. The Albuquerque basin, the largest, spans 160 km from south to north and 86 km from west to east. The Albuquerque southern basin contains pre-rift volcanic deposits while the central and northern basins contain volcanic rifting eruptions [7]. The sediment fill of the basins consists primarily of mafic volcanic flows (Figure 9) and (Figure 10) and alluvial fans. During rifting these sediments were deposited and are known as the Santa Fe Group. This group contains sandstone, volcanics and conglomerates. Eolian deposits are present in some basins. The Colorado Plateau acts as a microplate and may explain the creation of the Rio Grande rift [8], as a 1 to 1.5 degrees rotation in a clockwise direction, of the Colorado Plateau relative to the North American craton. A craton is defined as a stable and old part of the continental lithosphere and includes the crust and uppermost mantle of ancient crystalline rock covered by younger sedimentary rock.

The Rio Grande rift [9] is a north-trending continental rift zone. The interior of North American craton in the east is separated from the Colorado Plateau in the west. The rift (Figure 8) extends from the state of Chihuahua, Mexico in south to central Colorado in north (Figure 1). The Rio Grande Rift has been important for a long term because it parallels a major river and provides the north-south route.

The White Sands area is in the Tularosa basin, which had lakes with calcium sulfate (Ca SO₄) in the water. The lake water eventually evaporated in the arid climate. The dried calcium sulfate (gypsum) crystals (less than 2 mm in size) were blown by the wind to the northeast corner of the dried lakebed and re-deposited in a series of sand dunes. In a moist climate, the gypsum would have been dissolved by the rainfall. However, rainfall in the White Sands area is less than 20 cm and dunes deposits were only crusted and are not totally dissolved (Figure 11) and (Figure 12).

Numerous Native American peoples have settled in New Mexico. Their history of settlements, both modern and ancient, is interwoven with the culture and history of the state. Approximately 1500 years ago, ancestral pueblo people migrated south in search of more reliable water sources and built the elaborate cliff dwellings at Bandelier, Mesa Verde and elsewhere across the Four Corners region. Native American peoples eventually established permanent settlements, commonly known as pueblos (Figure 13). They were the ancestors of today’s Pueblo people, many of whom still occupy the original villages. Other ancestor groups such as the Apache and Navajo continued their nomadic lifestyles.

A Puebloan Native American tribal entity settled in the Ysleta Del Sur Pueblo section in El Paso, Texas. The Puebloan group includes the Southern Tiwa people (Figure 14) who were displaced during the Pueblo Revolt against the Spaniards in 1680-1681. There are 23 Indian tribes located in New Mexico with nineteen Pueblos, three Apache tribes (the Jicarilla Apache Nation, the Fort Sill Apache Tribe, and the Mescalero Apache Tribe) of the Navajo Nation. The nineteen Pueblos are comprised of the Pueblos of Cochiti, Acoma, Jemez, Isleta, Nambe, Laguna, Ohkay, Picuris, Owingeh, Pojoaque, Sandia, San Felipe, Santa Ana, San Ildefonso, Taos, Santa Clara, Santo Domingo, Zia and Zuni.

Native American people, the Karankawa, were concentrated along the coast of the Gulf of Mexico in southern Texas. Several independent seasonal nomadic groups shared the same culture and language. The tribe included the groups called the Guapites (Coapites), Cocos, Cujanes, and Copanes. During the Texan-Mexican War the Karankawa played an important role in the relationships between the Spanish, French and English empires. Commissioned as a captain in 1825, Stephen Austin, who led volunteers, expelled the Karankawa from the Austin land grant. In subsequent years, the Texan colonists repeatedly attacked the Karankawa, who drove them out of their native lands. By the 1840s, the Karankawa split into two groups, one of which fled to the Mexican state of Tamaulipas and the other settled on Padre Island. In 1858, Juan Nepomuceno Coortina led a group of Texan colonists who killed the remaining members of the Karankawa tribe on Padre Island. The Karankawa, as an organized tribe, had become extinct by 1891.

The first Europeans to explore the Rio Grande were part of an expedition tasked to survey the coast of the Gulf of Mexico in 1519. The name Rio Bravo was first used in 1536 on a map by a royal explorers when Alvar Nunez Cabeza de Vaca traversed the Rio Grande in 1535-1536. In 1540, Francisco Vazquez de Coronado explored the upper Pecos areas and middle Rio Grande. The Rio Grande basin was explored as an agricultural and mining settlement from 16^th to 18^th centuries. Earliest settlements 1563 in upper Conchos watershed were mining communities. The second colonization was in Lower Rio Grande area in New Mexico was in 1598 and the last settlement was in 1749 along the Lower Rio Grande. The exploration of the Rio Grande Basin was completed in 1779 with Mexican exploration by Juan Bautista de Anza in the San Luis Valley.

After Mexico won their independence from Spain in 1821 which extended their land holdings to as far north as the Oregon Territory, the secession of Texas in 1836 created a southern border shift and marked the start of the loss of Mexican territory. In 1844, President James K. Polk, who ran for office on the pledge to “fulfill America’s ‘Manifest Destiny’ which would extend the United States from

the Atlantic Ocean to the Pacific Ocean.” The United States attempted to buy New Mexico and California for $30 million. After Mexico refused the American offer, relations deteriorated. President Polk then sent 4000 troops into the land south of the Nueces River (near current city of Corpus Christi) and north of the Rio Grande River an area claimed by both countries. The Mexican cavalry attacked the US troops and killed or wounded 16 American soldiers. The United States then declared war on Mexico. After a series of bloody battles, the American forces captured the Mexican capital in September of 1847.

Mexico recognized the United States annexation of Texas under terms of the Treaty of Guadalupe Hidalgo. Mexico eventually agreed to sell additional land including more than one-third of Mexico’s territory for $15 million dollars. The United States purchased more than 1,300,000 km² of land that would become the states of California, New Mexico, Arizona, Utah and Nevada as well as parts of the present day Wyoming, Colorado, Kansas and Oklahoma. Therefore, the Mexican-United States War of 1846 resulted in the second greatest purchase of land and second only to the Louisiana Purchase of 1803. The purchase of Alaska from the Soviet Union was probably the third largest purchase of land that became part of the United States.

The Rio Grande has been the international boundary since 1848. The modern border took shape following the American-Mexican War. While the Rio Grande formed the dividing line between Mexico and Texas (Figure 7). The border was originally a straight line west from El Paso to the Gila River and then another straight line from Gila River to the Pacific Ocean south of San Diego. Following the Gadsden Purchase of 1853, the borders of New Mexico and Arizona were moved further south from the Gila River. The boundary commission completed its work in the late 1850s.

In 1899, the Rio Grande gradually changed position and a channel was dug creating Cordova Island, which became a Chamizal dispute. In 1944, the Mexico and United States signed a treaty regarding the Rio Grande River. The United States designated the Rio Grande as one of American Heritage Rivers in 1997. A 100 m wide sandbar formed in mid-2001 at the mouth of the Rio Grande River and the river failed to drain to the Gulf for the first time. The sandbar was dredged but quickly reformed. In 2002, spring rains flushed the sandbar out to sea. By 2003, the river once again flowed into the Gulf of Mexico.

Mexico and the United States share the river under a series of IBWC agreements. The IBWC traces institution roots to 1889 and the most notable agreements were signed in 1906 and 1944. The use of United States water is regulated by the Rio Grande Compact between Colorado, New Mexico and Texas. The Rio Grande water was over-appropriated including the Abiquiu reservoir (Figures 15-17). The section from El Paso downstream through Oyinaga was named “The Forgotten River”.

During the American War in 1846, the United States government requisitioned and moved steamboats from the Mississippi and Ohio rivers (Figure 18) and (Figure 19). The steamboats provided transportation for the United States Army

during the invasion of Nuevo Leon and Monterrey via Camargo Municipality, Tamaulipas. The USACE recommended only small improvements to make the Rio Grande River passable to El Paso rather than a canal and dam system to enhance shipping. However, these small improvement recommendations were not acted upon or funded. The Rio Grande is still not navigable by cargo barges, ocean-going ships, or smaller passenger boats despite its length and name. It is barely navigable for small recreational boats and only in a few places. At its deepest point along the 1600 km Rio Grande River, it is only 18 m deep.

The greatest physical challenges to year-around navigation are obtaining additional sources of water and the 1146 m elevation difference between the Gulf of Mexico and El Paso and Matamoros, which is a distance of approximately 1600 km (gradient of 0.75 m per km). The distance is similar to the Ohio River, the Gateway to the West, which has a distance of 1500 km (gradient of 0.20 m per km) [5]. The number of locks and dams on the Ohio River has recently been reduced to 20 [5] and the flow rate is much higher reducing the need for feeder lakes. The Ohio River is not in an arid climate and its flow rate is much easier to maintain with limited use of feeder lakes. The Lower Rio Grande would probably require approximately 40 to 60 lock and dams to permit shipping from El Paso and Ciudad Juarez to the Gulf of Mexico.

The Brownsville and Matamoros International Bridge (Figure 20), a swing bridge, built in the 1910s, is still used by automobiles to connect the two countries. After the early 1900s, a period of big steamboats, the Rio Grande was no longer open for navigation. At one time, the bridge had rail traffic. In 2015 a bridge, West Rail International Crossing was built about 24 km west of the Brownsville and Matamoros International Bridge to connect the United States and Mexico by railroad.

At the mouth of the Rio Grande on the Mexican side is the large commercial Port of Bagdad (Tamaulipas) (Figure 21). During the American Civil War (1860 to 1865), this port was used by the Confederacy. European warships anchored off shore managed to avoid conflict with the United States Navy. The port is a shallow-draft river port with the deeper-draft cargo ships anchored off shore and the smaller vessels hauling cargo to and from the deep-draft ships. The shallow sandbar at the mouth of the river could not be crossed by the deeper draft cargo ships. At the Port of Tamaulipas, bales of cotton were traded for European military supplies.

The Rio Grande Valley includes a river delta, stretching from the Gulf of Mexico to just east of Ciudad Camargo a distance of 200 km. The remaining 1400 km of the Rio Grande valley is a large river expanse. The Rio Grande Delta is agriculturally productive. The largest population centers on the south side of the Rio Grande are Matamoros, 0.5 million, Ciudad Acuna, 216 thousand, Nuevo Laredo, 375 thousand, Piedras Negras, 163 thousand and Ciudad Juarez, 1.5 million. The estimated total population of the Rio Grande Valley is approximately 5 million. The northern side of the Rio Grande Valley has a U.S. population of 1.3 million according to the U.S. Census. The largest population centers include Brownsville (Figure 22) and El Paso. With a total population of nearly 6 million, the Rio Grande Valley is a prime candidate for economic development, which could be enhanced by creating a navigation channel to El Paso, a distance of 1600 km, which could be used to ship goods locally, regionally and internationally. The port at Brownsville would probably need to be expanded.

The available water supply in the Rio Grande is disappearing. The Elephant Butte Dam in central New Mexico gates are only opened for a short irrigation season [4]. However, the International Boundary and Water Commission forced the release of stored irrigation water to save cottonwood trees on the riverbanks [10] as part of habitat restoration. Approximately 95 percent of the Rio Grande’s average annual flow is claimed for municipal and agricultural use.

Because of growing water needs for the population in Albuquerque and El Paso and Ciudad Juarez (Figure 23) and increased frequency of drought extremes, the river can run dry. As annual snow packs shrink and evaporation rates increase, the upper Rio Grande watershed will collect 30% less water. The Rio Grande silvery minnow is an endangered species. Its habitat is limited to the middle sections of the Rio Grande River near Albuquerque, New Mexico (Figure 24). The Rio Grande has an average annual yield of 3.2 billion m³ of which only 1/3 reaches to the Gulf (Figure 25) prior to building the Falcon Dam, near Rio Grande City in 1953. The discharge is now 85 m³.

Irrigation (Figure 26) has been practiced in the Rio Grande basin since prehistoric times, mostly by the Pueblo Indians of Mexico. Increased population made water treaties (1905-1907 and 1944-45) between United States and Mexico necessary. There was a Rio Grande Compact in 1939 between New Mexico and Texas. Essentially all of the 37,000 ha·m⁻¹ of the upper Rio Grande water (including 7400 ha·m⁻¹ allotted to Mexico by treaty) is consumed in the basin.

The international Amistad Dam, located near the confluence of Devils River, under terms of United States-Mexico treaty, produces considerable amounts of electricity. Excluding areas irrigated above the reservoirs, the river provided more than half of the irrigated lands in Mexico. The river provides water for 850,000 ha of cropland. The leading irrigated crops vary from alfalfa and potatoes in Colorado to pecans grapes and cotton in the southern New Mexico. In El Paso, Texas and Juarez, Mexico regions, cotton, citrus fruits and vegetables are grown and would benefit from reliable shipping to the Gulf of Mexico.

After animal husbandry (Figure 27) and agriculture, the leading industries are mining for petroleum, natural gas, uranium ore, coal, gold, lead, silver, gypsum and potash that could be enhanced by barge shipping on the Lower Rio Grande River. Recreation activities (Figure 28) and (Figure 29) are included year-around state and national parks (Figure 23), monuments, dude ranches, rafting, fishing, ballooning (Figure 28) and hunting.

Water conservation practices already in place will need to be enhanced for both the irrigated agricultural areas and the urban areas. Demand for water is exceeding the availability and the situation is only going to get worse as the population of the entire Rio Grande valley in both the United States and Mexico continues to rapidly expand.

The Rio Grande was historically used for navigation and the shipping of agricultural products (Figure 30) and goods on the Lower Rio Grande River near Brownsville and Matamoros. However, the small-scale USACE proposal to improve navigation to El Paso and Ciudad Juarez was never funded or developed. There are millions of people living in the Rio Grande valley between Brownsville and Matamoros and Ciudad Juarez and El Paso. The low cost shipping of produce and goods would provide an economic boom to the Rio Grande Valley.

Another mitigation objective is to get the USACE to build a lock and dam system on the Lower Rio Grande, which would permit year-around shipping of produce and goods similar to the Ohio River lock and dam system [5]. The Ohio River is approximately 1500 km long and has an elevation change of 300 m between Pittsburgh, Pennsylvania and Cairo, Illinois. It has a gradient of 0.20 m per km and which currently requires 20 locks and dams. The other environmental obstacles are to having sufficient year around water to supply the lock and dam system during dry periods. This could require water conservation practices in addition to more reservoirs and possibly even a pipeline to transport river water with low salt content from a point near the mouth of the Rio Grande River to avoid high salt water content to the Upper Rio Grande River, a distance approaching 2000 to 3000 km similar to the oil pipeline in Alaska [11] which extends a distance of 2000 km. Without additional urban and agricultural conservation practices, the extra water would either be used or evaporated in the arid climate and might not change the flow volume in the Lower Rio Grande Valley.

Stretches of the Rio Grande including sections of the Rio Grande near Dunn Bridge, New Mexico and Borlon Canyon, Texas were designated a National Wild and Scenic River (Figure 6). The Rio Grande River borders the Big Bend National Park and two National Monuments.

Hundreds of kilometers of the Rio Grande River is sucked dry and redirected to flow into irrigation before it reaches El Paso. The river is replenished by the Mexico’s Rio Conchos just upstream from the 190 km S-curve of the Big Bend Park and enough water is available to feed a 307 km segment of the 1978 of the Wild and Scenic River. Big Bend Park’s Santa Elena and Mariscal canyon has 450 m of towering limestone walls (Figure 31).

The construction of the American border wall is contributing to the destruction of the Thornforest habitat. The American Forests have planted 2 million thornscub trees and other native plants. The Thornforests traditionally have drawn millions of bird watchers each year. The birds include the hawk, green jays and Altamira orioles. In the Lower Rio Grande Valley, ecotourism generated more than $340 million in 2014. However, COVID-19 and border wall issues have eliminated most of the bird watching. Forest fires are difficult to control in the dry climate and can result in significant trees loss (Figure 32) with soil erosion (Figure 33). The situation requires mitigation including stream erosion controls and stream bank stabilization and gully structures (Figure 34) and (Figure 35).

Following the American-Mexican War, citizens from both countries traveled freely across the border. There were no federal limits on immigration for decades. After the passage of the Chinese Exclusion Act of 1882, it was Chinese, not Mexican; immigrants that vigilante groups and American authorities first sought

to keep from illegally crossing the southern border. “One of the ways that immigrants from China would try to get across the border was to learn a few words of Spanish and disguise themselves as Mexican” [12].

“Restrictions on the movement of Mexican citizens were not particularly enforced by the U.S. government until the decade of the Mexican Revolution in the 1910s when large numbers of refugees came to escape the war and there was a large demand for Mexican labor,” [12]. In 1916, the United States started enforcing refugee movement restrictions following Mexican revolutionary Poncho Villa’s deadly raid on Columbus, New Mexico and the subsequent publication of the Zimmerman Telegram, proposing a World War I military alliance between Germany and Mexico. The United States deployed soldiers to patrol the boundary along with government-sanctioned “home guards” and Texas Rangers to tightened border security.

The border between the Mexico and United States is nearly 3200 km stretching from the Gulf of Mexico to the Pacific Ocean and borders California, New Mexico, Arizona and Texas. The longest section of the border is the 2020 km border east of El Paso, Texas (Figure 7) to Brownsville, Texas. This part of the international boundary is a natural geographic barrier. However, the river often dries

up but continues to serve as the official border. Most of the 1130 km of chain link, barbed wire, post-and-rail and wire mesh fencing and wall (Figure 36) have been built along the 1290 km Rio Grande west of El Paso [13]. United States plans after 2018 were to expand the fence and wall building effort along the Rio Grande River (Figure 37) between Brownsville and El Paso (Figure 36).