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The Rio Grande ( or ; Río Bravo del Norte, or simply Río Bravo) is one of the principal rivers in the southwest United States and northern Mexico (the other being the Colorado River). The Rio Grande rises in south-central Colorado in the United States and flows to the Gulf of Mexico, passing through three U.S. states and four Mexican states. Its total length was 1896 mi in the late 1980s, though course shifts occasionally result in length changes. Depending on how it is measured, the Rio Grande is the fourth- or fifth-longest river system in North America. The lower 1254 mi of the Rio Grande, from El Paso–Juárez to Brownsville–Matamoros, form a significant portion of the Mexico–United States border.

The Rio Grande's watershed, while the fifth largest in North America at 335000 mi2, is mostly semi-arid or desert, and the natural discharge of the river is quite low for a river of its length. However, the river is prone to heavy flooding from both spring snowmelt and summer monsoon storms. The Rio Grande is often the only source of water for many miles around the desert valleys and canyons through which it flows, supporting a rich riparian zone in its floodplain. Today, the Rio Grande is an important source for irrigation and drinking water for the 14 million people who live along the river and its tributaries, especially in the heavily populated Lower Rio Grande Valley along the international border between Texas and Tamaulipas – as a consequence, the volume of its flow that reaches its delta and the sea has been greatly reduced over the past century.

Inhabited as early as 13,000 years ago by Paleo-Indians, the Rio Grande basin has been home to a diverse array of Native American groups, including the Puebloans of central New Mexico. The first contact between indigenous people and Europeans occurred in the 1500s with the arrival of Spanish explorers. In the 1600s, the region was settled by the Spanish, leading to brutal conflicts with Native Americans, before becoming part of Mexico after its independence from Spain in 1821. The Texas Revolution in 1836 preceded the Mexican–American War which resulted in the northern Rio Grande basin becoming part of the United States and the Rio Grande being established as the present international border. Throughout the 19th century, the Rio Grande country was considered one of the most lawless and remote parts of the American frontier. However, by the late 1800s the population of settlers along the Rio Grande had greatly increased on both sides of the border, causing conflict of water rights between the US and Mexico.

Treaties signed in 1906 and 1944 divided up the Rio Grande between the United States and Mexico. A number of large reservoirs and smaller diversion dams were subsequently built to control floods and distribute water for irrigation. Since damming, the Rio Grande has essentially been split in two – about 90 percent of water from the Upper Rio Grande river in the US is consumed before it even reaches the Mexican border. The 200 mi between El Paso–Juárez and the Rio Conchos, commonly known as the "Forgotten Reach", are dry except during large storms. Along the Lower Rio Grande, equally intensive water use has reduced the Rio Grande to 20 percent of its natural size by the time it reaches the Gulf. During droughts, it has sometimes failed to reach the sea at all. Water management remains a contentious issue today, despite the creation of a binational agency, the International Boundary and Water Commission (IBWC), to oversee the supply.

In addition to water supply, the Rio Grande is a central ecological, scenic and recreational resource in the states it flows through. The riparian forests and wetlands along its course have been much reduced due to development, but still support many species of fish, birds and amphibians, some endemic. The river's rugged canyons, and protected areas such as Big Bend National Park in Texas, are popular tourist destinations for hiking and whitewater rafting. Its many reservoirs support fisheries and water-based recreation that otherwise would not be possible in this desert region.

Names and pronunciation
Río Grande is Spanish for "Big River" and Río Grande del Norte means "Great River of the North". In English, Rio Grande is pronounced either or. Because río means "river" in Spanish, the phrase Rio Grande River is redundant.

In Mexico, it is known as Río Bravo or Río Bravo del Norte, bravo meaning (among other things) "furious" or "agitated".

Historically, the Pueblo and Navajo peoples also had names for the Rio Grande/Rio Bravo:


 * mets'ichi chena, Keresan, "Big River"
 * posoge, Tewa, "Big River"
 * paslápaane, Tiwa, "Big River"
 * hañapakwa, Towa, "Great Waters"

The four Pueblo names likely antedated the Spanish entrada by several centuries.


 * , Navajo, "Female River" (the direction south is female in Navajo cosmology)

Rio del Norte was most commonly used for the upper Rio Grande (roughly, within the present-day borders of New Mexico) from Spanish colonial times to the end of the Mexican period in the mid-19th century. This use was first documented by the Spanish in 1582. Early American settlers in South Texas began to use the modern 'English' name Rio Grande. By the late 19th century, in the United States, the name Rio Grande had become standard in being applied to the entire river, from Colorado to the sea.

By 1602, Río Bravo had become the standard Spanish name for the lower river, below its confluence with the Rio Conchos.

Course
Since the early 1900s, the Rio Grande has been traditionally divided into two sections, with the Upper Rio Grande extending from the Colorado headwaters to Fort Quitman, Texas, and the Lower Rio Grande from there to the Gulf of Mexico. This is done for convenience of water management, as the water in the Upper Rio Grande (originating mostly in Colorado) is almost totally consumed by irrigation and urban diversions, while the water in the Lower Rio Grande originates mostly from northern Mexico. As a result, with the exception of occasional floods, the Rio Grande today is essentially two hydrologically separate river systems. The length of the Upper Rio Grande is about 723 mi and the Lower, 1173 mi. To avoid confusion, this definition will be used for the purposes of this article.

The river is also divided into Upper, Middle and Lower sections within the individual states it flows through. In New Mexico nomenclature, "Upper Rio Grande" is the part of the river extending from the Colorado–New Mexico border to Otowi Bridge near Espanola; the "Middle Rio Grande" from Otowi Bridge to Elephant Butte Dam, and "Lower Rio Grande" from Elephant Butte to the New Mexico–Texas border. In Texas, the "Upper Reach" of the Rio Grande refers to that section between the New Mexico–Texas border and Fort Quitman, and the "Lower Reach" from Fort Quitman to the Gulf of Mexico. The Lower Reach is further divided into the Upper Rio Grande between Fort Quitman and Amistad Dam, the Middle Rio Grande from Amistad to Falcon Dam, and the Lower Rio Grande from Falcon to the Gulf.

Upper Rio Grande
The Rio Grande rises in the San Juan Mountains, in the western part of the Rio Grande National Forest in the U.S. state of Colorado. The river is formed by the joining of several streams at the base of 13478 ft Canby Mountain just east of the Continental Divide. The upper 100 mi of the Rio Grande are a clear, swift mountain stream, flowing eastward past Creede through a series of alpine valleys. The river emerges from the mountains at South Fork where it is joined by the South Fork Rio Grande and then continues eastward, past Del Norte, into the San Luis Valley. Despite its semi-arid climate and high elevation, the San Luis Valley is one of Colorado's main agricultural regions, with over 600000 acre irrigated using Rio Grande water. Oftentimes during the summer the river is reduced to a trickle, 10 percent of its original flow or less, by the time it reaches Alamosa in the southern part of the valley.

Past Alamosa the Rio Grande flows south, through the Alamosa National Wildlife Refuge and picking up the Alamosa and Conejos Rivers. It then enters New Mexico, where it traverses the Rio Grande del Norte National Monument and forms the deep Rio Grande Gorge. West of Questa it receives the Red River before entering the narrowest part of the gorge, the Taos Box, which is known for its 18 mi of difficult whitewater rapids. Downstream it is spanned by the steel arch Rio Grande Gorge Bridge, the 7th highest in the United States, which rises some 650 ft above the Rio Grande.

The Rio Grande receives the Rio Pueblo de Taos west of Taos and turns southwest, entering the Espanola Valley where the San Juan, Santa Clara and San Ildefonso Indian Reservations are located. Here it is joined from the west by the Rio Chama, the largest tributary of the Upper Rio Grande, which drains a large portion of the Carson National Forest. Below the valley the Rio Grande flows through White Rock Canyon along the eastern side of the Jemez Mountains before being stopped at the Cochiti Dam, which creates Cochiti Lake 20 mi west of Santa Fe. Designed to contain floods from the upper Rio Grande, Cochiti Dam is one of the largest earth-filled dams in the world.

From Cochiti the Rio Grande travels into the desert, flowing generally south by southwest through the Rio Grande Rift, a series of large, sediment-filled basins linked by narrow canyons through which the Rio Grande runs. The river first enters the Albuquerque Basin, where it is joined by the Jemez River from the west. It flows through the Albuquerque metropolitan area, the largest city in New Mexico. Both above and below Albuquerque the river is used heavily for irrigation, with the Middle Rio Grande Conservancy District managing the water supply. Despite channel modifications intended to control flooding, the Rio Grande supports a fragile bosque, or riparian forest ecosystem on the remaining part of its floodplain. From Albuquerque downstream the Upper Rio Grande has no perennial tributaries and is often the sole source of water in the arid desert of central New Mexico. This is the driest part of the entire Rio Grande basin with annual evaporation rates exceeding precipitation by ten times or more.

The Rio Grande receives the intermittent Rio Puerco at Contreras and the Rio Salado a short distance downstream at San Acacia. Below San Antonio the river flows freely until it reaches Elephant Butte Lake – the largest body of water in New Mexico – created by the Elephant Butte Dam near Truth or Consequences. Below Elephant Butte is Caballo Lake, formed by Caballo Dam. Both reservoirs store irrigation water for the Rio Grande Project, managed by the U.S. Bureau of Reclamation. From Caballo Dam, the Rio Grande flows south-southeast through the fertile agricultural region of Mesilla Valley, past Las Cruces; from Anthony it runs for roughly 20 mi along the border of New Mexico and Texas.

The river reaches the United States–Mexico border at the cities of El Paso, Texas and Ciudad Juárez, Chihuahua, which face each other across the Rio Grande. Below the International Diversion Dam near downtown El Paso, the Rio Grande is often dry – with 90 percent of its flow diverted in the US only a small fraction of its water flows to Mexico by treaty. With the exception of occasional large floods, the 200 mi Rio Grande border segment between El Paso–Juárez and the confluence of the Rio Conchos is usually a small stream or completely dry. It is commonly known as the "Forgotten Reach" because it was essentially ignored when the US and Mexico divided the Rio Grande's water in the early 1900s. The Upper Rio Grande officially terminates at Fort Quitman, Texas, where the last of the water from the upper reach is consumed.

Lower Rio Grande
Below Fort Quitman, the normally dry Rio Grande channel travels in a generally southeast direction through the Chihuahuan Desert, a remote region with only a few small settlements. At Ojinaga, Chihuahua the Rio Grande is joined from the south by its largest tributary, the Rio Conchos. Carrying runoff from the Sierra Madre Occidental in southwest Chihuahua, the Conchos essentially "revives" the Rio Grande, providing almost all the flow at their confluence except in wet years, as well as 35 to 40 percent of the total surface flow in the entire lower section of the Rio Grande. Below the Conchos the Rio Grande flows for about 100 mi southeast towards Big Bend National Park, where it forms the formidable 1500 ft deep Santa Elena Canyon, and the Mariscal and Boquillas Canyons.

The Rio Grande swings sharply northeast at the namesake "Big Bend" and flows a further 80 mi in that direction, along the eastern edge of the Sierra del Carmen, forming the border between Texas and Coahuila. In Texas this rugged region, known for its dynamic landscape and high mountains that create sky islands above the Chihuahuan desert, is known as the Trans-Pecos. The Lower Canyons of the Rio Grande, which extend for some 150 mi from La Linda, Coahuila to Langtry, Texas, traverse wild and isolated country with no towns or major roads.

At Langtry, the Rio Grande begins to widen into Amistad Reservoir, the largest of two major reservoirs on the river's border segment. The Amistad Dam is operated by the International Boundary and Water Commission, the joint agency formed by the US and Mexico to manage water in the agricultural region of the lower Rio Grande Valley and other places along the international border. The Rio Grande is joined in Amistad Reservoir by the Pecos River, its longest tributary, which flows for 926 mi through New Mexico and Texas. Despite its extensive watershed, the Pecos contributes a relatively small flow to the Rio Grande for most of the year, in part because most of its water is diverted for irrigation. However, the Pecos is also prone to irregular, but extremely large flash floods. The spring-fed Devils River also joins the Rio Grande in the reservoir, less than a mile (1.6 km) above the dam.

The Lower Rio Grande below the Amistad Dam is completely regulated in contrast to the wild river upstream, with the exception of large floods caused by hurricanes or monsoon storms. It flows in a generally southeast direction across the Texas Hill Country, passing Ciudad Acuña, Coahuila–Del Rio, Texas, and is joined by the Río San Diego from the west at Jiménez, Coahuila and then the Río San Rodrigo, also from the west, near Quemado, Texas. It then reaches Piedras Negras, Coahuila–Eagle Pass, Texas, where it receives the Rio Escondido. Further downstream, a short 10 mi segment of the Rio Grande forms the border between Texas and Nuevo León. From the ghost town of Islitas, Texas to the Gulf, it forms the boundary between Texas and Tamaulipas. After passing through Laredo, Texas–Nuevo Laredo, Tamaulipas the Rio Grande is joined by the Rio Salado in Falcon International Reservoir, created by the Falcon Dam near Nueva Ciudad Guerrero, Tamaulipas. Falcon is the last major reservoir on the Rio Grande and stores water for irrigation of the Rio Grande Valley, which extends from Falcon Dam to the Gulf of Mexico.

The Rio Grande Valley is not an actual geological valley but rather the floodplain of the Rio Grande as it meanders its final 150 mi across the Gulf Coastal Plain to the sea. Below Falcon Dam the Rio Grande turns east by southeast, past Rio Grande City, Texas where it is joined by the San Juan River. The river then flows between Reynosa, Tamaulipas–McAllen, Texas, where it encounters the Anzalduas Dam which diverts water for irrigation in both the US and Mexico. Below Reynosa–McAllen the much reduced Rio Grande continues for 16 mi to Retamal Dam, which serves for flood control. The dam regulates water flow into a system of floodways (bypasses) on both sides of the border, protecting Matamoros, Tamaulipas–Brownsville, Texas, the furthest downstream of the major urban areas on the Rio Grande.

The river terminates in a small sandy delta on the Gulf of Mexico, between the northern Laguna Madre in the US and the southern Laguna Madre in Mexico. The delta is believed to be of relatively young geological origin, forming only about 7,000 years ago when the Rio Grande established its present course to the Gulf. Because heavy water consumption in upriver states has diminished the flow and sediment load of the Rio Grande, the delta is slowly shrinking due to wave erosion. The reduction of flow is such that the Rio Grande, in recent decades, occasionally fails to reach the sea. During portions of 2001 and 2002, the mouth of the Rio Grande was blocked by a sandbar which was subsequently cleared by higher flows in 2003.

Watershed
With a total drainage area of 335600 mi2 in three U.S. and five Mexican states, the Rio Grande has the fifth largest watershed in North America. The watershed is approximately evenly divided between the two countries, with 54.7 percent in the United States. In the US, the Rio Grande watershed drains south-central Colorado, much of central New Mexico and southwest Texas; in Mexico it drains eastern Chihuahua and Coahuila, northern Nuevo Léon, and the northern edges of Tamaulipas and Durango. (The portion of Durango within the Rio Grande basin is the only state the Rio Grande itself does not touch or flow through; rather, it is drained by the Rio Florido, a tributary of the Conchos.) The area considered as the Rio Grande watershed includes a number of endorheic (closed) and semi-closed basins situated within or adjacent the basin that rarely, if ever, contribute water to the river. The actual area contributing water flow to the Rio Grande is only 173600 mi2, or 52 percent of the whole.

The Rio Grande rises in high mountains and flows for much of its length at high elevation; El Paso, Texas is 3762 ft above sea level. For the majority of its course below Espanola, New Mexico the Rio Grande flows through the arid Chihuahuan Desert, with the gallery forests along its floodplain often being the only significant vegetation for miles around, except in places with irrigated agriculture. For a significant portion of its length the Rio Grande flows through remote and inhospitable canyon country. Climate conditions vary widely along the Rio Grande, but with the exception of high mountain regions the basin is generally very dry. The part of the Rio Grande basin in Colorado has a cold temperate climate, the New Mexico reach has a cold to mild desert climate, and the Texas–Mexico border segment ranges from warm desert to subtropical. Annual rainfall is 8.5 in at Albuquerque, 8.6 in at El Paso, and 22.7 in at McAllen, Texas. Creede, Colorado, near the river's headwaters, receives an annual 13.5 in of rainfall and 47 in of snow.

The river's two major valleys, in central New Mexico and Southern Texas/Northern Mexico, are both commonly called the Rio Grande Valley. To avoid confusion, the New Mexico valley is often called the Middle Rio Grande Valley, while the valley shared by Texas and Mexico is called the Lower Rio Grande Valley. Other major valleys are the San Luis Valley in Colorado and the Mesilla Valley shared by New Mexico and Texas. The Lower Rio Grande Valley is the most densely populated region along the Rio Grande, with over 1.3 million people on the US side alone as of 2012. However, it also ranks among the poorest regions of the US, in part due to large amounts of illegal immigration. With five major cross-border metropolitan areas, it is a central point of trade between the United States and Mexico. The border segment of the river has doubled in population every two decades since 1960. The total population of the Rio Grande basin, including both the US and Mexico, was about 14 million in 1998, with an average density of 6 people per square mile (16/km2).

The vast majority of the Rio Grande basin consist of undeveloped desert and rangeland. The basin is about 43 percent shrubland and desert shrubland; 31 percent grassland and desert grassland; 14 percent forest, 7 percent urban and 5 percent agriculture. Due to the climate, dryland farming is impossible in the Rio Grande basin. However, more than 2 million acres (8,000 km2) of land are irrigated using Rio Grande water. The Rio Grande watershed is bounded on the west by the San Juan Mountains, the Jemez Mountains, and the Black Range in the United States, and in Mexico by the Sierra Madre Occidental. To the east it is bounded by the Sangre de Cristo Mountains (the southernmost range in the Rocky Mountains), and the Mescalero Escarpment (the western edge of Texas' Llano Estacado). Several mountain ranges run down the middle of the watershed, between the Rio Grande and Pecos River basins, including the Manzano Mountains, San Andres Mountains, Organ Mountains and Sacramento Mountains. Between the San Andres and Sacramento Mountains lie the endorheic Tularosa Basin, home to White Sands National Monument. In Mexico, the Sierra Madre Oriental form part of the southern boundary of the Rio Grande watershed.

Major cities in the Rio Grande basin not along the main stem of the river include Chihuahua City and Delicias, Chihuahua; Monterrey, Nuevo León; Monclova, Coahuila; and Roswell, New Mexico.

http://www.harcresearch.org/sites/default/files/Project_Documents/Rio%20Grande%20Apr15.pdf

About the Rio Grande

Tributaries
The largest tributary of the Rio Grande by discharge is the Río Conchos, which contributes almost twice as much water as any other. In terms of length and drainage basin size, the Pecos River is the largest. The Rio Grande's main tributaries are listed below from the mouth upstream.



https://books.google.com/books?id=ErkxBgAAQBAJ&pg=PA613&lpg=PA613

Natural flows
In terms of flow, the Rio Grande hardly resembles its natural state before large scale water development began in the late 1800s. The average virgin flow from the Upper Rio Grande Basin in the U.S. is about 2.5 million acre feet (3.1 km3) or 3400 cuft/s, but today the average flow past Fort Quitman, which marks the end of the Upper Rio Grande, is only about 140,000 acre feet (0.17 km3). This latter figure is the average of occasional wet years, as the Rio Grande below Fort Quitman is often dry.

Between Fort Quitman and Amistad Dam, unimpaired annual inflows from the American side – including the Pecos and Devils rivers – are about 698000 acre feet, and from the Mexican side (mainly the Río Conchos) about 837000 acre feet. From Amistad Dam to Falcon Dam, inflows from the US side are 70000 acre feet, and from the Mexican side 819000 acre feet. Between Falcon Dam and the mouth natural inflows are mainly from the Mexican side at the rate of 509000 acre feet annually.

As a whole, a total of 3.28 million acre feet (4.05 km3), or 60 percent, of the Rio Grande flow originates in the United States, and 2.17 million acre feet (2.67 km3), or 40 percent, originates in Mexico. The total annual runoff in the Rio Grande basin is in the vicinity of 5.45 million acre feet (6.72 km3), or about 7500 cuft/s. However, even in historical times not all this water reached the sea due to transpiration by riverbank vegetation and natural evaporation that the Rio Grande would experience during its long desert run to the Gulf of Mexico.

Seasonal runoff
Over its 1800 mi journey the Rio Grande experiences widely varying climates, and thus the Upper and Lower Rio Grande flood (or historically would flood) at different times of the year. Due to dams and diversions, very few stretches of the Rio Grande below the Colorado headwaters experience the seasonal runoff patterns they did in the past. These floods sustained the riparian ecosystem for up to miles on either side of the river. For example, the flood plain of the Rio Grande at Mesilla Valley near Las Cruces, New Mexico was up to 8 mi wide and would be mostly under water during a wet spring.

The Upper Rio Grande is essentially a snowmelt-driven system, with snow accumulating in the Rocky Mountains from December and April, and the peak of the melt between May and June. Historically, the snowmelt would come in one big rush in the late spring, with the majority of the year's water flow occurring in a roughly 40-day period. About 75 percent of the water in the upper Rio Grande is estimated to come from snowmelt. Before the construction of dams, it was not uncommon for the Rio Grande to reach flows of 20000 cuft/s in the spring as measured at Otowi Bridge, 74 mi above Albuquerque. The balance of the water comes from monsoon storms, which mostly occur in the late summer in the southern half of New Mexico.

The Lower Rio Grande receives most of its runoff from monsoon precipitation between August and October. Being a predominantly rain-fed system, it is more flashy than the Upper Rio Grande, prone to massive but relatively brief floods. The largest flooding events occur during hurricanes that strike the southern Gulf Coast, which have historically led to peaks on the lower river of 1000000 cuft/s or more – twice the average flow of the Mississippi River. In the Trans-Pecos region of West Texas, the river also takes in a significant volume of water from local artesian springs, giving it a substantial base flow during the rest of the year when the region receives little if any rain.

http://www.cdri.org/uploads/3/1/7/8/31783917/final_chapter_5_gutierrez.pdf

The U.S. Geological Survey maintains 19 stream gauges along the Upper Rio Grande, and six on the Lower Rio Grande, to monitor the river's flow rate, water level, temperature, turbidity or other factors. A further 18 stream gauges along the Lower Rio Grande are operated by the International Boundary and Water Commission, with fourteen on the US side and four on the Mexican side.

Snow

Geology
The sedimentary basins forming the modern Rio Grande Valley were not integrated into a single axial river system draining into the Gulf of Mexico until relatively recent geologic time. Instead, the basins formed by the opening of the Rio Grande rift were initially endorheic basins, or bolsons, with no external drainage and a central playa. An axial river existed in the Espanola Basin as early as 13 million years ago, reaching the Santo Domingo Basin by 6.9 million years ago. However, at this time, the river drained into a playa in the southern Albuquerque Basin where it deposited the Popotosa Formation. The upper reach of this river corresponded to the modern Rio Chama, but by 5 million years ago, an ancestral Rio Grande draining the San Juan Mountains had joined the ancestral Rio Chama.

The ancestral Rio Grande progressively integrated basins to the south, reaching the Palomas basin by 4.5 million years, the Mesilla basin by 3.1 million years, to Texas by 2.06 million years, and finally joining the Pecos River at 800,000 years to drain into the Gulf of Mexico. Volcanism in the Taos Plateau reduced drainage from the San Luis Basin until a spillover event 440,000 years ago that drained Lake Alamosa and fully reintegrated the San Luis Basin into the Rio Grande watershed.

Native Americans
Human habitation along the Upper Rio Grande dates to at least the Paleo-Indian period (11000–7000 BC), when people of the Clovis culture lived in the region,, but archaeological evidence from this early period is scarce. The Rio Grande valley in New Mexico was populated beginning in the 1200s–1400s AD by the Puebloan peoples, who had previously lived in areas of the Colorado Plateau such as the cliff dwellings of Chaco Canyon. Severe droughts caused by climate change forced them to abandon their traditional lands and migrate to the Rio Grande where water remained relatively abundant. These people are today divided into two language groups, Keresan and Tanoan.

The headwater region of the Rio Grande saw the arrival of humans in the Archaic period, with artifacts recovered in the northern San Luis Valley dating from at least 8000 BC. In the few centuries prior to the arrival of Europeans, the headwaters were visited at various times by the Ute, Navajo, Apache, Kiowa and Comanche people, but unlike groups further downstream, they practiced a hunter-gatherer lifestyle.

By the 1500s, Puebloans along the Upper Rio Grande had built a relatively advanced society based around irrigated agriculture. Although they did not have the engineering technology necessary to control water flows from the Rio Grande, they took advantage of the river's annual floods to irrigate some 20000 acre of the Rio Grande valley in present day New Mexico. The adobe pueblos for which the Puebloans were known and named for by Spanish explorers were large villages, often comprising many multi-story dwellings, built on high ground above the floodplain. A common architectural feature among many pueblos were kivas, large circular chambers built partly or wholly underground which were the center of religious life in the pueblos.

The highest concentration of pueblos – many of which are still prominent Native American communities today – is along the east side of the Rio Grande north of Santa Fe and west of the Sangre de Cristo Mountains. These include the well known Taos Pueblo, whose area was first settled around 900 AD but whose present-day adobe structures were constructed around 1400 AD. Taos was an important trading center for Native Americans not just of the Rio Grande pueblos but for the Plains Indians to the north.

The Lower Rio Grande was also certainly inhabited starting in the Archaic period, starting 8000–6000 BC, though some archaeological sites in southern Texas have been dated back to the Paleoindian period. Closer to modern times the area would be populated by the Coahuiltecans, Comanche and Lipan Apache. It is uncertain whether these peoples were descended from Archaic ancestors or migrated into the region later from the Great Plains, but evidence from the 1300s–1400s AD indicates the introduction of technologies such as bows and stone knives, which are characteristic of Plains tribes.

Prior to European contact, the Gulf Coast region surrounding the Lower Rio Grande is believed to have been populated by over 60 different tribal groups, with 49 of those in the Rio Grande delta alone in the mid-1700s. Unlike the people along the Upper Rio Grande, Native Americans in the delta region practiced no agriculture, because the extensive natural wetlands, freshwater lagoons and the Rio Grande's ecologically diverse estuary provided enough plants and game to meet their needs. Villages were small, generally composed of 120 to 300 people, each with a foraging territory of varying size. They were densely packed in the Rio Grande Valley but spread further apart inland, where the environment was drier and "they were forced to utilize almost every edible plant and animal food available" The term Coahuiltecan, often used to describe peoples of this region, is a blanket term coined in the 19th century long after the indigenous peoples had either been extirpated by colonists or assimilated into colonial society, and does not refer to any specific group.

http://www.ce.utexas.edu/prof/mckinney/ce385d/riogrande/landisfinalreport.pdf

https://riograndeguardian.com/acevedo-a-brief-history-of-the-rio-grande-valley-part-i/

San Luis Valley

San Luis Valley Project - USBR

Forgotten River

Wild West

IBWC

Spanish exploration
What is traditionally known as the Spanish entrada began circa 1535 when the explorer Álvar Núñez Cabeza de Vaca and his three companions arrived in the coastal region of the Rio Grande. The four were survivors of the ill fated 1527 Narváez expedition commissioned by Emperor Carlos I to explore what is now the southeastern United States. After most of the expedition's members had perished from starvation, disease or Native American attacks, Cabeza de Vaca, Alonso del Castillo Maldonado, Estevanico and Andrés Dorantes de Carranza were enslaved by Coahuiltecans for several years and eventually escaped by striking out west towards Sonora. During their journey they explored much of the country along what is now the Mexico–United States border, documenting the Devils and Pecos Rivers, Santa Elena Canyon and the Mesilla Valley. This was the first contact with Europeans for the natives living along the Lower Rio Grande, many of whom had heard stories from coastal tribes of "strange medicine men" capable of "wonderful cures" and celebrated in the Spaniards' arrival.

The next explorer to encounter the Rio Grande was the conquistador Francisco Vázquez de Coronado, who led 1,000 Spaniards and Native American allies in 1540 to find the fabled ruins of Cibola (the Seven Cities of Gold) north of Spanish Mexico. Expecting to find ancient treasures and gold, Coronado's expedition arrived at the Zuni Pueblo, the rumored location of Cibola, but discovered no wealth. The expedition continued east, reaching the Rio Grande in the last weeks of 1540, and spent the winter with the Puebloans in the Rio Grande valley. Coronado noted the native place-name "Tiguex" which would later be applied by the Spanish to the general region of the Rio Grande in New Mexico.

During the winter of 1540–41 food supplies were strained by Coronado's large host and led to conflicts. The Puebloans told Coronado – who at this point was desperate – of a much larger and wealthier city called Quivira to the northeast, though the natives may have "exaggerated its importance just to get rid of the troublesome Spanish visitors." After several more months Coronado reached Quivira and again found no treasure. Because the expedition had been financed by the Spanish government and many wealthy individuals in the hopes they would discover mineral wealth in the New World, it was regarded as a huge failure. Many soldiers deserted in shame and Coronado's reputation was ruined. In large part because of this, it would be more than 50 years before the Spanish returned to the Rio Grande country.

19th century development
http://www.riogranderestoration.org/manager/download_files/13.pdf

The New Mexico–Texas border does not precisely follow the Rio Grande, instead following the river bed as it was in 1850. The Rio Grande had shifted its course east since then, with the result that the "border segment" actually runs mostly within Texas. The boundary dispute between the two states was resolved in the Supreme Court case New Mexico v. Texas (1927) which fixed the official border on the 1850 course.

River modifications
"The Rio Grande is the only river I ever saw that needed irrigation." – Will Rogers

http://www.usbr.gov/watersmart/wcra/docs/urgia/URGIAMainReport.pdf

Parcher The Middle Rio Grande Valley in New Mexico is one of the oldest continuously farmed regions in North America, ever since the Puebloans introduced irrigated agriculture in the 1200s. The Puebloans relied on the annual rise of the Rio Grande to practice simple flood irrigation; once the spring floodwaters receded the saturated soil could be planted with their staple crops of corn, beans and squash. By bringing fresh layers of sediment, the annual floods maintained the fertility of the valley soils despite them being intensively farmed in many places. This is essentially the same method of farming as was practiced along the Nile in ancient Egypt, before the construction of the Aswan Dam. There is little evidence to suggest that the Puebloans (before contact with Europeans) constructed permanent irrigation works, such as canals (regionally called acequias) or storage reservoirs, unlike their ancestors in the Colorado Plateau. The total amount of land under irrigation, before Spanish arrival, was about 20000 to 30000 acre.

When the Spanish began settling the region in the 1600s they introduced modern irrigation systems to the region, such as canals, reservoirs and floodgates, which were necessary for growing European staple crops such as wheat. By the 1700s about 40000 acre were under irrigation. When American settlers came to the Colorado and New Mexico parts of the Rio Grande in the mid-1800s, they greatly expanded the irrigation system, to about 1248000 acre in New Mexico by the 1880s, although this decreased sharply in a few years due to drought and mismanagement of water supplies. Irrigation began in the Lower Rio Grande Valley between Texas and Mexico some years later, around 1898, and the population of the area grew rapidly on both sides of the border following this new economic opportunity.

The International Boundary and Water Commission (IBWC) was established in 1889 as an effort to divide more equitably the waters of the Rio Grande and the Colorado River between the US and Mexico. One of the main reasons for the IBWC's creation was the conflict over the Upper Rio Grande, which was almost totally diverted by Colorado and New Mexico farmers by the time it reached the Mexican border at El Paso. The 1906 Boundary Water Convention guaranteed Mexico 60000 acre feet of water from the Upper Rio Grande, less than 2.5 percent of the natural flow. In 1938 Colorado, New Mexico and Texas signed the Rio Grande Compact, which established rules for the management of Upper Rio Grande waters.

Due to the vulnerability of the region to flood and drought, and growing cross-border water conflicts, at the turn of the 20th century the US and Mexico governments began to publicly fund large scale dam and irrigation schemes. The Boquillas Dam in Chihuahua, constructed across the Rio Conchos, was one of the largest masonry dams in the world at its completion in 1913. Although it stored water for irrigation, its primary purpose was hydroelectric power generation. Elephant Butte Dam in New Mexico, when completed in 1916, was the second largest irrigation dam in the world, exceeded only by the Aswan Dam in Egypt, and was the largest dam in the Western Hemisphere. Smaller water projects were constructed in the following decades but the pace of development picked up again starting in the 1950s, with Falcon Dam (1954), Abiquiu Dam (1963), Amistad Dam (1969), and Cochiti Dam (1973). Dams were also built on the Pecos River in New Mexico and Texas, and the Salado and San Juan Rivers in Coahuila, Nuevo Leon and Tamaulipas.

By the 1980s there were fewer new developments and the focus turned to maintaining existing ones and making them more efficient. This was simply because there was no water in the Rio Grande left to develop; in fact in many place water rights were (and still are) over-allocated, with more "paper water" than real water that is available for use.

The most recent major development has been the Closed Basin Project, authorized in 1972.

Boquillas Dam

http://www.waterassembly.org/archives/9th%20Assembly/KevinFlannigan-5%20-%20Impacts%20of%20Over-Allocation-Transcr.pdf

https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.highlight/abstract/857/report/F

http://www.nytimes.com/2015/04/13/us/mighty-rio-grande-now-a-trickle-under-siege.html

http://www.texascenter.org/publications/rioconchos.pdf

https://www.fas.org/sgp/crs/row/R43312.pdf

Upper Rio Grande
The Boundary Waters Convention and Rio Grande Compact primarily govern the use of water along the Upper Rio Grande: Colorado, New Mexico and a small corner of western Texas, and a small portion of northern Chihuahua. Although the Rio Grande Compact was created to ensure an equitable distribution of water among the four states, it has not been followed consistently throughout its history, with significant "water debts" accruing that have resulted in long running lawsuits. Under the compact, Colorado must allow at least 400000 acre feet per year to flow into New Mexico, and 790000 acre feet per year must be released from the Elephant Butte Dam to satisfy water users in southern New Mexico, Texas and Chihuahua. The Boundary Waters Convention guarantees some water will flow to Mexico but only a small portion – 60000 acre feet of the total 2500000 acre feet that originates in the Upper Rio Grande Basin.

Rio Grande Project
The U.S. Reclamation Service (now the Bureau of Reclamation) was created in 1902 to develop the water resources of arid western states. Elephant Butte Dam, completed in 1916, was the key feature of the Rio Grande Project, which now irrigates about 178000 acre in New Mexico, Texas and Chihuahua. One of the key purposes of the Rio Grande Project was to ensure a water supply to the Mexican side of the border, which prior to damming was frequently starved of water due to upstream diversions in the United States. Of the total 790000 acre feet Rio Grande Project allocation, 92 percent is used in the states of New Mexico and Texas. Between the 1910s and 1930s the Bureau of Reclamation also constructed the San Luis Valley Project, which helps to irrigate the San Luis Valley of Colorado.

Middle Rio Grande Project
In 1925 the Middle Rio Grande Conservancy District was formed to manage irrigation systems around Albuquerque, New Mexico as the Middle Rio Grande Project. The District built El Vado Dam in 1935 to store water from the Rio Chama, which flows into the Rio Grande above Otowi Bridge. It also built four diversion dams and a canal system along the Rio Grande to deliver water to farms and six pueblo communities along the Middle Rio Grande Valley. From the 1950s to the 1970s the project was rehabilitated by the Bureau of Reclamation and the Army Corps of Engineers.

Rectification and Canalization Projects
The Rio Grande Rectification Project, a joint undertaking between the United States and Mexico, was constructed between El Paso–Juárez and Fort Quitman, Texas, in response to un-anticipated geomorphological changes caused by the construction of Elephant Butte Dam. Regulation of water flows made the Rio Grande slower and shallower, causing sediment to build up and creating a flood threat by reducing the channel capacity. The project was begun in 1933 with the signing of the Rio Grande Rectification Treaty, and was completed in 1938 at a cost of $5 million ($84 million in 2016 dollars), of which 88 percent was paid by the United States. The Rectification Project widened and straightened the river bed and created a 560 ft wide floodway protected by levees. The project shortened this winding segment of the river from 155.2 mi to just 85.6 mi, and also included the construction of three international bridges and the Caballo Dam. Situated a few miles below Elephant Butte, Caballo's main purpose was to store water released from Elephant Butte during the winter for electricity generation, which would otherwise have been wasted. This increased the total available water supply for the summer irrigation season.

The Rio Grande Canalization Project was constructed between 1938 and 1943 in southern New Mexico, for essentially the same purpose. About 130 mi of levees were constructed to create a floodway between Caballo Dam and the Mexican border, and a "low-flow channel" was created to carry the normal flows of the Rio Grande. By speeding up the river flow it also reduced water losses due to evaporation and growth of vegetation along the river banks, making the delivery of irrigation water much more efficient. Both the Rectification Project and the Canalization Project were built in part to fulfill conditions set by the 1906 Boundary Waters Convention.

San Juan–Chama Project
The San Juan-Chama Project in northern New Mexico was first proposed under the Rio Grande Compact in 1938, and an official report was released in 1946 by the Bureau of Reclamation for a trans-basin diversion from the Colorado River system to the Rio Grande. However, the project was not authorized by Congress until 1962, and was completed in 1976. It is now the largest of many such water diversions across the Continental Divide from the Colorado River Basin to the Rio Grande. A tunnel system collects water from tributaries of the San Juan River and delivers it to Heron Lake and into the Rio Chama. The project delivers about 110000 acre feet annually, mainly for the city of Albuquerque and farmers in the Middle Rio Grande Valley. San Juan-Chama water can be legally stored in Heron, El Vado and Abiquiu reservoirs.

Water rights issues
During dry years, the Middle Rio Grande Project uses natural flows of the Rio Chama and Rio Grande for irrigation, while the Rio Grande Project depends on water stored in Elephant Butte and Caballo reservoirs. During wet years water unused by the Middle Rio Grande Project flows south and replenishes these downstream reservoirs. In July 2002, the Rio Grande compact was amended so that the Middle Rio Grande Conservancy District is only allowed to store additional water in its upstream reservoirs (El Vado and Heron) when the combined storage at Elephant Butte and Caballo exceed 400000 acre feet. By giving the Middle Rio Grande Project first right to streamflow, and the Rio Grande Project first right to storage, the move was meant to ensure a sufficient water supply is available for both projects. However, this system still favors the Middle Rio Grande Project during periods of extended drought as most of the streamflow is consumed long before it reaches Elephant Butte Reservoir; in turn, it has contributed to the accruing of New Mexico's water debt.

Water use in Colorado has also been a contentious issue, with the state essentially ignoring Compact requirements until 1966, when New Mexico and Texas successfully sued to get back their water rights. A series of heavy snowmelt years in the 1980s subsequently filled Elephant Butte Reservoir and erased Colorado's outstanding water debt. Since then, the main problem has been the timing of delivery of irrigation water. Colorado consumes nearly the entire flow of the Rio Grande during the summer irrigation season but allows the river to flow unimpeded during the winter months. Even though Colorado delivers the correct volume of water on an annual basis, it starves New Mexico's Middle Rio Grande Project of its supply during the critical summer months, due to the lack of a storage dam on the Rio Grande between the Colorado state line and Albuquerque. (Although Cochiti Dam is located in this area, its function is not irrigation, but flood-control.)

Chamizal Dispute

U.S.–Mexico Treaty (1944)
The full name is Treaty relating to the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande.

Mexico water debt

Floodways
Rio Grande Project & international perspectives

Water rights controversy

Lower Rio Grande

Drought

IBWC flood control systems

IBWC Upper Rio Grande projects

Anzauduas Dam

Texas water adjudication