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Background

Lower Rio Grande EditionThe Bosque Education Guide

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Background

BackgroundRefuge in the Desert: Life on the Lower Rio Grande

by Nancy Stotz

Terms in bold are defined in the glossary.

For centuries, the Rio Grande has been a focal point for human activities in the arid landscapes of what is now south-central New Mexico. Traversing the northern reaches of the Chihuahuan Desert, the Rio Grande is the only through-flowing stream in the region, and its waters have had a significant effect on trade routes, settlement patterns and population growth. In turn, these human activities have affected the Rio Grande, changing the volume, timing and location of the river’s flows and negatively impacting the native plant and animal communities dependent upon those flows.

Prior to significant human impacts, the river had a sinuous path, enhanced by riffles, pools and backwaters, and its broad floodplain boasted numerous marshes, ponds and a riverside forest known as the “bosque” in Spanish. These native habitats have been largely eliminated along the river’s course through southern New Mexico, but in recent years, public interest in these dramatic changes to the Rio Grande has grown. Efforts are under way to restore some of the natural features of the river and to educate the public about the river’s natural environment and its historic role in the region.

This Background provides an overview of the river’s history and ecology, and it details three distinct phases in the river’s history, which are used as the underlying theme for many of the activities in the Bosque Education Guide. In the Guide, the phrase “Rio Bravo” refers to the “wild river” in its natural state, prior to significant human impacts. The phrase “Rio Manso” refers to the “tamed river” as altered by human activity, especially from 1850 to the present. And the phrase “Rio Nuevo” refers to the “new river,” the one we can strive for and one that incorporates elements of Rio Bravo and Rio Manso, allowing for continued and necessary human use of the river while also recognizing and providing for ecological needs.

Voices from the Past

Although humans have lived along the Lower Rio Grande for thousands of years, some of the most significant human impacts began to occur during the late nineteenth century. One way to get a sense of how the Lower Rio Grande has changed through time is to read the journals of travelers who visited the region prior to some of the most drastic changes to the river’s system. Spanish exploration and settlement of the region began in the late 1500’s, and Anglo-American exploration and travel intensified in the mid-nineteenth century.

Formatted sidebars like this will be used in this Background to present direct quotations from such sources, so these early travelers can tell you, in their own words, what it used to be like along the Lower Rio Grande. Complete citations for the quotes are provided in the References list at the end of this Background.

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The Lower Rio Grande

The information in this edition of the Guide will focus on the Lower Rio Grande in New Mexico, which runs from Elephant Butte Dam, near Truth or Consequences, to the southern end of the Mesilla Valley, just upstream of El Paso, Texas.

The Lower Rio Grande runs through two broad valleys, separated by a rocky, narrow reach known as Selden Canyon. Starting at Elephant Butte Dam, the Palomas Valley is roughly 50 miles (80 km) long and two miles (3.2 km) wide. The upper portion of this valley now contains Caballo Reservoir, as well as several small towns including Garfield, Hatch and Rincon. Selden Canyon begins at the base of San Diego Mountain, also known as Tonuco, just southeast of Rincon. The canyon ends at the head of the Mesilla Valley, where the town of Radium Springs is now found. The Mesilla Valley is also about 50 miles (80 km) long, but it is much broader than the Palomas Valley, reaching a maximum width of almost eight miles (12 km) near the city of Las Cruces.

B e c a u s e t h e re a re n o significant tributaries of the Rio Grande in this reach, most of the water in the Lower Rio Grande comes from farther upstream. Precipitation in northern New Mexico and parts of southern Colorado, especially winter snowfall in the high mountains, provides much of the water we see in the Lower Rio Grande. Summer thunderstorms also provide short-term, though sometimes significant, pulses of water in this reach.

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Background

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Rio Bravo: A Naturally Dynamic River

Prior to significant human impacts, the Lower Rio Grande was a dynamic river whose shifting course and variable flows supported a diverse array of habitats. The river followed a meandering path—a curving, sinuous shape that is common to many rivers and that results in fairly continuous, small-scale changes in channel location. Meanders tend to migrate laterally and downstream because water cuts into the outside of each curve while sediments are deposited on the inside. The extreme curve of a meander can also get cut off from the river when a shorter, straighter channel develops between the upstream and downstream ends of the meander’s arc, leaving behind an oxbow lake in the old curve of the meander.

Meanders create fairly small-scale changes to a river channel’s location, but along the Lower Rio Grande, large-scale channel shifts also occurred. For instance, in the 1840s the river was located well east of the town of Mesilla, passing through the area we now know as Mesilla Park. Flooding in 1862 left the town of Mesilla surrounded by water on both sides, and in 1865, the entire river moved into a new channel west of Mesilla.

Such large-scale channel shifts were most often associated with flood events. The river’s natural hydrograph, the amount of water flowing in the river at different times of year, was highly variable, both within and between years. In general, the river’s lowest flows were during winter and early spring. The peak flows came in late spring and early summer, when the water from snowmelt reached the Lower Rio Grande. Low-intensity flooding associated with this annual pulse enriched the river’s floodplain by depositing water, nutrients and fine sediments onto the valley floor. Some years, more intense floods occurred, typically after winters with especially heavy snowfall in the mountains or after severe summer storms.

Flooding associated with the annual snowmelt pulse, as well as more extreme events, helped sustain a variety of wetland habitats. The river’s floodplain held numerous ponds, marshes and sloughs. Some of these were in abandoned river channels, and all could be recharged with new surface water during flood events. Such wetlands were also maintained by the relatively high water table in the river valley; the high water table was recharged by water that seeped into the ground during periods of high flows, and it helped sustain the river and its associated wetlands during seasons when surface flows from upstream diminished.

meander

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Rio Bravo: A Mosaic of Habitats

The Lower Rio Grande’s shifting course and variable flows supported a diverse array of habitats. The annual spring snowmelt pulse and occasional scouring floods were critical to the establishment of the riverside bosque. The bosque was dominated by cottonwoods (Populus deltoides subsp. wislizenii) and willows (Salix spp., including the shrubby coyote willow, S. exigua, and the tree-form Goodding’s willow, S. gooddingii). Cottonwoods and willows require bare, moist, sandy soils in order for their seeds to germinate. Their small, wind-borne seeds are released in late spring and early summer, which increases the likelihood that the seeds will land on a bank or sandbar that has been recently cleared and moistened by the river’s natural snowmelt pulse.

Some of these germination sites would have been created regularly on point bars, by the continuous, gradual movement of the meandering river. Others would have been created more suddenly, following a significant channel shift or scouring flood that exposed or deposited new soil appropriate for cottonwood and willow germination. Germination of seeds did not guarantee establishment; seedlings and young plants growing along active river channels ran the risk of destruction over the next few years if the spring snowmelt pulse or other flood events were strong enough to dislodge them. Long-term establishment of new cottonwood and willow stands thus depended on a series of events: proper conditions for germination, followed by several years of conditions conducive to

continued growth.

The end result was a patchy distribution of cottonwoods and willows, with different-aged stands occurring along both active river channels and old channels or other suitable sites created by floods. Bands of flood-tolerant seepwillow (Baccharis spp.; a shrubby member of the sunflower family, not closely related to the true willows) often grew along active river channels where cottonwoods and willows failed to get established.

Evidence of Flooding

In 1851, a government official entered the lower end of the Palomas Valley near San Diego Mountain and observed: “From the water marks on the trees, the river rises about four feet above its banks, inundating the bottom lands to the base of the hills which border them, and rendering the valley impassable.”

Bartlett 1965: 217

Bosque and Wetland

A government official described the bosque and wetlands near Hatch in 1851 this way:

“We pitched our tents in a thick grove of large cotton-woods...on the opposite side was a pond or laguna, extending a mile or more. As this body of water was not wider than the river, and presented many sinuosities, I think it must have been formerly the channel of the Rio Grande....The laguna is now supplied by overflows from the river. There were many wild fowl in it; but its banks were so open, we could not approach the game.”

Bartlett 1965: 217–218

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Farther from the river, the common shrubs and trees of the floodplain included arrowweed (Pluchea sericea), seepweed (Suaeda nigra), saltbush (Atriplex canescens), honey mesquite (Prosopis glandulosa) and screwbean mesquite (P. pubescens). The latter is often referred to by its Spanish name, “tornillo,” which means “screw”; both the Spanish and English names refer to the plant’s distinctive coiled seed pod.

The mosaic of habitats along the Lower Rio Grande’s floodplain included a variety of wetlands, including wet meadows, marshes, oxbows and other ponds. Although early explorers’ accounts rarely described wetland species in detail, we can assume that many of the native species would have been the same as those found today. The high water table created saturated, saline soil conditions, where saltgrass (Distichlis spicta) was probably the dominant plant. Wet meadows probably supported

additional plant species including rushes (Juncus balticus) and yerba mansa (Anemopsis c a l i f o r n i c a ) . Marshes and some pond edges were likely populated by plants such as cattails (Typha latifolia), sedges (Schoenoplectus americanus, S. acutus and Eleocharis spp.) and reeds (Phragmites australis).

Aquatic habitats were varied as well. Some, such as marshes, oxbows and backwaters along the river’s edge, featured still water. Riverine habitats within a single meander curve ranged from the relatively deep, fast-moving outer edge of the curve to the slower, shallower inside of the meander. The various aquatic habitats provided opportunities for a diverse assemblage of aquatic animals.

Mesquite

Mesquite played a role in military events in the Mesilla Valley. On the eve of the Battle of Brazito, a Mexican escaped capture by American forces by taking “…shelter in the chaparral, or thick mesquite bushes (which somewhat resemble locust but [are] stunted)…”

In 1857, military use of mesquite took a new twist: a chronicler of a military expedition exploring the feasibility of using camels in the desert southwest noted at a campsite near Ft. Fillmore: “Grass indifferent; mesquite wood abundant, especially a kind of which the camels are particularly fond, the fornia [tornillo] or screwbean.”

Bieber 1936: 228; Lesley 1949: 171

screwbean mesquite with “tornillo”

wetland vegetation

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Rio Bravo: Native Wildlife

The aquatic habitats of the Lower Rio Grande supported at least 21 native species of fishes. Some, such as the shovelnose sturgeon (Scaphirhynchus platorynchus), which can reach lengths of up to three feet (1 m) and prefer river channels with sandy bottoms and strong currents, are limited to large river systems.

Other native fishes depended upon the river’s connectivity and variable flows. The American eel (Anguila rostrata) lives in fresh water for most of its life, but breeding occurs in salt water, so eel populations require unfettered passage to and from the sea. A suite of smaller fishes from the Cyprionid family (minnows and their relatives) spawned when they sensed the river’s pulse of spring snowmelt. Their eggs drifted with the river’s current, and young fish developed in relatively still waters downstream. Once they grew, the fish could move back upstream to begin the cycle again.

The native fishes in the Lower Rio Grande were joined by three species of aquatic turtles and two aquatic mammals (beavers, Castor canadensis, and muskrats, Ondatra zibethicus). More than 40 species of birds dependent upon open water (including waterfowl such as ducks and geese, as well as less familiar water birds like grebes, coots and gulls) have been observed along the Lower Rio Grande; when you add in species that are dependent upon marshes or shoreline habitats (including herons, rails and sandpipers), the total climbs to more than 90.

The bosque and other terrestrial habitats closely associated with the river supported even more species. Overall, there were 58 species of reptiles and amphibians along the Lower Rio Grande, 38 species of mammals and more than 250 species of birds.

Terrestrial Wildlife

Most accounts of wildlife in the journals of early travelers focused on species they could use for food. The following journal entries come from a member of Doniphan’s Army, which crossed the Jornada del Muerto in 1847:

“November 1. Still laying by at San Diego....Beautiful view of the river and surrounding hills from our camp ground. Deer, wolves, and turkeys killed and eaten; fish and turtles caught...”

In 1848, another traveler noted an extensive list of mammal and bird sign along a single stretch of the river, in the narrow valley that has been flooded to create Elephant Butte Reservoir: “In passing along the river, I saw the tracks of the otter, the catamount, the wildcat, the bear, the raccoon, the polecat, the crane, the duck, the plover, the deer, and the California quail.”

Bieber 1936: 337; Johnston 1848: 575

Beavers

After the opening of Santa Fe trade to Americans, the trapping of fur-bearing mammals along the Rio Grande accelerated greatly. In 1826, an official complained that “The taking of peltries of beaver is a branch of trade profitable only for the Anglo-Americans, who make up hunting parties and also establishments for them which last several months; as a result the specie [sic] will soon be destroyed.”

To avoid such officials, in 1827 American trappers used rafts to float down the Rio Grande to El Paso; after trapping beavers along the river, they dismantled the rafts, sold the valuable wood, and sneaked out of Mexican territory with their furs. By 1838, official concern about the survival of beaver populations had reached El Paso, where authorities wrote to Chihuahuan officials about the imminent extinction of both beavers and otters along the Rio Grande.

Madrid quoted in Marshall 1916: 259; Weber 1971: 157, 224

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Historic OverviewHuman occupation along the Lower Rio Grande has been documented as early as 11,500 years ago, with bands of Paleo-Indian hunters following herds of now-extinct mammoth and bison. Hunting and gathering continued to be a large part of subsistence for subsequent Archaic cultures, who lived in this area from 7,000 years ago to around AD 200. Archaic people hunted smaller animals and harvested seasonal plants, and the Rio provided rich resources. Towards the end of the period, they began to build subterranean structures called pithouses and initiated the earliest forms of agriculture.

In AD 200, wetter conditions may have encouraged domestication of crops, particularly corn, and dry-land farming was undertaken throughout the region. The Pueblo people who occupied the area were primarily from the Jornada Mogollon culture. Pithouse villages grew to larger pueblo villages by AD 1300. Many archaeological sites of the period contain pottery, stone tools and remnants of their pueblo dwellings. Many Pueblo period sites have been recorded on the mesas and ridges overlooking the Rio Grande.

Historic Apache use of this region has been documented as well. Apache groups were mobile, using hunting and gathering strategies and establishing settlements called rancherias throughout southern New Mexico. These groups occupied the area from as early as the 1500s through the 1880s. Many Apaches from this and other parts of New Mexico were sent to the Mescalero Apache reservation in the late 19th century. They continue to be an active presence in southern New Mexico.

El Camino Real, established by Don Juan de Oñate in 1598, linked Mexico City to Santa Fe and facilitated European settlement and trade. This internationally significant trail traversed the eastern edge of the Mesilla Valley and was strongly linked to the Rio Grande. The main branch of the trail left the river at the northern end of the valley and crossed the treacherous desert of the Jornada del Muerto, or “Journey of the Dead,” to avoid rugged canyons and valleys of this stretch of the river. Archaeological sites can be found along the southern Rio Grande, including large remnants of El Camino Real campsites called parajes, which provide insight into the large groups of Hispanic travelers along the trail. These travelers brought enormous numbers of livestock and carts laden with trade goods with them. El Camino Real has been nominated as a national historic trail, and segments of the trail are being preserved and interpreted.

Although early Hispanic settlements were created in the El Paso Valley in the 17th century, the Mesilla and Palomas Valleys were settled later. Populations were impacting the Rio Grande, however, due to heavy traffic along El Camino Real and upstream development. Livestock grazing in the watershed and the need for timber and other resources may have had impacts on the lower Rio Grande.

In 1680 Pueblo people and other native groups rebelled, forcibly expelling Spanish colonists from all of New Mexico. Refugees fled down El Camino Real, using the paraje system on their retreat southward to El Paso. By the 1690s, the

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Spanish returned to New Mexico and reestablished towns and villages along the middle and upper Rio Grande.

The first early settlement in Mesilla occurred in 1848, and by 1850 it was a firmly established colony. Due to its importance as a supply center, its proximity to the Rio Grande, as well as the signing of the Gadsden Purchase, Mesilla became a part of the United States in 1854. Las Cruces was established in 1848 and outgrew Mesilla due to the establishment of a railroad station in 1881.

With settlement came large changes in land use along the southern Rio Grande. Bosque habitats were replaced with agricultural fields. Acequia systems were created in order to channel water from the river to farmer’s fields. Many remnants of this historic system remain today, including the Picacho Drain that runs through Mesilla Valley Bosque State Park.

The drought and flood cycle of the Rio Grande had large impacts on farmers throughout the 19th century. In 1906, the U.S. government entered into an agreement with Mexico for the Rio Grande Project, which guaranteed and specified water deliveries to Mexico. In 1916, Elephant Butte Dam was constructed to ensure the proper delivery of water, and it helped to stabilize the water supply for farmers. Other significant dams constructed along the Rio Grande include Caballo Dam, Percha Dam, Leasburg Dam and Mesilla Dam. From 1938 to 1943, the Rio Grande Canalization Project removed the bosques and built levees along 105 miles of the Rio Grande, from Percha Diversion Dam to El Paso. The same length of river was dredged to create a deeper channel and meanders were removed. Regular mowing was initiated to control floodplain vegetation, which engineers feared might exacerbate flooding by blocking the rapid movement of the river during flood stage. These dams, levees and canals have worked in concert with Elephant Butte Dam to control flows along this stretch of the Rio Grande.

Agriculture in the lower Rio Grande changed after the construction of these dams and levee systems. With more predictable flows, farmers could grow commercial crops of cotton, alfalfa, chile, onions and pecans. While the farming culture continues to thrive along the Lower Rio Grande today, rapid and sustained population growth in and around Las Cruces and El Paso is increasingly turning farm fields into subdivisions, and river water currently dedicated to agricultural use will likely be diverted to urban use.

The Plight of Farmers

During his pastoral visit to the Mesilla and El Paso valleys in 1902, the Bishop of the Diocese of Tucson lamented how the drying of the river was ruining agriculture. The difficulties faced by farmers helped lead to the construction of Elephant Butte Dam.

In La Mesa the Bishop wrote, “We go to wander in the fields. These—alas!—lacking water, are almost all fallow. How is this poor village going to feed itself? Black misery is in sight for this year.”

Granjon 1986: 80

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Changes to the River’s Flows

The earliest numerical data on the volume of the river’s flows comes from gaging stations established by the International Boundary and Water Commission. One of the first stations they established was at El Paso, in 1889, allowing us to graphically examine the effects of Elephant Butte Dam on the volume and timing of flows along the Lower Rio Grande.

The first graph below shows the average volume of flows for the earliest five years available. As you look at this graph, keep in mind that by the early 1890s, human influences had already had a significant impact on the river.

Notice the timing of peak flows. The sharp peak in May represents the spring snowmelt pulse. In earlier times, that pulse might have been somewhat later. After the end of the US Civil War, the removal of timber from the mountains where the river’s snowpack develops every winter accelerated, exposing the snowpack to more sunlight, which may have led to an earlier spring snowmelt.

Notice the volume of peak flows, almost 350,000 acre feet (over 420 million cubic meters). By the early 1890s, agriculture above Socorro, New Mexico was estimated to be reducing the river’s summer flows to less than half of what they would have been without agriculture

upstream.The second graph

shows the average flows in the first 10 years following the dam’s construction. To allow a direct comparison, the vertical axis is the same as the first graph, highlighting the reduction in peak flows, which dropped to less than 90,000 acre feet (slightly over 100 million cubic meters). Notice also the change in the timing of peak flows, which were spread out over the entire irrigation season.

IBWC 1956

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Rio Manso: A Tamed River

The Rio Grande and its surrounding environment have changed significantly during the past 150 years. The controlled timing and volume of the river’s flows has allowed farmers to plant large fields of crops where there were once willows, cottonwoods, wetlands and a meandering river. The Rio Grande itself now very rarely floods and is controlled by levees and dams. Lower Rio Grande cities are growing, impacting the watershed and wildlife corridors. The mosaic of bosque habitats, including floodplain wetlands, willow thickets and different-aged stands of cottonwoods, are scarce.

A few patches of riverside forest remain—the most extensive can be found between Elephant Butte Dam and Caballo Reservoir, in Selden Canyon, along the western edge of the river, which lacks levees, adjacent to the Robledo Mountains in the Mesilla Valley and in a handful of state parks along the Lower Rio Grande (the riverside portions of Elephant Butte and Caballo state parks, Percha Dam State Park, Leasburg Dam State Park and the Mesilla Valley Bosque State Park).

Unfortunately, the dominant tree in most of these locations is the non-native saltcedar (Tamarix chinesis, also known as tamarisk). Introduced from the Mediterranean, this exotic was used as an ornamental and a windbreak, as well as for erosion control. It was present, in small numbers at least, in the Albuquerque area by 1908, in Mesilla Park by 1910 and in El Paso by 1926. Like cottonwoods and willows, it produces seeds which germinate best on open, moist soil but, unlike the native species, it produces seeds for several months each year and the seeds are viable for a longer period, giving them many more opportunities for germination. Saltcedar grows quickly and often forms pure, dense stands, which generally support many fewer species of animals than the native forests they have replaced.

However, there are public and private agencies working to protect and conserve remaining bosque habitats along the Lower Rio Grande. Removing saltcedar is a major task, and other efforts include preserving parcels of land along the river, reintroducing native willow and cottonwood and restoring wetland habitats. Examples of such management endeavors can be seen at Bureau of Land Management (BLM) properties, Selden Canyon and at the five New Mexico State Parks along the lower Rio (Elephant Butte Lake, Caballo Lake, Percha Dam, Leasburg Dam and Mesilla Valley Bosque).

cottonwood leaf

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Rio Manso: Native Wildlife

The native fish populations of the Lower Rio Grande have suffered from impacts associated with human activities. Of the 21 fishes native to the area, only seven remain, and they now share the river with 15 non-native species introduced by humans. The sturgeon and eels are gone, and six of nine Cyprionids native to the Lower Rio Grande have disappeared from this reach, along with the long-nosed gar (Lepisosteus osseus), blue sucker (Cycleptus elongatus), gray redhorse (Scartomyzon austrius), Mexican tetra (Astyanax mexicanus) and two species of catfish. Of the 14 fish species that have disappeared from the lower rio, one is listed as endangered by the federal government (the Rio Grande silvery minnow, Hybognathus amarus), and four are listed as threatened or endangered by the State of New Mexico (the silvery minnow, the gray redhorse, the blue sucker and the Mexican tetra).

Most amphibians are directly tied to water because they lay their eggs in water and their young develop there, and their responses to habitat changes along the Lower Rio Grande have been variable. Some appear to have shifted into habitats created by water manipulations associated with agriculture and water deliveries. For instance, Woodhouse’s toads (Bufo woodhousii) are still abundant along the river, and an unusual population of plains leopard frogs (Rana blairi), otherwise found only in the eastern third of the state, thrives in spots along the river near Truth or Consequences. However, at least one amphibian species, the northern leopard frog (Rana pipiens), is thought to have disappeared from the Lower Rio Grande. Populations of northern leopard frogs elsewhere have also declined, and several potential causes have been identified, including water pollution, habitat changes and increased predation by bullfrogs (R. catesbeiana) and non-native fish.

Changes in Fish Abundance

Quotations from visitors to El Paso document early changes in fish populations in the Rio Grande.

Upon reaching the river near modern El Paso, the chronicler of Oñate’s 1598 journey explained that the river’s “bountiful waters teemed with many fish and we easily caught a number.”

A 1773 description of life in El Paso noted “…the river abounds in fish, known as rock fish, although some call it bream. Other delicious kinds are the corazon and the enguila [eel], all of more than medium size. The enguilas are found more often in the ponds formed by the overflow of the river than in its channel.”

In the late 1890s, when agricultural development along the upper reaches of the Rio Grande was significantly impacting flows in the lower rio, a government biologist surveying plants and animals near El Paso observed that “[o]wing to the lowness of the water in this portion of the Rio Grande at certain seasons, the fish fauna is limited and variable….”

Villagra 1933: 127; Hackett 1902: 508-509; Mearns 1907:80-81

Rio Grande silvery minnow

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In general, terrestrial wildlife has fared better, and the populations of reptiles along the Lower Rio Grande appear relatively stable. Reptiles that still occur regularly include eastern fence lizards (Sceloporus undulatus), New Mexico whiptails (Aspidoscelis neomexicanas), garter snakes (Thamnophis spp.), gopher snakes (also known as bullsnakes; Pituophis catenifer), spiny softshell turtles (Apalone spinifera), Big Bend slider (Trachemys gaigeae), ornate box turtle (Terrapene ornata), lightning-fast coachwhip snakes (Masticophis flagellum) and western diamondbacked rattlesnakes (Crotalus atrox).

Mammals that can still be regularly encountered along portions of the Lower Rio Grande include muskrat (Ondatra zibethicus), striped skunk (Mephitis mephitis), gray fox (Urocyon cinereoargenteus), Botta’s pocket gopher (Thomomys bottae) and raccoon

(Procyon lotor). Evidence of beaver (Castor canadensis), coyote (Canis latrans) and javelina (Pecari tajacu) activity are commonly encountered at the Mesilla Valley Bosque State Park, and rock squirrels (Spermophilus variegatus) are seen regularly during the warmer months. Mule deer (Odocoileus hemonius) are still present at some state parks along the Lower Rio Grande (Percha Dam, Caballo Dam and the riverside portion of Elephant Butte Lake State Park).

During certain seasons, geese, ducks and other migratory water birds (including pelicans and grebes) can be quite abundant along the Lower Rio Grande, especially at Elephant Butte and Caballo lakes and in the few remaining wetlands along the river’s course. Great blue herons (Ardea herodias), green herons (Butorides virescens) and black-crowned night herons (Nycticorax nycticorax) are found along most reaches of the Lower Rio Grande. Agricultural lands in the river valley support many types of raptors (predatory birds like hawks and falcons), including American kestrels (Falco sparverius) and red-tailed hawks (Buteo jamaicensis). Bald eagles (Haliaeetus leucocephalus) and osprey (Pandion haliaetus) can occasionally be seen fishing along the Lower Rio Grande. Percha Dam State Park is one of the most popular bird-watching destinations in the state during migration, and a variety of migratory songbirds breed in the remnant bosque and wetlands found there, including vermilion flycatcher (Pyrocephalus rubinus), phainopepla (Phainopepla nitens), summer tanager (Piranga rubra), black-headed grosbeak (Pheucticus melanocephalus), Bewick’s wren (Thryomanes bewickii) and red-winged blackbird (Agelaius phoeniceus).

Many of these breeding birds, as well as some other types of terrestrial wildlife, are supported by a diversity of insect life that occurs along the Lower Rio Grande, including long-legged craneflies and elegant mayflies that hover near the water and dozens of species of colorful butterflies and dragonflies. Shiny black carpenter bees nest in dead wood and pollinate nearby chile crops. Huge tarantula hawk wasps (the state insect ) prowl the undergrowth for tarantula burrows.

However, the populations of many terrestrial animals have declined over time, as some are restricted to remnants of native habitats along the Lower Rio Grande. Among the mammals and birds that occur, or used to occur, along the Lower Rio Grande, eight are listed as threatened or endangered by the State of New Mexico:

javelina

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bald eagle, neotropic cormorant (Phalacorax brasilianus), peregrine falcon (Falco peregrinus), southwestern willow flycatcher (Empidonax traillii extimus), interior least tern (Sterna antillarum athalassos), Bell’s vireo (Vireo bellii), spotted bat (Euderma maculatum) and Mexican gray wolf (Canis lupus baileyi). Of these eight, three are also listed as threatened or endangered by the federal government: the wolf, the least tern and the southwestern willow flycatcher.

The wolf’s decline is not unique to the Rio Grande; these large predators have been exterminated virtually everywhere that humans have encountered them. Though some 19-century accounts of “wolves” along the Lower Rio Grande may have referred to coyotes (Canis latrans), which were sometimes called “prairie wolves,” verified specimens of gray wolves do exist for the Lower Rio Grande. The interior least tern is a water bird that nests on sandbars and open shorelines; it is currently a rare migrant along the Lower Rio Grande, and its population declines along

major river systems in the central United States are thought to have been caused, in large part, by habitat destruction associated with river management and dam construction. The southwestern willow flycatcher is a small, insectivorous bird that breeds exclusively in thickets of young riparian trees such as willow; a handful of breeding pairs of willow flycatchers have recently been found in remnant patches of forest along the Lower Rio Grande, all upstream of the Mesilla Valley.

Though not designated as threatened or endangered species at a state or federal level, many other bird species have been

negatively impacted by human activities along the Rio Grande. Numerous bird species that breed in the bosque are now quite rare along the Lower Rio Grande, including yellow-billed cuckoos (Coccyzus americanus), painted buntings (Passerina ciris) and wild turkeys (Meleagris gallopavo).

A snowy egret and a shorebird in the Rio Grande at Caballo Lake State Park

southwestern willow flycatcher

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Rio Nuevo: Opportunities and Challenges for the Future

A variety of initiatives are under way which could help the Lower Rio Grande regain some of its historic ecological values. The following list is necessarily incomplete, but it provides at least a glimpse into the many organizations, both public and private, that are working on such projects. Almost all are collaborative efforts.

For instance, a thirty-member stakeholder group is working with the International Boundary Water Commission (IBWC) and the U.S. Army Corps of Engineers on a “conceptual restoration plan” for the Lower Rio Grande. This plan could result in some changes in the way IBWC manages the river’s channel and the floodway between the levees, and it could also allow for restoration of some habitats.

To support such restoration efforts, World Wildlife Fund is working with the Elephant Butte Irrigation District (EBID), which represents the farmers along the Lower Rio Grande) to facilitate the transfer of water rights from agricultural and municipal uses to environmental uses.

New Mexico State Parks has become a big player along the Lower Rio Grande. The division manages many of the most extensive remnants of riverside habitats, and habitat conservation and restoration figure prominently in the management plans of each park. The recently established Mesilla Valley Bosque State Park, dedicated to habitat restoration and environmental education, is a prime example of extensive collaboration and cooperation. The park is centered on two small wetlands that were created prior to the park’s establishment, through a cooperative effort involving the Southwest Environmental Center (SWEC) and the City of Las Cruces. The park includes properties owned by IBWC, EBID, an adjacent farming family and the New Mexico Department of Game and Fish. Partnership opportunities also exist with the federal Bureau of Land Management (BLM) and Bureau of Reclamation (BOR) and with local organizations like SWEC, the Mesilla Valley Audubon Society, the Native Plant Society of New Mexico–Las Cruces Chapter, New Mexico State University and the Doña Ana County Associated Sportsmen, all working to provide educational materials for the park.

The fact that so many organizations are coming together to work on projects along the Lower Rio Grande is very encouraging, but future efforts will face numerous challenges. Every project will be constrained by water delivery obligations created by international and multi-state agreements, as well as issues relating to flood control. Although all of the river’s water is already appropriated for specific uses, population growth in the region will increase the demand for water. The City of Las Cruces has already taken preliminary steps toward the construction of a water treatment plant that would allow surface water from the river to be used for residential and industrial applications, to supplement the ground water the city currently relies upon. That increased demand for river water is likely to be accompanied by decreased supply.

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Many long-term climate models predict that changing precipitation patterns will result in a shift to a more arid climate in the region, including a decrease in winter precipitation in the mountains of the watershed of the Rio Grande. The spring snowmelt pulse is the lifeblood of the Lower Rio Grande—historically, it supported the diverse plant and animal communities of the river’s floodplain; and in recent years, its storage in and release from Elephant Butte Reservoir has supported the agricultural productivity of the Palomas and Mesilla valleys. Continued cooperation and many innovative ideas are going to be required to ensure a healthy future for the Lower Rio Grande.

AcknowledgmentsMost of the text in the section entitled “The Lower Rio Grande,” as well as the two “Rio Bravo” sections, the two “Rio Manso” sections and the “Voices from the Past” sidebars, was extracted directly from or derived from information presented in Historic Reconstruction of the Ecology of the Rio Grande/Río Bravo Channel and Floodplain in the Chihuahuan Desert, by Nancy G. Stotz; Report prepared for the Chihuahuan Desert Program, World Wildlife Fund, 14 June 2000, 158 pp. The Chihuahuan Desert Program of World Wildlife Fund has generously given permission for the Reconstruction to be used as a basis for large portions of this Background.

Christy Tafoya, Education and Resource Protection Program Manager for New Mexico State Parks, and Alex Mares, Heritage Interpreter for New Mexico State Parks, provided information and editorial support for the Historic Overview section. Rob Yaksich, Instructional Coordinator for New Mexico State Parks, and Jim Stuart, a biologist with the New Mexico Game and Fish Department, provided information and editorial support for the descriptions of wildlife currently found along the Lower Rio Grande. Kevin Hansen, State Parks Region 3 Interpretive Ranger, reviewed the document.

Ocotillo in bloom above Elephant Butte Lake State Park

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ReferencesBaldwin, P.M. 1938. A short history of the Mesilla Valley. New Mexico Historical Review 13: 314-324.

Bartlett, J.R. 1965. Personal narrative of explorations and incidents in Texas, New Mexico, California, Sonora, and Chihuahua, connected with the United States and Mexican Boundary Commission, during the years 1850, 1851, 1852, and 1853. Volume I. Chicago: Rio Grande Press. [Reprint of 1854 publication.]

Bieber, R.P., ed. 1936. Marching with the Army of the West, 1846-1848. Glendale, CA: Arthur H. Clark Co.

BISON-M. n.d. Biota Information System of New Mexico. Available at http://www.bison-m.org/

Blue Earth Ecological Consultants. 2007. Mesilla Valley Bosque State Park management plan: public review draft. Report prepared for New Mexico State Parks, 5 November 2007, 124 pp.

Couchman, D.H. 1990. Cooke’s Peak, pasaron por aqui: a focus on United States history in southwestern New Mexico. Cultural Resources Report No. 7. Las Cruces: Bureau of Land Management, Las Cruces District.

Degenhardt, W.G., C.W. Painter, and A.H. Price. 1996. Amphibians and reptiles of New Mexico. Albuquerque: University of New Mexico Press.

Findley, J.S., A.H. Harris, D.E. Wilson, and C. Jones. 1975. Mammals of New Mexico. Albuquerque: University of New Mexico Press.

Granjon, H. 1986. Along the Rio Grande: A pastoral visit to southwest New Mexico in 1902, ed. M.R. Taylor and trans. M.W. Lopez. Albuquerque: University of New Mexico Press (published in cooperation with the Historical Society of New Mexico).

Hackett, C.W., ed. 1902. Historical documents relating to New Mexico, Nueva Vizcaya, and Approaches thereto, to 1773, Volume III. Collected by A.F.A. Bandelier and F.R. Bandelier. Washington D.C.: Carnegie Institution.

International Boundary and Water Commission. 1956. Summary Water Bulletin, 1889-1955.

Johnston, A. R. 1848. Journal of Captain A.R. Johnston, First Dragoons. In Report of Lieutenant J. W. Abert, of his examination of New Mexico in the years 1846-’47. 30th Congress, 1st Session, House Executive Document 41. Washington, D.C.: United States Engineering Department.

Kelley, P. 1986. River of lost dreams: Navigation on the Rio Grande. Lincoln: University of Nebraska Press.

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Lesley, L.B. 1949. Uncle Sam’s camels, the journal of May Humphreys Stacey supplemented by the report of Edward Fitzgerald Beale, 1857-1858. Cambridge, MA: Harvard University Press.

Marshall, T. M. 1916. St. Vrain’s expedition to the Gila in 1826. Southwestern Historical Quarterly 19: 251-260.

Mearns, A.E. 1907. Mammals of the Mexican Boundary of the United States: A Descriptive catalogue of the species of mammals occurring in that region; with a general summary of the natural history and a list of trees. Washington, D.C.: United States National Museum Bulletin 56/Smithsonian Institution.

Scurlock, D. 1998. From the rio to the sierra: an environmental history of the Middle Rio Grande Basin. Rocky Mountain Research Station General Technical Report RMRS-GTR-5. Ft. Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Seager, R., Ting, M., Held, I., Kushnir, Y., Lu, J. Vecchi, G., Huang, H., Harnik, N., Leetma, A., Lau, N., Li, C., and Naik, N. 2007. Model projections of an imminent transition to a more arid climate in southwestern North America. Science 316 (25 May 2007): 1181-1184.

Stotz, N. G.. 2000. Historic reconstruction of the ecology of the Rio Grande/Río Bravo channel and floodplain in the Chihuahuan Desert; Report prepared for the Chihuahuan Desert Program, World Wildlife Fund, 14 June 2000, 158 pp.

Sublette, J.E., M.D. Hatch, and M. Sublette. 1990. The fishes of New Mexico. Albuquerque: University of New Mexico Press.

Villagra, G.P. 1933. History of New Mexico, trans. G. Espinosa. Los Angeles: The Quivera Society.

Weber, D.J. 1971. The Taos trappers: the fur trade in the far southwest, 1540-1846. Norman: University of Oklahoma Press.

Weber, D.J. 1982. The Mexican frontier, 1821-1846: the American Southwest under Mexico. Albuquerque: University of New Mexico Press.

Williamson, M.A., P.W. Hyder, and J.S. Applegarth. 1994. Snakes, lizards, turtles, frogs, toads and salamanders of New Mexico. Santa Fe: Sunstone Press.

gopher snake

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Illustrationsjavelina, p. 32: Burt, William H., and Richard P. Grossenheider. A field guide to the mammals of North America north of Mexico, third edition. New York: Houghton Mifflin Company

snowy egret, page 33: New Mexico State Parks Archive

ocotillo, p. 35, and Mesilla Valley Bosque State Park pond, this page: New Mexico State Parks Archive

others: The Bosque Education Guide, Middle Rio Grande Edition

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Basic Ecological ConceptsBy Lisa Ellis, Ph.D.

Department of Biology, University of New Mexico

Biology, the study of life on Earth, is a multi-faceted academic field. Even the relatively young branch of biology known as ecology is complex, with many areas of specialization. It is impossible to provide a thorough explanation of these disciplines here, but the following essay includes some key concepts that will help you in your study of the Rio Grande bosque.

Levels of Taxonomic Classification

Taxonomy is the science of classification of organisms. Biologists classify organisms based on their evolutionary relationships, using a hierarchical system of grouping by shared features.

The fundamental unit of life on Earth is what biologists refer to as the species. Although the concept of a natural unit for classification is critical to many fields within the biological sciences, biologists are far from agreeing on its exact definition, and the meaning of a species may vary somewhat depending on the type of biologist you question. For our purposes, the widely accepted “biological species concept” is quite appropriate. The biological species concept states that a species is a population or series of populations whose members are able to interbreed freely under natural conditions and who do not breed with other species. Although there are exceptions to and difficulties with this definition, it works for most organisms. A simpler way to define species, which is not complete but will work for young audiences, is that a species is a unit of classification that refers to a population (group) or series of populations (groups) of closely related and similar organisms.

Closely related species are combined into a genus, while closely related genera form a family, and so on up the classification hierarchy. This classification hierarchy, listed from the largest or most inclusive category, is shown below. Taxa for the white-footed mouse are listed in the right-hand column as an example. Note that the genus and species names are always either italicized or underlined, with the genus name capitalized and the species name given in lower case.

Kingdom .............................................................................AnimaliaPhylum (animals) or Division (plants) ...........................Chordata subphylum ............................................................VertebrataClass .....................................................................................MammaliaOrder ...................................................................................RodentiaFamily ..................................................................................MuridaeGenus ...................................................................................PeromyscusSpecies ................................................................................. leucopus

white-footed mouse

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Levels of Ecological Classification

Ecology is the study of the relationships between organisms and the environment. Ecologists use a number of levels of classification that reflect ecological relationships. These terms have specific meanings in ecology. As in taxonomic classification, the fundamental unit of ecological classification is the species. For example, the most common species of mammal within the bosque is the white-footed mouse, whose scientific name, which is unique to this species, is Peromyscus leucopus. A population is a group of organisms of the same species that live in the same area at the same time. Thus we can talk about a population of white-footed mice living within a particular forest. A community is an association of organisms (plants, animals, microorganisms) that live in a particular place or habitat. These organisms affect each other in various ways, such as through a food chain or web (see below). For example, coyotes may eat white-footed mice; this predator–prey relationship is part of the community dynamics.

An ecosystem includes all the living organisms, the non-living components, a source of energy and the interactions among these in a given area. One difficulty in defining an ecosystem is in choosing the area you wish to discuss. An ecosystem can be as small as a drop of water or as big as the planet, the global ecosystem. Thus the term “ecosystem” sometimes seems a bit vague, or may seem like a catchall to include a wide variety of concepts. Throughout this guide, we will refer to the “riparian ecosystem.” By this we mean the living organisms (bacteria, fungi, plants, animals) that are found along the river, specifically the Rio Grande, and the interactions that occur among them and with the various non-living components, such as soil, as well as processes such as the hydrology and chemical reactions in the system and the input of sunlight energy.

A landscape is an area of land containing a patchwork of ecosystems. We can take a landscape perspective to look at the Rio Grande Valley in New Mexico, which would include several ecosystems such as the riparian forest, wetlands (marshes, wet meadows), grasslands and agricultural fields.

Some Basic Ecological Concepts

The term habitat refers to the kind of place where an organism normally lives. It includes the arrangement of food, water, shelter and space that is suitable to meet an organism’s needs. You can think of this as the “address” where an organism lives. In contrast, a niche is the “occupation” of an organism. It defines the role of an organism in an ecosystem, such as a “fish-eating wader” for a heron, or a “plant-juice-sipping summer buzzer” for a cicada. An organism’s niche may change during different life stages. For example, a tadpole typically lives in the water and eats plant material, while the adult frog may catch insects from the shore.

The source of energy for all life on Earth is the sun. Green plants (and some bacteria) are the only organisms that can directly capture the sun’s energy and change it into a form that other organisms can use. Through the process of photosynthesis, plants

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use sunlight to change carbon dioxide and water into sugar and oxygen. The oxygen is given off into the air, where it is available to other organisms including humans. Simple sugar molecules make energy available to plants and, by forming the basic units of complex carbohydrates, contribute to plant structure. Other organisms then eat the plants, or eat organisms that eat plants, and in doing so indirectly gain the benefit of the sun’s energy to run their bodies. The flow of sunlight energy is therefore passed from producers (green plants) to primary consumers (animals that eat plants, such as leafhoppers) to secondary consumers (animals that eat other animals, such as birds); this sequence is known as a food chain. As energy is passed along the food chain, much is used up at each level as it works to run each organism. This energy is given off as heat and results in less energy being available at each stage along the food chain. It takes a lot of grass to support one rabbit, and many rabbits to support one hawk. As a consequence, there are many, many green plants on the Earth, fewer animals that eat plants, and even fewer animals that eat animals; this is known as the energy pyramid. In the bosque, the cottonwoods and other plants trap the sunlight energy and provide it in a form usable by the entire collection of other organisms found there. They provide the foundation for life along the river.

Although sunlight energy is used up as it is passed along the food chain, fortunately there is an abundant supply of this energy. In contrast, the materials from which all living things are made are limited in supply and must be used over and over. The primary building blocks of all living things include only six materials: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. When an organism dies and decomposes, these materials are returned to the system and are used again. The carbon that was once part of a dinosaur’s tail may now be in the tomato that you eat for dinner! If these compounds are removed from the cycle in some way, they may become limited in supply. For example, if a tree dies but the wood does not readily decompose, carbon and other nutrients are trapped in the wood and are not available to other organisms. This appears to be happening in the bosque. Without the annual flooding that once inundated the forest, wetting the fallen wood and increasing the rate of decomposition, undecomposed wood is now building up and trapping nutrients. This affects the health of the entire ecosystem.

One very important cycle is the water cycle. Rain that falls on a hillside percolates down into the ground water, or may flow aboveground into a lake or the ocean. Water in the lake or ocean then evaporates, and drops join together into clouds, to eventually fall again as rain. Our use of water greatly affects the water cycle. In New Mexico we remove water from the underground aquifer (water present in the bedrock below ground) much faster than it is replenished. Much of this water evaporates directly into the atmosphere while we use it, and may then fall again somewhere else on the planet, thus reducing the amount of water available locally.

We also impact the cycling of materials by introducing poisons. As materials are cycled over and over, toxins build up. Concentrations of toxins increase along food chains, since a predator eats many prey with the toxin, a process known as

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biomagnification. These increasing concentrations of toxins often have devastating effects. Some well-known examples include top-predator species such as bald eagles and peregrine falcons that nearly became extinct due to the effects of DDT or other chemicals. Awareness of these problems may go a long way towards helping to keep our cycles clean.

Through the flow of energy and the cycling of materials, all living things are interrelated. A mouse not only gets energy from the seeds that it eats, but also gets materials that will help to build more mouse tissue. The mouse breathes out carbon dioxide which is taken in by plants, which in turn give off oxygen used by the mouse. The mouse also depends on plants for finding shelter, and it provides food for a snake or owl. The components of the bosque are interrelated with connections extending to the surrounding uplands as well. Some connections are obvious, such as birds that fly between the bosque and uplands at different times of day or during different seasons, moving materials from one place to another. Others are more subtle, such as water flowing underground. But these connections make our actions even more important. Pesticides applied to our fields may add toxic materials to the river, affecting not only the water itself but also all the organisms that depend on that water.

Change is an integral part of the natural world. Changes may occur over geologic time, such as the transition of the Rio Grande from a series of lakes to the river that we know today, or they may occur over much shorter time periods, such as the transition of a seed to a tree and finally to a fallen log. Change was once an integral part of the natural Rio Grande riparian ecosystem, as the river wandered across the floodplain leaving behind its ever-changing mosaic of vegetation. However, human-induced changes have much different effects on the ecosystem. The rate at which we are causing changes on Earth is much greater than has been known previously, and we do not yet know the ecological consequences of most of our actions. By understanding the ecological systems in which we live, and how we interact with them, we can begin to lessen our impact on Earth.

Biodiversity and Conservation

Two phrases that have become commonly used by biologists and natural resource managers, and now also by policy makers, the media and the general public, are biological diversity (typically shortened to biodiversity) and biological integrity. These phrases represent concepts that are critical to developing strategies for the preservation of biotic systems, yet they are often used imprecisely, without a true understanding of the underlying concepts. The Office of Technology Assessment, in a report titled “Technologies to Maintain Biological Diversity,” defines biological diversity as “the variety and variability among living organisms and the ecological complexes in which they occur.” Thus biological diversity, or biodiversity, is

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really much more than just the species richness (the number of species) or species diversity (a measure of both the number of species and their relative abundances) at a given site. Biological diversity is a more comprehensive concept that considers several types of classification. For example, we can consider not only the number of species at a site, but also the representation of different higher levels of taxonomic classification such as the number of families or orders present. Likewise, we can consider genetic variability within a population or among different populations. Genetic variation among populations is reflected in “subspecies” designations, and these local populations or subspecies are often threatened or endangered. Finally, we can consider ecological diversity at a number of different levels. One example of this is the number of ecosystems represented within a landscape, such as the presence of the habitat mosaic within the Rio Grande Valley. All of these concepts are important when considering biodiversity.

Biological integrity refers to the wholeness of a system, which includes the presence of all of the appropriate components and of all appropriate processes. Thus not only must the correct native species of plants, animals and microorganisms be present, but also the numerous processes, such as nutrient cycling or energy flow, should be occurring at their correct rates. This reflects the capacity of the system to recover after disturbances. The biological integrity of the Rio Grande riparian forest has been greatly compromised since the water in the river has been regulated. Natural processes such as flooding, which affects, for example, rates of decomposition of dead plant material and recruitment of young woody plants, have been altered. Thus the riparian forest now is not self-sustaining. Conservation and restoration efforts focused on promoting the biological integrity of a system are much more effective than those designed simply to save individual species. This is an important concept along the Rio Grande.

Keystone species are important to the maintenance of local biodiversity. A keystone species is a species that has strong regulatory effects on the composition of the community, even though it may be in relatively low abundance compared to other members of the community. The presence of the keystone species in some way determines which other species of organisms can be present. For example, in montane river systems beavers act as keystone species through their engineering feats: the construction of beaver dams and ponds creates habitat for many other aquatic plants and animals. When beavers are removed, the ponds are not maintained and many other species are eventually lost. Other examples are large herbivores such as elephants and rhinos in the African savannas, sea otters in the great kelp forests off the western coast of North America, and starfish in the intertidal zone. In the local bosque, the Rio Grande cottonwood is considered a keystone species because it provides the physical structure for the forest itself. Without cottonwoods, certain species of animals, such as cavity-nesting birds or insects that feed on cottonwoods, will not be supported. The presence of the cottonwoods creates the physical conditions that allow other bosque plants to grow. A complete loss of cottonwoods will certainly change the structure and composition of the local riparian forest.