Anthropomorphic Effects on the Rio Grande: A Biogeographic Assessment Lauren Bilbe, River Systems Institute, Texas State University, 2006
Figure 1. Rio Grande subbasins.
1 Draft Geographic Course of the River
Depending on how it is measured, the Rio Grande ranks as the forth or fifth longest river in the United
States and approximately the twenty-second longest in the world. The river flows 1900 miles from southwestern Colorado through central New Mexico and on to El Paso, then flows south-east, making the
border between Texas and Mexico, and empties into the Gulf of Mexico. The portion of the river which flows
through Colorado is 175 miles and in New Mexico it runs 470 miles in length; the Texas-Mexico border
portion is almost 1300 miles (Metz. 2004). The watershed is a total of 335,500 miles², though only
approximately 176,000 mi² are made up of open water basins that serve as tributaries. About 88,000 mi² of
the contributing areas come from each
country, with 38,000 mi² of the US portion
below El Paso (Levings et al. 1998).
Colorado
The Rio Grande in Colorado stems
from western Colorado near Silverton and
flows east through the San Juan
Mountains. The main climatic sources of
the Rio Grande’s flow come from Figure 2. Colorado Watersheds, Rio Grande Basin. Colorado and northern New Mexico
snowmelt from April to May and general
summer monsoons happening from July to
late August.
Three main tributaries flow into the
San Luis basis from the San Juan Mountain
and other nearby mountain ranges. These
include Alamosa and Conejos Rivers from
2 Draft
Figure 3. Rio Grande Basin in Colorado. the west and Trinchera and Culebra Creeks from the east. Also, two channels import water from the upper
San Luis closed river basin and its two tributaries. These are the Rio Grande Canal and Conveyance Channel.
There are four towns in the basin, with Alamosa at the conveyance of Alamosa River and the Rio Grande, as the largest. The basin is surrounded by mountains to the east and west, where the mountain headwaters in
Colorado have an average potential evapotranspiration of 70% (Eaton and Anderson. 1987;Levings et al.
1998). This can be seen in Figures 2, 3, and 4.
Figure 4. Underground hydrologic water basins of the Rio Grande Rift.
Figure 5. Watersheds of New Mexico.
New Mexico
From the San Juan Mountains in Colorado, the river flows through the mountains and highlands of north central New Mexico. The length of the Red River located in New Mexico joins the river from the east 3 Draft near Taos. Downstream Rio Varechos enters Rio Chama and the tributary, then conjoins with the main stem
of the river at Espanola. From there, it flows southwest
through the Espanola Basin west of Santa Fe, to the central and
southern plains of the state, bordered on both sides by various
mountain ranges. The central valley, known as the Middle Rio
Grande Valley, stretches from below the Cochiti Reservoir to
just above Elephate Butte. Rio Puerco and Rio Salado join the
river from the west in this stretch below Albuquerque. Below
Elephant Butte, Caballo Dam, and Truth or Consequences, is
the Mesilla Valley. No major tributaries are located in this
region. Las Cruces is the only major city. From Las Cruces,
the river flows southeast thirty miles to El Paso. The flow in
southern New Mexico is low in the fall and winter due to
alluvial seepage at delayed water returns from irrigation.
Flows are higher in the spring and summer as water is released
from Elephant Butte and Caballo reservoirs for the irrigation
season.
East of the major Rio Grande basin in New Mexico lies
the Pecos River basin, a major tributary to the Rio Grande
conjoining in Texas. The Pecos basin stretches from just
northeast of Santa Fe to southeast New Mexico. It has four
main contributing streams in New Mexico. Two are in the
Figure 6. Precipitation. north. In the south flowing west are Rio Hondo and Rio Penasco, north of Carlsbad. The river flows past Carlsbad on
its way to Texas (Eaton and Anderson. 1987).
4 Draft
Texas and Mexico
Once the main stem of the Rio Grande reaches Texas at El Paso and Juarez, Mexico it forms the international boundary between Texas and Mexico all the way to its entrance in the Gulf of Mexico. From El
Paso, the river flows southeast in a flat basin quickly reaching Fort Quitman. Water is diverted at America
Dam for El Paso’s use, and from International Dam for Juarez. Water is added back to the river at the Haskell
R. Street wastewater treatment plant, resuming perennial flows. Shortly downstream, water is taken out again at the Riverside Diversion Dam. After flowing through the Riverside Diversion Dam, the flow of the river is
seasonally intermittent. The flows in this portion
come from irrigation returns, alluvial inflows, and
mountain springs towards the convening with the
Rio Conchos. After Fort Quitman, the river flows
through valleys and canyons at a trickle for 212
miles until it reaches the confluence with Rio
Conchos just above Presidio and Ojinaga.
The Rio Conchos is the largest tributary in
the southern half of the river making up about
2/3rds of flow on the border. The Rio Conchos
starts in the Tarahumara Mountains of Chihuahua
and Durango states in Mexico. The river consists
of seven rivers and streams. To the south the
tributaries include Rio Florido, Del Valle de Figure 7a. Rio Conchos Basin in Mexico. Allende, Rio Del Parral, and De Aguiar/De Belleza.
In the northern basin there are Rio San Pedro and Rio Chuviscar. Rio Concho’s two main stems join in the northern half of the basin, east of Ciudad Chihuahua. The river is perennial and provides most of the water
5 Draft for the Rio Grande from its confluence in the
south. The quality of the river from this portion
down is mostly influenced by the waters of the
Rio Conchos, which provides a perennial flow to
the Gulf. Flows from the Conchos are regulated
by its dams in Chihuahua (International
Boundary and Water Commission and et. all.
1994). Figure 8. Pecos River Basin, Texas. Downstream of Presidio/Ojinaga, the river flows through a wide valley, until it reaches Big Bend National Park where the river travels through a series of canyons. After the park, the river flows northeast 135 miles through limestone canyons to Langtry,
Texas. Along the way, Alamito and Terlingua Creeks have joined the river. After Langtry, the river flows around small hills and through narrow valleys until it arrives at Amistad Reservoir. Amistad is the confluence point for the United States’ Pecos and
Devils Rivers.
The Pecos River enters Texas in Loving County, after flowing through New Mexico where it is immediately dammed. It flows southeast in the vicinity of Pecos, Fort Stockton, and a handful of small towns.
The Pecos river’s main tributaries in Texas are
Delaware, Toyah, and Independence Creeks.
Devils River is a smaller tributary flowing generally south from northeast Crockett County. It Figure 9. Rio Grande, Pecos, and Devils Rivers in south west Texas.
6 Draft joins the southern portion of Amistad Reservoir, where it doubles in surface area. Its main tributaries are Big
Satan Creek, Rough Canyon, and Dolan Springs.
7 Draft
Figure 10a. Upper Rio Grande Basin of Texas.
Figure 10b. Lower Rio Grande Basin of Texas.
The flow downstream of Amistad Dam is greatly influenced by the water it releases, with its tributaries impacting flows and quality less. Just below Amistad Dam on the Rio Grande the sister cites of
Del Rio and Ciudad Acuna straddle the river and the Texas-Mexico border. Further downstream are Eagle 8 Draft Pass and Piedras Negras, also sister cities. The US tributaries, entering in this stretch are San Felipe Creek,
Sycamore Creek, Pinto Creek, and Las Moras Creek, all of which are minor. Also in this region, Rio San
Diego, Rio San Rodrigo, Rio Escondido, and Rio Las Vacas, all from Mexico, deposit into the mainstem. The headwaters of San Diego, San Rodrigo, and Escondido come from the Del Burro Mountains in the west. The river then flows through Laredo/Nuevo Laredo. Eighty miles south of those cities, the river passes by Zapata and is dammed again at Falcon Reservoir.
In this stretch, Rio Salado, a very long tributary, also joins in. Rio Salado begins far south of Langtry and has many important tributaries. These include Sabinas, Salados de la Nadadores, Sabinas Hidalgo, and
Cameron Creek in the states of Coahuila and Nuevo Leon.
Figure 7b. Lower Rio Grande tributaries in Mexico.
9 Draft
The sister cities
of Eagle Pass and
Piedras Negras change
the quality and quantity
of the river downstream
through irrigation,
power generation, and
returning waters.
Laredo and Nuevo
Laredo impact water
quality with significant
effluent discharges.
During tests in 1994, the
Figure 11. Texas rivers and cities. river flows were
increased after International Falcon
Dam at four times the natural flow.
The water was released for use in
Mexico .
Below Falcon Reservoir,
the river flows through a narrow,
flat basin for 250 miles to the Gulf.
In the next 50 miles downstream of
10 Draft Figure 12. Total Annual Precipitation of Mexico.
Falcon, Rio Alamo and Rio San Juan
Catarinas join in from Mexico. Rio
Alamo, the smaller of the two rivers,
has the following tributaries: Macho,
Lajitas, Reyes, Yrias, San Jeronimo
Creeks, and Rio Sosa. San Juan’s
tributaries reach far into Nuevo Leon
and include Rio Sabinas, Rio
Pesqueria, Rio Santa, Rio Ramos,
Rio Pilon, and Mohinas Creek.
Figure 13. Actual Evapotranspiration. After Falcon Dam, the
International Anzalduas Dam diverts most of the excess releases at Mission and Reynosa for Mexican human and agricultural supply. Downstream the flows are reduced with many small diversions and change with agricultural and urban uses and releases
(International Boundary and Water Commission and et. all. 1994). The river travels through Rio Grande
City/Camargo and Brownsville/Matamoros. It also passes near McAllen, Pharr, San Benito, and Harligen, and is then released into the Gulf of Mexico near Point Isabelle. See Figures 10c and 11.
11 Draft Figure 10c. Rio Grande Bays and Estuaries of Texas
Major Ecoregions
Figure 14a.
12 Draft Southern Rocky Mountains Province
The headwaters region in Colorado is mainly composed of
mountainous forests and semiarid steppe. Elevation ranges from
6,000 feet below sea level to mountain peaks above 14,000 feet
above sea level (Levings et al. 1998).
Flora
The vegetation in the mountainous areas of Colorado and
Northern New Mexico are a mix of evergreen needleleaf forest,
deciduous forest, mixed forest, and grasslands. The areas highest in
elevation are characterized by cool, wet conditions of the alpine
tundra, with woody shrubs, such as willows, herbs, lichens, and
mosses. The subalpine areas consist mostly of Engelmann spruce Figure 15a. Climate regions of Upper Half of RG. and subalpine fir. Below in the montane zone, douglas-fir occupies the upper, more moist areas and ponderosa pine occurs in the lower, drier zones. After fire disturbances, aspen and lodgepole pines are first in succession in the subalpine and montane zones. In the ponderosa pine forests, undergrowth consists of a variety of oaks, including Gambel, gray, and wavyleaf oaks.
Grasses and sagebrush also occur in tree openings in Ponderosa Pines. The foothill woodlands are characterized by mountain-mahogany and scrub oaks. Along the Colorado Plateau Province, ponderosa pine and pinyon-juniper vegetation alternate. In the San Juan basin where the various tributaries of the Colorado and Northern New Mexico portion of the basin join, the vegetation is a drier scrubland, particularly sagebrush and antelope bitterbrush, with some peripheral grasslands. The lower montane riparian and woodland and shrubland areas of Colorado make up only 1% of the landscape. However 70% of the state’s wildlife species depend on this habitat. In the river watershed, cottonwoods and other deciduous trees, including Rio Grande,
Narrowleaf, Plains, and Lanceleaf Cottonwoods inhabit areas where the roots can reach the water table (U.S.
Geologic Survey. ).
13 Draft
Fauna
Information about fauna of this environment is limited and fragmented in scope and research methods
(U.S. Geologic Survey. ). Birds normally found in this ecoregion are mountain bluebirds, chestnut-backed chickadees, gray jays, Steller’s jays, Clark’s nutcrackers, pygmy nuthatches, ruby-crowned kinglets, and red- breasted nuthatchs. Also, hawks and owls live in most of these areas. Cutthroat trout are one of the variety of native fish to this region. Common small mammals include mice, bushytail woodrats, mountain cottontails, chipmunks, martens, and squirrels. These animals are known to use riparian areas: mountain voles, pocked gophers, pocker gophers, field mice, shrew, mink, and ground squirrels. Large mammals consist of the black bear, porcupines, beavers, bobcats, mountain lions, bighorn sheep, deer, moose, and elk. These are also found in riparian areas (Rocchio. 2005;U.S. Forest Service. ).
Chihuahua Desert
Southern New Mexico and west Texas make up part of the
Chihuahua Desert region. The middle Rio Grande, Pecos River,
Devils River, and Rio Conchos flow through this desert, where
Annual precipitation is less than eight inches per year and the
annual potential evapotranspiration may exceed 1000% (Levings et
al. 1998). Figure. 16. Chihuahua Desert of the United States and Mexico. Flora
The Chihuahuan Desert is characterized by scrub brush and
sparce grasses. Shrubs that inhabit this area include creosotebush, mimosa, acacia, mesquite, ocotillo, and
others. Riparian vegetation is characterized by the introduced salt cedar, as well as cottonwood, and desert
willow. The grasses of the desert occur in deep soil basins and include the following grasses: the dominant
blue grama, Tobosa in low areas where water collects, and beargrass and sotol on the hillsides. Some trees
which inhabit the grassland areas include little walnut, netleaf hackberry, and oak. Woodlands also occupy
14 Draft slopes and valleys with these trees: oak, juniper, pinyon pine, Texas madrone, and bigtooth maple. The
woodland shrubs are Texas mountain laurel, scarlet
bouvardia, and mountain sage. It is especially
interesting to note that this ecoregion contains the
highest diversity of cacti in the world, and some
believe that this type of plant originated here,
radiating out. Common cacti are prickly pear,
hedgehog, living rock, nipple cacti, and Corys.
Coniferous forests are found in high elevations.
Fauna
The extreme climate of the Chihuahuan desert
has led to the creation of a large number of
specialized and endemic species.
This region hosts a great variety of fish,
Figure 14b. including many endemic species, like its especially-
large variety of pupfish.
Invertebrates include the desert tarantula, whip
scorpions and other scorpions, the giant centipede, and
the desert millipede. A diverse number of butterflies
and moths also call it home.
Amphibians common to the region include the Rio
Grande frog, spadefoot toads, and the barred tiger
salamander. Bull frogs have recently been introduced
and have been successful along the Pecos and the main
stem of the Rio Grande. Lizards found here are
15 Draft
Figure 14c. whiptails, collareds, honeds, spinys, and geckos. Common snakes are the Mexican kingsnake, the Trans-
Pecos ratsnake, and many varieties of rattlesnakes. The diurnal ones are garter and coachwhips.
The largest number of different species of birds in a U.S. National Park, 425, has been counted at Big
Bend. They are usually in the process of migrating or live there seasonally and are found in low densities.
Some of the rare birds found there are the Colima warbler, lucifer hummingbird, varied bunting, band-tailed pigeon, elf owl, Mexican jay, black-chinned sparrow, green kingfisher, and pyrrhuloxia.
Mammals in the region are mainly diurnal as well. A highly diverse number of bat species inhabit the area, with 18 species found in Presidio County. Mexican freetail bats are the most common throughout the
Rio Grande Basin. Other notable species are the Brazilian Freetailed, the pallid, and the western pipistrelle.
Other mammals are a large variety of nocturnal mice, woodrats, many kinds of skunks, the spotted ground squirrel, black-tailed jackrabbit, ringtail, collared peccary, and coyotes. Larger predators are the mountain lion, gray wolf, and the black bear (Chihuahuan Desert Research Institute. ).
South Texas Brushlands or Tamaulipan Thorn Scrub
Flora VEGETATION TYPES OF TEXAS Below Del
Rio/Acunas, the river flows
into the Brush/Scrubland
section until its entrance into
the Gulf of Mexico. The
ecoregion characterizes
nearly all of south Texas and
into central Nuevo Leon and
Tamaulipas. The vegetation
is mainly thorn scrub with
neotropical biotic Figure 17.
16 Draft influences. The brushlands were formerly mesquite and acacia savanna with up to three feet tall somewhat open scrub areas and grasslands. Other vegetation types include oak forests, including live oak mottes, and
some tall riparian forests. Caliche capped
ridges are also characterized by purple
sage. Along the coastal areas, native
vegetation includes coastal marsh,
floodplain forests, and upland grasses.
Fauna
This region is generally considered
one of the most, if not the most,
biologically diverse area of the United
States.
Of representative insects, there are
70 species of dragonflies and damselflies,
around 300 species of butterflies,
including 70 endemic to this region, and
over 900 species of beetles in the
Figure 14d. Audobon Sabal Palm Grove Sanctuary alone.
Fish found in this portion of the river are diverse in their heritage. Some of the tropical fish found here are the Mexican tetra, and the Rio Grande cichlid. Identified within the same areas are northern fish like freshwater drum and buffalofish. Estuarine species found upstream include the striped mullet, gizzard shad, and the American eel. Introduced fish are the blue tilapia, common carp, Oscar, and peacock bass.
Amphibians include mainly frogs and toads, with 21 different species, making up over 25% of the species found in the United States. Unusual species include the giant toad, Mexican treefrogs, Mexican
17 Draft burrowing frogs, sheep frogs,
Rio Grande chirping frogs,
White-lipped frogs, the Rio
Grande lesser sirens, and
black-spotted newts.
Reptiles include 11
species of turtles, the
American alligator and the
introduced spectacled
caiman, 13 lizards, including
the keeled earless, mesquite, Figure 18. Land Use and Modified Vegetation of Mexico. rosebelly, four-lined skink, and Laredo striped whiptail, and two introduced species. Also, there are 33 types of snakes, representing 40% of all species of Texas snakes.
The diversity of birds in the Valley is the greatest in the United States, totaling 485 species. Endemic birds include the Texas red-shouldered hawk, Zapata Carolina wren, and the Brownsville common yellowthroat. Some of the tropical birds that make this region their northernmost home are the least grebe, muscovy duck, gray and white-tailed hawks, green parakeet, red-crowned parrot, tinged and green kingfisher, northern beardless-tyrannulet, great kiskadee, green and brown jays, cave swallow, tropical parula, and altamira and Audobon’s orioles. Many other birds have been found in the region, some anomalies and over
200 species common to the United States.
83 mammal species have been counted in the lower Rio Grande valley or offshore of the confluence with the Gulf of Mexico. Endemic species include the Mexican long-tongued bat, southern yellow bat, Texas pocket gopher, Gulf Coast kangaroo rat, Mexican spiny pocket mouse, Coues’ rice rat, eastern hog-nosed skunk, ocelot, and jaguarundi. Offshore species found are the bottle-nosed dolphin, several large whales, and others (Fermata Incorporated. ).
18 Draft Transitional Ecoregions
Areas of ecotone overlap can be found between the major ecological regions in the Middle Rio Grande
Valleys of New Mexico and Texas/Coahuila. Both of these regions have high biodiversity that is a
combination of the flora and fauna characteristic of the surrounding ecological areas. These ecotone
environments are important and will be discussed more thoroughly in other portions of this report.
Human Impacts to the Land and Water
Prehistoric impacts of Native Americans on the southwest landscape are not well known. However,
evidence indicates that Native Americans significantly impacted populations of mammals through hunting
Figure 19. Map of Land Use in 1881.
and flora by intentional burning of the landscape in the Rocky Mountains and plains (U.S. Geologic Survey. ).
It has been suggested by some that firewood foraging may have caused a shift from juniper and pinyon
dominated areas to grasslands((Figures 22, 23, 24) Allen, Craig D., Betancourt, and Swetnam. 1998). When
the Spanish arrived at the river valley in the sixteenth century, they found that Native Americans were living
in relatively large villages, had cleared bosque areas, and were using irrigated agriculture systems in west
Texas and New Mexico (Levings et al. 1998).
19 Draft In the Rocky
Mountains, European impact
was first felt when beavers
were trapped almost to
extinction by the 1840s.
Then discovery of gold
brought an influx of mining
operations and railroads. The
cropped land use map of
Colorado (Figure 19) shows a
large number of silver mines
Figure 20a. Land Use Map of Central Northern New Mexico in 1880s. in use in the headwaters area
of the west. The
southwestern headwaters area
shows gold and coal mines.
In the San Luis Valley,
grazing was taking place and
agriculture was occurring in
the river valley. Logging
soon followed.
Following the
Spanish settlement of New
Mexico in the 1700’s, the
landscape dominated by
grasses was altered with the Figure 20b. Land Use Map of Central New Mexico in 1880’s.
20 Draft introduction of sheep and cattle grazing as well as fire suppression. Figures 15 through 17 are maps of land use in the Rio Grande Valley of New Mexico. Cattle grazing and timber industries are clearly shown.
Agriculture is not, though it
was likely occurring in the
arable river valleys.
Additionally, the loss of
southwest grasslands has been
attributed to decadal climate
cycles and increased CO2 in the
soils triggering a shift to shrubs
that grow in the cool seasons
from warm weather grasses
Figure 21. Land Use Map of Central Southern New Mexico in 1880s. ((Figures 22, 23, 24) Allen,
Craig D., Betancourt, and
Swetnam. 1998). Europeans also expanded the area of irrigated agriculture bringing wheat, barley, and oats
to the upper half of the river valley. Little riparian area was left after their expansion throughout the 19th century, as woody vegetation was cleared for agriculture and wood fires, and surrounding areas were heavily overgrazed. Their impacts to the land were more significant than the Native Americans.
21 Draft
Figure 22b. Locations of tree ring samples analyzed in Figure 7.
Today, crops growing in New Mexico
and Colorado include potato and alfalfa. In
Southern New Mexico and West Texas Peppers,
cotton, pecans, and onions are being grown. Figure 22a. Tree ring fire record for the southwestern Little information is available on the effects of U.S. Yellow marks record local fires. Red marks record regional fires current agricultural practices. The dominant
issue faced by farmers in the region is the pressure to reduce water use due to demands by Albuquerque, Santa
Fe, Las Cruces and other cities in that highly arid region (Metz. 2004).
Fire records dating back to 1600, based on tree ring
samples, in the Southwest show that regional fires occur on dry or
average years after two or three wet years, which allow fuel loads
to build up. Also, the regional fire cycles fall in step with La Nina
(dry) years. Figure 19a shows these records and Figure 19b shows
where the tree ring samples came from. Fires are believed to
have been set by Native Americans up to the 20th century.
However, the high frequency of lightning in the southwest has led
scientists to believe that fuel loads and climate regulate fires more
so than the cause of the initial spark. Increase of intensive grazing,
Figure 23. Change of spatial distribution of montane grasslands over near Jemez, NM 22 Draft over time. starting in the late 1800’s, appears to be the main reason for the lack of fires in the early 20th century.
Community and governmental efforts and abilities to suppress fires became significant in the second half of the 20th century.
Tree ring samples in the mountains of New Mexico have shown an increase of conifers in former
grasslands. An example of this is shown near Jemez, New Mexico, in Figure 20, which shows the spatial
decrease of grassland hectares ((Figures 22, 23, 24) Allen, Craig D., Betancourt, and Swetnam. 1998).
The dominant shrub of the Chihuahuan Desert, the creosotebush, is found in southern New Mexico
packrat records from 2,500 years ago. However, creosote did not become a dominant feature until after the
arrival of the Spanish. The grazing and fire suppression practices of the European settlers allowed snakeweed
to eventually move in, followed by creosotebush in southern New Mexico. In west central New Mexico, the
encroachment of Juniper is attributed to these practices ((Figures 22, 23, 24) Allen, Craig D., Betancourt, and
Figure 24a. 1899. Acoma Pueblo and Figure 24b. 1977. Acoma Pueblo and Enchanted Enchanted Mesa, west of Albuqueque Mesa, west of Albuqueque approximately 100km. approximately 100km.
Swetnam. 1998). Figures 21a and b record the increase in Juniper over time.
The shift to a shrub-dominant landscape in the southwest has been hard to stop, because rich soil
collects under the shrubs as the grasses become fragmented. Efforts to reverse the process began in the 1960s
and 1970s when some of these new woodlands were cut down and burned. The vegetation shift has caused
mass erosion throughout the southwest, cutting out channels, or arroyos, on a large scale during the 19th and
20th centuries that had not existed before. Arroyo cutting was particularly severe in the Rio Puerco tributary 23 Draft basin. These deep arroyos caused deterioration of wetlands and their riparian vegetation, causing more arroyo cutting. The degradation of wetland vegetation and its effect on native fauna remains a serious problem throughout the southwest. In recent times, arroyos have been refilling and have had some succession of tamarisk in the bed/riparian areas. However, the sediment loads from the lack of vegetation continue to be a problem today, particularly in the Rio Puerco basin ((Figures 22, 23, 24) Allen, Craig D., Betancourt, and
Swetnam. 1998;U.S. Geologic Survey. 2004).
Forestry in the southwest began with the introduction of trains in the mid to late 1800’s. Through the
early twentieth century,
indiscriminant cutting was
standard practice, resulting in
soil erosion and major habitat
loss. By the mid twentieth
century, selective cutting was in
practice. From the 1960’s to the
1980’s, cutting was performed
to keep forests at even ages.
Also, occurring at that time, was
the removal of snags, dead trees
on the forest floor, which
reduced habitat for small
animals and birds. Up through
this time period, selection for
old growth stands and
individuals, especially the
highly valuable ponderosa pine,
24 Draft
Figure 25.
Figure 26. Economic based land use of Texas in 1935. was standard. The result has been extensive erosion of soils over time and highly dense young forests, prone to insect outbreaks. Furthermore, long term overgrazing and fire suppression practices have contributed to the degradation. The combination of young forests, the removal of grass breaks by overgrazing, and fire suppression leave the forests highly prone to damaging fires (U.S. Geologic Survey. ).
Grazing also began early in Texas with the introduction of sheep. By the mid to late 1700s, sheep grazing was well established with as many as 90,000 of them herded from Laredo to McAllen. Cattle and goats were grouped together in the landscape and were a booming industry by the late 19th century, then
continued to through the first half of the 20th century. The effects on the Texas landscape have been similar to
those experienced in New Mexico, though there is less literature is available. Decrease in grasses, succeeded
by small scrub brush has occurred in the southwest portion of the state. The cattle industry remains the main land use in the arid regions of Texas today, though their density has decreased out of necessity (Carlson. ).
The Spanish also discovered that Native Americans were using irrigated agriculture in the arid West
Texas areas of the Rio Grande and the Pecos, though they were living in smaller communities. The Spanish settled around 1500 at the Indian community in Presidio and carried on the agricultural tradition. It is claimed
25 Draft that Presidio is oldest known, continuously used irrigated agriculture community in the United States. In the
1868, the first large irrigation project in Texas was constructed at Del Rio. In the 1870’s, the next major project was built at Fort Stockton on the Pecos. El Paso and the coastal area of the river received major irrigation improvements the 1890s (Smith. ).
Agriculture has continued to be an important economic, land, and water use factor in Texas. In 1948,
10% of the irrigated cropland was utilized for agriculture in the state; in 1973, 72 % of this land was utilized, and of the irrigated lands within the basin, Marfa, Van Horn, and Pecos areas are watered mainly from the ground. The other areas of the basin depend on surface river water.
In south Texas, the lower Rio Grande Valley remained rural grazing ranchland until around 1900. The first American settlement, Fort Brown was established in 1846, however, large scale irrigation agriculture did not happen here until 1898. This and the arrival of the railroad in 1904 brought about quick changes to the urban landscape in of the delta. Within the next thirty years, the population in Starr, Cameron, Hidalgo, and
Willacy counties had reached 176,000, and it has not slowed down since. Today, land uses of the Lower
Valley in both the United States and Mexico are mostly agriculture and urban areas, though large cattle ranches remain. The rich soils of the river delta allows for a wide variety of crops including citrus fruits,
Figure 27. Agriculture of Mexico in 1885.
26 Draft cotton, grain sorghums, and sugarcane. The conversion of the Tamaulipan thornscrub for these two purposes is a major concern for wildlife. In 1988, 95% of this brushland had been cleared to make way for cities and farms, including 90% of the riparian vegetation (Bloodworth and Gillett. ).
The Rio Grande’s exit into the Laguna Madre has important effects on the water and the condition of the laguna is important to the health of the fish. Laguna Madre is one of a couple of hypersaline areas like this in the world. 75% of Texas seagrass meadows exist in the Laguna. Human impacts to this ecologically sensitive are include the loss of 150 kilometers squared due to excavation of a canal connecting the upper and lower portions. Also, 190 kilometers squared were lost due to saline moderate inflows from Arroyo Colorado, an irrigation return flow canal on the Rio Grande, and others like it. Overall, there was a 30% decline in seagrass from the mid 1960s to 1988. Finally, algal blooms, better known as “brown tide” occurred in more recent times. Causes include limited flushing of the Laguna, added nutrients, and low zooplankton. All of these causes could have come partly from the Rio Grande. The health of the aquatic life in the Rio Grande is affected by the Laguna’s health when the mouth of the river at the Gulf is open (most years). In addition, the
Rio Grande has a greater affect on the Laguna health (U.S. Geologic Survey).
Little information about the land use history of Mexico was found, but it likely has had similar land use patterns as in Texas. Mexico is also well known for excessive use of land for agriculture in dry regions causing vegetation breakdown. Today croplands are less viable in arid regions, which has contributed to the movement of people to the border lands. However, the Rio Conchos basin is particularly productive in agriculture, as is the Lower Valley.
Water Law and Hydrologic Modifications
The degradation of aquatic ecosystems and decline of particular organisms in the southwestern United
States is attributable to dams and other watershed construction projects including diversions and wetland drainage. These projects also lead to fragmentation of habitats, reducing wildlife viability. The Rio Grande has also been dammed, channelized, and changed in many ways and is no exception (U.S. Geologic Survey).
27 Draft The boundary between Mexico and the United States at Texas was defined under the Treaty of
Guadalupe Hidalgo in 1848 as the center of the deepest channel of the river. However, the natural state of the river is to meander, in a constant state of change. This caused problems for determining the proper boundary, and land owners periodically had land and property seized. In 1884 and 1889, the International Boundary
Commission was created to administer the treaty. The shifting of the river continued, and oxbow lakes, or
now isolated wetlands that used
to be the main river channel,
were created. In 1905, the law
determined that when the lakes
were created (then called
bancos and now known as
resacas), ownership would
belong to the country that it
ended up on the side of
(Timm.).
In 1916, Elephant Butte
Dam was built to retain water
for irrigation use in the lower
Rio Grande Valley of New Figure 28. Mexico and El Paso/Juarez.
The creation of the dam worsened the river’s crop-destroying meandering due to low flow sediment build up.
It also increased scrub brush invasion with the upset of riparian areas. So, the Rio Grande Rectification
Project was created in 1933, channelizing the river from El Paso to Fort Quitman. A year earlier, the Lower
Rio Grande Valley Flood Control Project was agreed upon to raise and reinforce levees, as well as to dredge the floodways and channel. Soon thereafter, the Rio Grande Canalization Project was constructed,
28 Draft straightening the 100 mile portion from Caballo Dam (finished around that time) and south to the Texas border. Also agreed upon in 1933, were a series of levees and other controls approved for the 300 miles between Donna and Brownsville. The 1944 treaty apportioning the river (discussed below) below Fort
Quitman also agreed to the building of Falcon and Amistad dams. Arroyo Colorado is the last major project of notable size on the river. It catches the agricultural return at Brownsville and impounds it for recreation.
The Arroyo is seriously polluted and stops the inflow of the freshwater to the Boca Chica estuary (Timm. ).
Many other dams and channelization projects of smaller scale exist on the Rio Grande and its tributaries.
Figure 28 shows the main dams in the basin and their completion dates.
The river shifting problem continued with the Chamizal agreement of 1963, where a large piece of downtown El Paso was ceded to Juarez. In 1970, the last land switch occurred when over 8 miles was cut out of the Presidio area, straightening and stabilizing the river. These channelization and dam projects have proven beneficial economically, especially for farmers, but at the cost of river and riparian health, which is explored further in the Ecological Effects section (Metz. 2004).
29 Draft
Figure 29. International Treaties and Hydrologic Modifications on the Rio Grande.
The permanent apportioning of water rights first occurred in 1906 when it was determined that a maximum of 60,000 acre feet a year had to be provided to the Juarez valley at Fort Quitman as long as
Mexico waved their right to water above that point. This amount is proportionally reduced in years of drought. The 1944 treaty apportioned the water south of Fort Quitman such that the United States kept all water from its tributaries and received 1/3 of the water from most Mexican tributaries totaling no less the
350,000 acre feet a year average over five years. The treaty also stated that the water was to be split evenly below the last international dam. These treaties remain in effect and appropriated amounts regularly exceed actual flows (Timm. ).
Information regarding the total average flows of the river was not found, probably due to the variable flow rates caused by the many dams and their releases in the watershed. However, the combination of
30 Draft hydrologic construction and diversions is known to reduce average flow of the lower reaches of the river by
30% to 50%. Though channelization has not been studied in the basin, the effects on the riparian ecosystem
are well known. The altered flows and historically normal flooding has contributed with the replacement of
mesic riparian wooded areas with xeric plants, such as mesquite, and the lack of periodic flooding has
required native vegetation to adapt or be replaced with non-indigenous ones (U.S. Geologic Survey. ).
Current Water Use Problems
Table 1. Table 2.
Table 3.
Urbanization The first major shift towards
urbanization in the basin began with the arrival of
the railroads. The Rio Grande saw its first trains
in the 1850s in the Lower Valley in Texas and
possibly in Colorado. By the 1890s, the railroads
had reached El Paso, and by 1904 had reached the
lower valley and many other places in the
watershed ((Tables 1,2,3) Hinojosa, J. R. 1998).
The population boom has been experienced throughout the United States, especially along the border, and has
had significant impacts on the environment ((Figures 22, 23, 24) Allen, Craig D., Betancourt, and Swetnam. 31 Draft 1998). Particularly in the 1970’s, seven U.S. cities on the U.S.-Mexican border had growth rates at three to five times the national average. Mexican cities during this period had growth rate of 67% to 97% (same as tables). In 1980, the total population of people living near the river exceeded 5 million. In 1994, 8 million
Mexicans alone lived on the river, four times the number of people on the entire U.S. portion of the basin
(U.S. Geologic Survey. ). New Mexico and Colorado have seen significant increases in population size and urbanization, as well. Though, their rates are no where near as high a border cities, due to Mexican immigration on the border. Major cities on the river in New Mexico include Santa Fe, Albuqueque, Socorro,
Truth or Consequences, Mesilla, and Las Cruces. Texas cities are El Paso, Presidio, Del Rio, Eagle Pass,
Laredo, Rio Grande City, McAllen, and Brownsville. The main Mexican cities on the river are Ciudad
Juarez, Ojinaga, Cuidad Acuna, Piedras Negras, Nuevo Laredo, Camargo, Reynosa, and Matamoros (Metz.
2004).
Impacts of urbanization include increased pollution from the main sources of maquiladoras, waste water, especially unincorporated towns or colonias, and runoff from city streets and lawns.
The rapid immigration to the border
NUMBER OF BORDER RESIDENTS WITHOUT ADEQUATE has found cities unprepared. Lack of WASTEWATER TREATMENT FACILITIES BY COUNTY, 1996 necessary infrastructure, particularly
inadequate waste water treatment facilities
is a significant problem. Most Mexicans
on the border live without proper sewage
treatment. 30 towns and cities from both
sides of the border were listed as needing
improvements to their wastewater systems
by the Army Corps of Engineers in 1996.
Figures 30 & 31 illustrate the problem in Figure 30a. Texas. Hidalgo county was the worst off with 136,000 residents with inadequate sewage treatment. El Paso County was next with 75,000. Cuidad
32 Draft Juarez produced the most improperly treated effluent discharged in the river per day at 67 million gallons;
Reynosa and Matamoros were next UNTREATED OR PARTIALLY TREATED WASTEWATER DISCHARGED ALONG THE TEXAS-MEXICO BORDER FROM releasing 30 million gallons per day MEXICAN CITIES, 1998* each.
Colonias are an important part of
the inadequate wastewater treatment
situation on the border. Colonias are
unincorporated villages that often do not
have running water or effluent treatment.
There were approximately 1,400
colonias with approximately 340,000
Figure 30b. residents in south Texas alone in 1995,
and they exist on both sides of the
border. Figures 31 and 32 show the
distribution of colonias in the Texas.
Impacts from effluent released by cities
and/or colonias are serious problems for
surface, groundwater, and human health,
and they are discussed in the water
quality section ((Tables 1,2,3) Hinojosa,
J. R. 1998;U.S. Geologic Survey. ). Figure 31. Locations of Colonias in Texas.
33 Draft
Figure 32.
Maquiladoras
Pollution from industries is
particularly bad in the lower
half of the basin due to the
increasing size number of
maquiladoras. Maquiladoras
are factories owned by U.S.
or other foreign companies.
The Mexican government Figure 33. Concentration of Maquiladora Factories along the Border. initiated the industry in the
1960’s to promote economic development on the border. Figures 33 and 34 show recent distuributions of maquiladoras and people employed by them in Mexico. The cheap labor, tax-free importing to the home 34 Draft countries, and lax
environmental laws have
attracted corporations, and
NAFTA has promoted an
influx of new maquiladoras
on the border. All types of
pollution can be discharged
by these factories and
regulations in Mexico are
less stringent than the
United States. Data Figure 34. accounting for point source pollution by maquiladoras is scarce. However, it has been estimated that as little as 30% of maquiladora toxins are brought back and properly disposed of as required by United States-Mexico agreements, and 98% of maquiladoras lack proper waste water treatment systems. Indirectly, maquiladoras contribute to environmental degradation by attracting people to the already overwhelmed border for the jobs (Metz.
2004;Williams. 1995).
Overall, the most important human impacts to the environment over the last 30 years are hydrologic changes to the river, brushland clearing, immigration to the border, and urban related contaminants. Urban settlements and agriculture have reduced and fragmented wildlife habitats. The damage to water quality and the Rio Grande ecosystem is examined in the following sections.
35 Draft Water Quality
The water quality in the upper half of the Rio Grande Basin was assessed from 1993 through 1995 in two studies, one concerning the upper half of the river through El Paso, and the other from El Paso and
downstream. Based on the results of the water quality testing, general contributors to water quality deterioration in the Rio Grande basin are as follow. Urban centers pollute through waste water, organic
chemicals, and temperature change.
Agricultural uses and fertilizers add
pesticides. Mining pollutes by contributing
trace elements. Additional pollution not
addressed in this paper comes from nitrate
and phosphorus deposited from the
atmosphere (Levings et al. 1998).
Figure 15b.
36 Draft
Figure 35a. IBWC Study Testing locations.
Traditional Water Quality Standards
Overall, natural water quality tests were mostly healthy in the lower half of the Rio Grande, except for localized high concentrations of nutrients, bacteria, and total dissolved solids.
Nutrients and Microorgranisms
Nutrients are natural part of the river processes necessary for life, and there is a limit to healthy amounts of nutrients in water that are often exceeded through additions by humans. Two common nutrients added by people are nitrogen and phosphorus. These and others chemicals are contributed through wastewater effluent, agricultural runoff, rain, and dissolved matter from natural minerals. The accumulation of nutrients leads to eutrophication, or overgrowth of aquatic plants that causes oxygen depletion in the water.
Eutrophication can significantly disrupt aquatic ecosystems, killing off oxygen-sensitive species. 37 Draft The studies of the western half of the river have found wastewater releases to be far more significant than natural nutrient additions (Levings et al. 1998). And the southeastern half has many communities with inadequate wastewater treatment facilities (International Boundary and Water Commission and et. all. 1994).
Dissolved nutrients in the upper Rio were found mainly below urban areas, particularly wastewater treatment plants. Drains in agricultural valleys found only small amounts of dissolved nutrients. The numbers may be small due to quick biological use, though it was still believed that urban sources of dissolved nutrients exceed the agricultural. Overall, stream levels of phosporus from over 500 testing sites found that nearly half exceeded recommended levels from government agencies.
Total nutrient counts in the river and its tributaries are also closely related to nutrients contained in sediments. The largest source of nutrients overall came from Rio Puerco where high amounts of sediment discharge have occurred in the past. Of all portions tested in the upper half of the river, 35% of sites had ratings of eutrophic or highly eutrophic due to nitrogen and phosphorus levels. Eutrophication decreases dissolved oxygen concentrations in the river. Historic data of nutrient counts on the Rio Grande show that nutrient counts decrease at, and downstream of reservoirs, due to settling out.
Ground water testing showed leaching of nutrients from agricultural sites exceeded standards in 1/6 to
1/3 of the shallow water testings. Shallow wells under Albuquerque were known to intake significant amounts of effluent nutrient loading. Nitrate tests there were low though, due to low oxygen levels causing a lack of conversion from nitrogen. Minimal leaching into deep wells indicates little recharge in those areas
(Levings et al. 1998).
In a 1992 study by the Texas Natural Resource Conservation Commission (TNRCC), now the Texas
Commission on Environmental Quality (TCEQ), five of six test sites below the border sister cities on the Rio
Grande showed bacterial contamination and some overly high levels of nutrients. Their presence was mainly attributed to waste water, including some untreated, and non-point runoff (International Boundary and Water
Commission and et. all. 1994).
38 Draft Dissolved Oxygen
Little data was discovered about the dissolved oxygen concentrations in the upper portion of the river.
In the lower half of the river, dissolved oxygen met the 5mg/L standard at all but 1% of the tests. The depressed oxygen levels were low mainly upstream from the confluence of the Rio Conchos and downstream of Falcon Reservior where significantly decreased releases were occurring (International Boundary and Water
Commission and et. all. 1994). In tributaries, low dissolved oxygen counts were found in four tributaries out of 23. These four carry wastewater from the following urban areas: Ciudad Juarez, Piedras Negras, Nuevo
Laredo, and Reynosa.
Dissolved Solids
Dissolved solids are minerals mixed into the river water solution. Salinity is one of the most important types of dissolved solids and can generally be measured through these counts. Dissolved solids make their
way into the water systems through evaporation,
transpiration, and dissolution. Each of these
processes is a major contributor in the Rio
Grande. Overall, quantities of solids increase as
the Rio Grande flows south and east, with small
counts in the upper portions, and progressively
larger counts downstream. Locations with
particularly high amounts of dissolved solids for
their location on the river were Trinchera Creek
in Colorado, Rio Puerco in New Mexico, and El
Paso, Texas. Figure DS1 shows counts taken in Figure 15c. {{2475}}. the USGS study (Levings et al. 1998). In the lower reaches of the river, total dissolved solid (TDS) levels were exceeded consistently above
Presidio/Ojinaga before Rio Conchos, and below Brownsville/Matamoros between 1976 and 1993. In
39 Draft between those areas, five tributaries had high TDS counts in the 1993 study. These sites in included the Pecos and four others ranging from just north of Laredo/Nuevo Laredo to midway between Falcon and
Brownsville/Matamoros. Only the Pecos and Rio Salado contributed significant amounts of TDS to the main
Rio Grande channel.
Sources of salt include soils in areas with high evapotranspiration rates returned in water agricultural fields and from ocean water. The heightened salinity makes it difficult for nonindigenous fish to survive and has allowed the blue tilapia to become dominant in the Brownsville area, for example. It is also more difficult for humans to use and may require filtering for some purposes (U.S. Geologic Survey. ).
Additional Tests
The lower half of the river was also tested for chloride, sulfate, total organic carbon, pH, and temperature. All tests from the main channel of the river came back fine except for high chloride counts above Presidio/Ojinaga, high sulfate below Brownsville/Matamoros, and elevated levels for both on the Pecos river. High organic carbon quantities came back for the four tributaries with low dissolved oxygen counts in:
Ciudad Juarez, Piedras Negras, Nuevo Laredo, and Reynosa. A high organic carbon count was also found at
Zacate Creek north of Laredo. An elevated level of organic carbon was only found in the main stem of the river downstream of the Juarez site (International Boundary and Water Commission and et. all. 1994).
Toxic Pollution
In the lower half of the river, many toxic chemicals exceeded fish and human health standards and many others were above the 85th percentile of toxicity in United States’ rivers. Many more chemicals were detected. In the mainstem portion of the river, six toxic chemicals exceeded quality standards in the water column itself (International Boundary and Water Commission and et. all. 1994).
Pesticides
Pesticides were found in 94% of the surface waters, tributaries, and drains of the upper half of the river during the USGS study. However, most pesticides found in that study were barely detectable in the lab and no levels exceeded state or national standards.
40 Draft The use of these organochlorine pesticides were banned in the early 1970’s in the United States due to their high toxicities and lack of biodegradation. Theses chemicals are hydrophobic in that they bond to carbon in sediments instead of breaking down in water. The persistence of these substances leads to bioaccumulation in sediments and in all of those that consume its waters.
There is build up of organochlorine pesticides in upper reaches of the Rio Grande. DDE was found in
10 of 11 composite fish samples taken. DDE, DDT, and DDD were found in 1/3 of sediment sample and more in whole body fish samples. Presence of cis-chloridane, trans-chloridane, and trans-nonachlor were also found in whole body fish samples. Overall, the organochlorine detection on fish samples were more diverse, high in amounts detected, and detected more often. However, amounts of these substances were below national river averages and quality guidelines for theses chemicals. Sediment cores for Elephant Butte also showed a significant decrease in toxicity levels since the 1970’s.
Modern pesticides have been designed to break down quickly to lessen the toxic effects. However, they were still detected in the basin’s waters. In the surface waters of the upper portion of the basin, 156 samples were taken and 125 of those samples, a total of 332 occurances of 23 different pesticides were detected. The chemicals found most often were DCPA, metolachor, prometon, and simazine.
These pesticides were also found in the shallow aquifers of the basin. Shallow ground water wells
were tested in the top 10 to 15 feet of the water table throughout the upper half of the Rio Grande. In these tests, at least one pesticide was found in 29% of the wells. The pesticides most commonly found were prometon and metolachlor. No levels of pesticides detected exceeded government standards, though there are no standards for some of these chemicals.
Deep water wells are the main source of drinking water for the Rio Grande watershed. In all, only one pesticide was detected in all of the deep wells in the upper portion.
In general, the few pesticides found and the small concentrations of them in the waters of the upper half of the basin show that the pollution occurring is not significant enough to be of major concern. Maps of specific site ratings of pesticides detected can be found in Figure 15 (Levings et al. 1998).
41 Draft In the lower half of the basin, only particularly toxic pesticides were tested for. The pesticides determined to be of high concern in this half were chlordane, DDE, dieldrin, and lindane. Diazinon was determined to be of medium concern.
Pesticides exceeded standards or were in the US Fish and Wildlife Service’s 85% ranking in the US in four of the five main use areas of the lower river. No pesticides exceeded standards in the El Paso/Juarez area. However, just before the confluence of the Rio Conchos, in the mainstem and in the Conchos, there were high levels of DDT and lindane in fish tissue. Below Amistad and above Del Rio/Cd. Acunas, diedrin and DDT were found in the mainstem and a tributary. Above Laredo/Nuevo Laredo to below the city, the tributaries brought in high amounts of five pesticides, including DDE, diaznon, dieldrin, and chlordane.
Diaznon was present in Zacate Creek at levels that were toxic for aquatic life in acute and chronic levels. The only chemical found in high levels downstream in the main portions of the river was chlordane. Finally, toxic chemicals were found in one US and one Mexican tributary and at two locations in the river below Falcon
Dam. Each site had different chemicals identified, including DDE, DDT, diazinon, and chlordane. All chemicals found to be in excess of national standards in the lower half of the basin were found in fish tissue or sediment samples, except for one test with high amounts of diazinon in the waters of Zacate Creek
(International Boundary and Water Commission and et. all. 1994).
Volatile Organic Chemicals (VOCs)
Volatile organic compounds are common household, industrial, and agricultural chemicals that easily evaporate in normal air pressures and temperatures. A portion of these carbon-containing chemicals are possible carcinogens causing health problems for those whose are in contact with them. VOCs leach into water systems and then are removed by absorption into organic matter and clays where bacteria and microbes break the VOCs down.
VOCs were found in a total of 11% of the shallow ground water tests in the northwest half of the river.
Most of these wells were in urban areas. VOCs detected in this portion include methyl tert-butyl ether
(MTBE), xylene, solvents, and metal degreasers. None were in high quantities (Levings et al. 1998).
42 Draft In the Texas-Mexico portion of the river, volatile organics exceeded standards in the stretch from
El Paso to the confluence with Rio Conchos and at one location south of Laredo. Methylene chloride was the most persistent of the volatiles, found in sediment tests at six out of the seven sites, including a tributary from
Juarez and one from Texas near the Rio Conchos entrance. It exceeded the standard for aquatic life by sixteen times in El Paso and twenty three times at the downstream of El Paso. In addition, toluene was high in sediment below El Paso and in the Juarez sewage discharge channel. Both of these chemicals were high south of Laredo in the river. Also, chloroform was found in high quantities in the water coming from El Paso’s
Haskell Street wastewater treatment plant. Methylene chloride and toluene were listed in a high priority toxics of concern list. Chloroform was low priority (International Boundary and Water Commission and et. all. 1994).
Semivolatile Organic Compounds
Semivolatile organic compounds are toxic and often carcinogenic substances polluted by humans.
Their presence can be a long-term source of pollution. Cresol and phenols come from coal and wood tar distilling, oil, gas, and other chemical plants, livestock industry, and effluent.
These compounds were found in bed sediments of the river and its tributaries in the upper portion.
The main chemicals found were polycyclic aromatic hydrocarbons (PAHs), phenols, and phthalate esters.
Four main locations contained the majority of the contaminants. These spots were downstream from two urban settlements, a mining location, and a forest (Levings et al. 1998).
In the southeastern half of the basin, polychlorinated biphenols (PCBs) were listed as a high priority toxic of concern, two semivolatile organics were listed as medium priority two, including a cresol and other phenols. The other two phenolics recoverable and phthalate, were of low priority. Cresol and phenol were found in the effluent canals coming from sediment in El Paso and the water in Juarez. Phenolics recoverable was also in the sediment in El Paso Haskell canal. Downstream, from above Del Rio/Cd. Acuna to
Laredo/Nuevo Laredo, PCBs were extremely high in fish tissue in the river at five locations and in the San
Felipe Creek south of Del Rio. The toxicity level at its highest was 36 times the fish health limit just above
Eagle Pass. From there south, three tributaries had excessive phthalate amounts in sediment, all coming from
43 Draft Mexico, including Arroyo el Coyote which also had a high count of cresol, Rio Salado, and Anhelo Drain where is was found in chronic levels in water tests (International Boundary and Water Commission and et. all.
1994).
Trace Elements
Trace elements are naturally occurring chemicals that in many cases are necessary in nature. Some though, are long-lasting toxic substances, such as mercury, cadmium, and arsenic, and can be introduced through human use. Exposure to these chemicals can kill in large doses, and create severe chronic health problems with long term exposure.
Significant quantities of trace elements were found in the Rio Grande from various human and natural sources. Concentrations in the north were strongest near agricultural and mining sources of the pollution, instead of urban centers, and pollution decreased progressively downstream.
The Creede Mining District in Colorado was the main source of pollution in Willow Creek and the Rio
Grande headwaters region. Testing found “highly elevated” levels of antimony, arsenic, cadmium, copper, lead, mercury, silver, and zinc in sediment downstream from the district. Some of these elements were also found in the water column, and in fish and bryophyte (moss) tissue samples.
Also in Colorado, “highly elevated” amounts of cadmium, cobalt, copper, iron, manganese, nickel,
strontium, uranium, and vanadium were found in the Alamosa River water column and in sediments where it
entered the San Luis Valley. The presence of these chemicals has surpassed the Colorado chronic aquatic-life
standard for trout.
The Red River in New Mexico tested positive for beryllium, cadmium, cobalt, lithium, manganese,
molybdenum, nickel, and zinc in the water column and in sediments. One location exceeded New Mexico
cold water chronic standards for fisheries. At its confluence with the Rio Grande, the levels of these elements
were “moderately lower”.
The Rio Puerco confluence sediment tests found the maximum detectable amounts of almost all metals
tested for. Mesilla and Rincon Valleys had the highest concentrations of uraninum, molybdenum, lithium,
44 Draft chromium, boron, barium, and arsenic dissolved in the water column. Their presence can be contributed to by geothermal springs, irrigation return flows, and urban non-point source pollution.
Groundwater tests found four chemicals exceeding standards. Theses chemicals were uranium, molybdenum, manganese, and iron. Uranium exceeded standards in 15 wells, while molybdenum exceeded in two. Manganese and iron consistently exceeded standards, probably due to septic effluent. Overall, the ground water contamination is not bad on a widespread level (Levings et al. 1998).
The southern half of the river has extensive amounts of trace elements, especially metals polluting its waters. Eleven trace elements were named on the list of high priority for toxics of concern. These are residual chlorine, arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, and zinc. The list of low priority chemicals includes antimony and thallium. Every test site in the lower half of the river showed at least one trace element exceeding river health standards. Four sites out of 45 showed only one element over the standards; three were Mexican tributaries and one was a US tributary. At the upper limits, El
Paso/Juarez had the most, with nine. The average number per site on the river itself was five, while for tributaries it was 3. However, toxic levels were higher in tributaries due to undiluted effluent releases. There was no significant difference between the number of trace elements exceeded on the US versus Mexican tributaries.
Though their average number of chemicals exceeding limits was lower than the Rio itself, the sites with the most alarming concentrations of elements were in the tributaries. In El Paso/Juarez area, residual chlorine, which is used to sterilize effluent, was being released from the wastewater treatment plant at a level in the water column that causes acute and chronic health levels for aquatic life. These levels are 63 and 100 times higher than the health standard. High levels are believed to have impacted the macrobenthic community in the El Paso Haskell Wastewater canal. Arsenic and mercury were also above human health standards in this region in the water column. Arsenic comes from arsenic in bedrock, industrial waste, air pollution, and pesticides and is also know to come from pesticide in the Rio Conchos basin. Copper was twice as high in El
Paso/Juarez than was found anywhere else in the lower basic. A copper smelting plant used to operate on the river in El Paso, and copper is also common in urban runoff.
45 Draft Silver was found in the water column of the river and tributaries near Del Rio/Cd. Acunas and near
Eagle Pass/Piedras Negras at levels that could possibly cause acute and chronic health problems in aquatic life. Silver does is not naturally prevalent in rivers, but comes from industrial sources. It is likely to be the cause of the depressed fish and macrobenthic community in the area. From Eagle Pass to Laredo/Nuevo
Laredo selenium was at chronic aquatic health levels and at levels harmful to humans in tributaries, with selenium at acute levels in the river at Laredo. Chromium is a byproduct of industrial releases and from cooling systems. Chromium was at chronic aquatic health levels in the sediment just above Falcon Dam. Also in tributaries near these cities, arsenic has exceeded human standards, though seemed to have less of an impact in the main stem. From Laredo/Nuevo Laredo to below Falcon Dam diazanon is at acute and chronic aquatic levels. Also, between Falcon and Brownsville, a tributary has arsenic and mercury exceeding human health standards. So listed are the worst cases of metals/elements in the river and tributaries. Richness of trace element loads were common the bottom half of the river (International Boundary and Water
Commission and et. all. 1994).
Radon
Radon is a radioactive product of the uranium-radium breakdown cycle. It was found naturally in bedrock in the upper Rio Grande region. This compound poses threats to human health through common exposure from tapwater sources releasing radon into the air and through ingestion. The EPA proposed a limit that has been withdrawn of 300 pCi/L as the water quality standard. Many wells throughout the agricultural areas of the northwest portion of the river were found to have counts over the proposed limit. They were highest in the San Luis Valley where the average was around 1200 pCi/L. The radon amounts are believed to be from natural sources versus human pollution (Levings et al. 1998).
Inorganic
Un-ionized ammonia was listed as a medium priority on the list of toxics of concern for the southern half of the basin. It was found in four Mexican tributaries at chronic aquatic levels including the Juarez
46 Draft effluent channel, one upstream from Ciudad Acunas, one downstream of Piedras Negras, and one southeast of
Nuevo Laredo. Impacts were significant in the river downstream of the Juarez channel. Un-ionized ammonia comes from nitrogenous organic matter coming from wastewater of gas, metal, and other chemical industries.
Cyanide was found in the river at a chronic aquatic health level downstream of Anzalduas Dam. The fish and macrobenthic communities were abundant, though exhibited a slightly higher number of abnormalities
(International Boundary and Water Commission and et. all. 1994).
Biological Effects
Macrobenthic Communities
Macrobenthic tests were performed in the lower half of the river basin, using two measurement
systems. One type was the Mean Point Score system (MPS) where the following six parameters were measured: species richness, standing crop, Epheneropter-Plecoptera-Tricoptera (EPT Index), diversity, equitability, and community function. Community function was based on the number of functional feeding groups, dominance of largest feeding group, and abundance or organisms that feed on small organic matter.
The overall scoring of abundance ranged from 1 to 4 with 4 being exceptionally high and 1 being limited.
The other test was the Invertebrate Community Index (ICI), ten measurements of community structure based on taxa diversity and richness. Scores range from 0 to 60 where 0-10 was limited, 11-35 was intermediate, 36-44 was good, and 45 to 60 was excellent.
High macrobenthic counts were found at 17 out of the 18 sites tested on the river. Tributaries were not tested. No limited aquatic life use ratings were given out which would have indicated severe life impediments. Sites were divided into four categories of community quality ratings. Category 1 was the best with MPS ratings of 3.17 to 3.67, ICI of 36 to 46, species richness from 33 to 65, and standing crop densities of 440 to 22,000 (average of 6,000) individuals per meter squared. Category 2 had MPS rating of 2.50 to 3.17,
IPI 22 to 30, species richness 15-51, and standing crop of 500-35,000 individuals. Category 3 was rated as follows: MPS of 1.50 to 2.33, IPI of 27-30, species richness of 19 to 34, and standing crop of 5,000 to 42,000.
47 Draft Finally there was only one Category 4 site, below Laredo/Nuevo Laredo. The ranges for this site were MPS of 2.17-2.67, ICI of 14-22, species richness of 21-44, and standing crop of 14,000 to 18,000.
The river above and at El Paso were Category 2 sites. The site above El Paso was high in the category with very little chemical-induced stress, but had low growth possibly because of scouring of the river bottom from reservoir releases. The site in El Paso was at the lower range due to channelization, high chemical ratings, and little substrate to test. The potential of chemical impacts to macrobenthic community was determined to be moderate. Just before the river reached the confluence of Rio Conchos, the quality had deteriorated to a Category 3 site due to high dissolved solids amounts, return flows from irrigation, sedimentation, turbidity, and variability of flows. Chemical stress was slight.
Between the Rio Conchos confluence and Falcon Reservoir, the macrobenthic levels were high. The geographic location with an overlap of three ecoregions has been attributed to this, as well as very slight to no impacts from toxic chemicals. At Presidio/Ojinaga the river is a Category 1. In Big Bend, it is a Category 2 due to inflows from Terlingua Creek. A mile downstream showed improved environment and macrobenthic communities. South of Langtry and before the confluence of the Pecos and Devils Rivers, it is a Category 1.
This site had the highest ratings by both MPS and ICI standard and was the only site with superior ratings on both indexes. Below Amistad and above Del Rio/Cd. Acuna it was also a category 1. Below Del Rio/Cd.
Acuna, the site was a 2 with very little chemical stress. The river was back up to a Category 1 again from above Eagle Pass/Piedras Negras to Laredo/Nuevo Laredo. The ratings at these last two sites are surprising due to toxicity levels in the river. Below Laredo/Nuevo Laredo, the river was Category 4. The site had good habitat quality. So, it was determined that the poor quality was due to high amounts of organic enrichment and toxic chemicals.
Between Falcon Reservoir and the confluence with the Gulf, six sites were tested. The river was a
Category 2 site with river bed scouring, though few chemicals caused stress near Rio Grande City.
Downstream, the river tested as Category 1 and a high 2 again downstream. The last two sites were surprising because of poor water quality samples. Near McAllen and Pharr it was a Category 3. It went back to a low 2 below Anhuelo Drain, south of Las Milapas. Poor quality at this site was due to inflows from the drain
48 Draft scouring the bottom, causing decreased macrobenthic counts. Finally, the last two sites near San Benito
Brownsville/Matamoros were Category 3. The three Category 3 sites were in the upper portion of the
Category range. They were probably knocked down to a level three because of highly variable flows and monotonous environment with little variablility in the bed substrates of sand and silt. Toxic chemical stress was believed to be low for these sites (International Boundary and Water Commission and et. all. 1994).
Fish Community Structure
The vertebrates most endangered vertebrates in the United States are fresh water fish. Rio Grande’s
fish community is a variation of Mississippi River plains fishes. The native fish community in the New
Mexico reaches of the river has between 16 and 27 species. Three of the six endemic species are believed to
be extinct. These are the Rio Grande shiner, phantom shiner, and the Rio Grande bluntnose shiner. Also, the
Rio Grande chub is now found only in tributaries and the Rio Grande silvery minnow is only in the mainstem.
These fishes’ habitats mainly consisted of isolated outlier populations that remained viable with emigration.
Their decline is attributable to dams built in the early to mid 20th century which stopped their migration
between communities. The silvery minnow used to inhabit all of the river basin except Colorado. Today, it is
only found in New Mexico.
In the USGS study, “Environmental perturbation” in the fish communities was discovered in the upper
half of the river. Conclusions were based on counts of introduced, pollution-tolerant, omnivorous, and
deformed fish at 10 sites in the New Mexico and Colorado reaches of the river. 29 species of fish were found
from a breadth of 10 taxanomic families, 13 native and 16 introduced. Species richness per testing location
ranged from 1 to 13 different types of fish. The site with only 1 fish type was Mosca, Colorado where the
only fish found was the Rio Grande cutthroat trout. The US Fish & Wildlife service removed all other fish to reintroduce the trout. Generally, the number of fish found per testing location increased downstream as water temperature increased and the number of contributory water sources increased. In the upper half of the river,
El Paso had the greatest species richness possibly due to the warm waters of Elephant Butte and Caballo Dam
49 Draft upstream providing new generations of fish. General decline in fish populations occurred between 1993 and
1995.
Four of six sites sampled for diversity showed predominance of introduced fish likely from stocking practices. Omnivore dominance at four locations was an indication of ecological stress, but also that of a river growing in size. These sites were at Taos, Rio Chama at Chamita, Isleta, and El Paso. 2% “external anomalies” per fish were found at most sites; But Islesta had a 14% fish deformity rate and the worst site for fish community health in all was Isleta (Levings et al. 1998).
No direct link was found between fish community health and riparian health at testing locations.
Rather, fish community health was linked to general climate conditions affecting water temperature and DO.
The study of the lower Rio Grande found three major breaks in fish communities indicated by species discontinuity. The breaks were associated with the inflow of the Rio Conchos, Amistad Dam, and Falcon
Dam. A total of 53 species were counted throughout the Texas-Mexico portion.
At El Paso study sites species were widely distributed and salt-tolerant. Species included: gizzard shad, red shiner, common carp, river carpsuck, channel catfish, western mosquitofish, green sunfish, bullheaded minnow, and longear sunfish. At the station just upstream of the Rio Conchos entrance, the influence of the river was apparent due to influx of species.
Amistad proved to be a distinct boundary for fish influenced by turbidity upstream caused by modifications, irrigation return flows, runoff, and Rio Conchos. Downstream, the flow of the river was visually clear from water leaving the reservoir and somewhat from springs. There was a gap in species in the stretch between Amistad and Falcon. Missing in this segment were river carpsucker and blue catfish. Unique to this segment were the Texas shiner, gray redhorse, and smallmouth bass. In addition, roundnose darter and
Rio Grande darter, both traditionally creek species, were in the mainstem at this stretch.
Below Falcon, the water had significantly higher salinity than the middle reach. This caused a high percentage of estuarine and marine species found at all sites, though more so as the river came closer to the mouth at the Gulf. Species found here were American eel, Atlantic needlefish, gulf killfish, sheepshead minnow, Amazon molly, Sailfin Molly, Mountain Mullet, Striped Mullet, and Big Mouth Sleeper. The lower
50 Draft the flow, the higher upstream these fish were found, due to the higher percentage of salinity. At Arroyo los
Olmos, 200 miles upstream of the Gulf, there were 96% estuarine and marine species. Absent from this section of the river were several species of minnow, Tamaulipas Shiner, Rio Grande Shiner, and the Speckled
Chub, all of which were observed there historically. Another study around that time found no freshwater fish below Brownsville (International Boundary and Water Commission and et. all. 1994).
Riparian Habitat
Riparian vegetation is SEGMENTS WITH HIGH AND MODERATE POTENTIAL FOR TOXIC CHEMICAL IMPACTS ALONG THE RIO GRANDE, PHASE 1 OF RIO GRANDE TOXIC SUBSTANCES STUDY dependant on an intact hydrological regime including
natural surface water flows and
floods. They are very important
habitats for species that rely on
the river, a large percent, and the
filter sediments. In addition,
they maintain vegetation
groupings unique those areas. In Figure 30c. the southwest, cottonwoods, willows, and other trees are only found along the river. The altering of the flooding regimes, overgrazing, construction of roads and other projects, and increased levels of salt caused by irrigation agriculture have changed the sensitive riparian vegetation dynamics and the communities overall. The flooding and shifting of river channels is necessary for establishment of tree seedlings, especially cottonwood. As a result, the tree communities are not reproducing and the existing communities are dying due to old age and fires. Therefore, dominance of saltcedar, Russian-olive, and Siberian elm has occurred. Almost all riparian areas are endangered in the southwest. The USGS study found significant habitat degradation based on the following
51 Draft features: stream modification, bank erosion, bank vegetation stability, and riparian vegetation density. The only site with no degradation found was Rito de los Frijoles in Bandelier National Monument, New Mexico
(Levings et al. 1998). Additional information regarding the biological status of the river can be found in the following figures.
Cheatgrass is an introduced species that has been replacing native grasses in juniper, ponderosa, and douglas-fir woodlands. Cheatgrass burns easily, which aids in succession over native perennial grasses. Purple loosestrife is invading wetland and streamsides in Colorado, crowding out native plants necessary for animal food and shelter. It has no natural enemies in the U.S. keep it in check.
In virtually all of the river basin, salt cedar has taken over riparian areas. It is highly successful because it is drought resistant and it has a deep root system. The plants take over areas by lowering the water table, making it harder for other plants to get ground water. They also increase soil salinity and are highly flammable, increasing wildfire frequency and heat. The influx of salt cedar is promoted by disturbances to native vegetation, such as overgrazing.
Vascular plant species richness is high in the southwest, with New Mexico having 3,900 taxa alone.
However, endemism is not high, only about 4%. These include 24 cacti, 10 annuals, 9 woody shrubs, and 3 biennials. Regional sensitive plants include cacti, prickly poppies, milk-vetches, wild buckwheats, paintbrushes, penstemons, saltbushes, sagebrushes, and there are more. Families that account for over half of the species New Mexico include sunflower, pea, cactus, and figwort families. Threats to these plants include habitat change and all inhospitable land uses discussed earlier. Species subject to commercial exploitation and poaching include penstemons, agaves, yuccas, and cacti (U.S. Geologic Survey. ).
The list of threatened and endangered species, included in a series of charts that follow, has plants only from Texas. All are on the Federal lists. Nine cactus species are included: six endangered, three threatened.
Four trees, shrubs, and subshrubs are included: three are endangered and one is threatened. Seven wildflowers are listed: six endangered, one threatened. Finally, two grasses are included, though only lists
52 Draft any information about its status: the Little aguja pondweed is endangered ((Table 4) Gil, K. and Wilkins.
2003). The lack of plants listed from Colorado and New Mexico probably means that more research needs to be conducted in the Rio Grande watershed in those states.
Figure 15d.
53 Draft
Figure 15e.
54 Draft
Figure 15f.
55 Draft
Figure 15g.
56 Draft
Figure 35b.
57 Draft
Figure 35c. Number of fish species collected.
58 Draft Fish Texas Endemics In the United States, fresh water fish are the
most endangered vertebrates. In the United States,
approximately 20% of fish are threatened or are
already extinct, compared to 7% of birds and
mammals. Overall, Rio Grande’s fish community is
a variation of Mississippi plains fishes.
Rocky Mountain aquatic ecosystems are
influenced by nonindigenous populations,
especially of brown and rainbow trout. This has led Color Key (number of endemic taxa in county) : to decline in native populations and their genetic
1-19 20-38 39-57 58-75 76-93 diversity. Native trout have bred with the
Figure 36. introduced species and most of the populations are now hybrids. Most Cutthroat trout species have seen catastrophic declines in the last century and two are now extinct. The Rio Grande cutthroat trout declined to the point of needing reintroduction in the river in
Colorado (U.S. Geologic Survey. ). This is evidenced by its listing under species of special concern, though it is not listed as threatened or endangered. Other Rio Grande species of special concern in Colorado are the
Rio Grande chub and sucker ((Table 4) Gil, K. and Wilkins. 2003).
In New Mexico, the Rio Grande fish species count is between 17 and 27 species and 30% are endemic.
Three of the six endemic species do not occur in New Mexico anymore. These are the Rio Grande shiner, phantom shiner, and the Rio Grande bluntnose shiner. The Rio Grande chub is now found only in tributaries and the Rio Grande silvery minnow is only in the mainstem. The fishes’ habitats mainly consisted of isolate outlier populations that remained viable with emigration. The decline of these species is attributable to dams built in the early to mid 20th century which stopped their migration between communities. The silvery
minnow used to inhabit virtually all of the river. Now, it is only found in New Mexico and is on the Federal
59 Draft list of endangered species (U.S. Geologic Survey. ). Fish on the New Mexico endangered species list are the
Rio Grande and blue suckers, the Pecos gambusia, and the Rio Grande cutthroat trout. The state listed threatened species are the Pecos pupfish, Mexican treta, Pecos bluntnose shiner, gray redhorse, green throat darter, and the bigscale logperch.
Texas’ list of threatened and endangered fish is the longest. The Federally listed endangered fish include: the Rio Grande silvery minnow, Leon Springs pupfish, Comanche Springs pupfish, Big Bend
Gambusia, and Pecos Gambusia. All of the fish on the state list, not included on the national list, are threatened. These are the shovelnose sturgeon, Mexican stoneroller, Devils River minnow, Rio Grande chub,
Chihuahua shiner, blue sucker, Conchos pupfish, Pecos pupfish, Blotched Gambusia, Rio Grande darter, river goby, and blackfin goby ((Table 4) Gil, K. and Wilkins. 2003).
Many of these fish are endemic to or prefer springs as their main habitat. The warm, low oxygen water leads to fish adaptations to survive. Such fish include pup, pool, spring, and killi-fishes, along with livebearers. Their decline is caused mainly by over-pumping of ground water.
General causes of fish decline in the river are the building of canals and dams, loss or degradation of habitat, chemical pollution, over exploitation, and introduction of nonnative species to this region. The factor of greatest effect is the physical barrier and unnatural regulation of water by dams.
The disappearance pattern of fish generally is as follows. The most sensitive fish, usually with limited range and/or specific habitat requirements die first. This is followed by the collapse of the entire population of fish. Known fish that are now extinct in the river are the Amistad gambusia and the phantom shiner. The recovery of fish populations can only be achieved by habitat preservation. Stocking can help the process, though it is likely limited to specific geographic ranges (U.S. Geologic Survey).
Invertebrates
In 1994, only 13% of Federally listed threatened and endangered species were invertebrates.
However, of all species on earth, invertebrates make up 90%. This is evidence that more research into
60 Draft invertebrates must be conducted. In 1995, 90,000 different species of invertebrates were counted in the north
America and 75,000 more were expected yet undiscovered. They have only recently been added to endangered species lists.
Little is known about invertebrates in Colorado and no threatened or endangered species are listed in the Rio Grande basin area of the state. Butterflies, moths, and skippers are highly successful still, with about
2,000 known species still part of the community. Threats to these invertebrates include overgrazing, urbanization, and habitat disturbances (U.S. Geologic Survey. ).
In New Mexico, 410 species have been counted. Invertebrates are only listed as threatened or endangered on the state list. There are two crustaceans endangered. The mollusks at risk include two endangered and three threatened. No insects were listed. Finally, all gastropods listed in the Rio Grande basin were in New Mexico. These include four endangered, all Pyrgulopsis, and nine threatened.
Texas’ list of endangered invertebrates were all included in the Federal list. There was one crustacean, and one mollusk. No gastropods were listed. However, all of the insects listed in the basin were in Texas and all were beetles. The total of listed beetles was nine ((Table 4) Gil, K. and Wilkins. 2003).
Aquatic habitats house the most sensitive invertebrates. These species are effected by the same environmental changes as southwestern fish. Generally, invertebrates, like other animal species, are threatened by natural water cycle changes, alteration to habitat, pollution, urbanization, and the introduction of nonnative species to that environment (U.S. Geologic Survey. ).
Amphibians and Reptiles
World wide, amphibian success is declining due to habitat lass, predation of introduced fish, logging, and other factors. Threats faced by these species are unique compared to other types of animals. Amphibian
61 Draft and reptiles are collected for the pet trade and snake populations, particularly rattlers, are disturbed by roundups.
The Rocky Mountain populations are also seeing this decline. Western toads now occupy less than
20% of their former range in Colorado and New Mexico (U.S. Geologic Survey. ). The western toad and the
Texas horned lizard are the only amphibians listed in Colorado. They are on the state endangered species list.
In New Mexico, the western toad is also on the state endangered species list, along with the lowland leopard frog. The plainbelly water snake, gray-banded kingsnake, and the Gila monster are included on this list. The state threatened list includes two snakes: the mottled rock rattlesnake and the narrow head garter snake. In addition, one turtle, the western river cooter, one lizard, the sand dune lizard, and two salamanders,
the Jemez Mountains and Sacramento mountain salamanders, were on the list.
The Texas list is more extensive. There were five Federally listed sea turtles, three endangered and two threatened. Two fresh water turtles were on the state endangered list. Four of the five lizards listed were
from Texas, as state threatened species. Seven snakes were listed as endangered, though only one was on the
Federal list. This was the Concho water snake. Two salamanders and three frogs/toads were on the state
threatened list ((Table 4) Gil, K. and Wilkins. 2003).
The two New Mexican salamanders are sensitive upper montane species disturbed by logging. The
New Mexican ridge-nosed rattler is an example. It is collected as a novelty and is affected by mining and
logging.
In lower elevation conifers, where more amphibians and reptiles live, species need not be as
specialized to survive. As of the mid 1990s, their habitat was still sufficient, though urbanization has affected
the landscape.
62 Draft RARE SPECIES IN TEXAS
The Chihuahua Desert has had few
studies performed recording these species.
However, it is believed that the desert
grasslands have been turned to shrub with
the overall decrease in grazing. However,
studies have only been observing a short
time period.
In aquatic areas, these species
depend on flood plains. Since dams have
reduced flooding over river banks, there has
been a corresponding decline in species. Figure 30d. These species include: leopard frogs, the western and southwestern toad, garter snakes, the Big Bend slider, and others. Additional causes include habitat change, introduced predators, pollutants, disease, the lowering of the water table, and other causes.
Introduced species are not as well documented as with fish. The bullfrog has outcompeted the leopard frog and garter snake in Arizona and California. Nonnative fish have also led to the decline of amphibians in
California. The effects of the introduced nonnative turtle’s effects are unknown (U.S. Geologic Survey. ).
Birds
Populations of large birds were nearly extinct in Colorado in the 1970’s due mainly to DDT and other pesticide use. These birds include the bald eagle, peregrine falcons, white-tailed ptarmigans, and trumpeter swans. In the mid 1990’s, populations had all grown. However, the trumpeter swan was the only bird with a population near or over 1,000 (U.S. Geologic Survey. ). The Mexican spotted owl and bald eagle were on the state threatened list and the peregrine falcon was of special concern as of 2003 ((Table 4) Gil, K. and Wilkins.
2003).
63 Draft Generally, woodland and forest bird declines in the southwest are not well known due to a lack of data. The species known to decrease in number include ladder-backed woodpecker, Bendire’s thrasher, gray vireo, which has suffered significant declines, and the buff breasted flycatcher. Theses species have been declining possibly due to overgrazing and logging.
Neotropical migrant songbirds in the Rockies have seen a decline in population due to predation and possibly by brood parasitism by cowbirds, which increases with nearby logging. Rare species have declined due to the cutting of old growth forests (U.S. Geologic Survey. ). However, the only species listed in
Colorado was the state designated endangered willow flycatcher, which was also listed in New Mexico and
Texas.
Additional birds were listed in Colorado. These include the least tern, also listed as endangered in
New Mexico and Texas, and the piping plover as threatened. In New Mexico, the least tern, piping plover, bald eagle, and peregrine falcon were also listed, all as threatened. Additional birds include the eastern brown pelican, Federally listed as endangered, common black-hawk, and northern aplomado falcon.
Overall, 47% of all birds whose habitat is deciduous vegetation near water will only inhabit these
areas. This habitat is particularly important for migrant birds. These include neotropical birds and short
traveling birds that nest in the southwest in winter.
In New Mexico, four more neotropical migrant songbirds were designated by the state as endangered
and four were threatened ((Table 4) Gil, K. and Wilkins. 2003). 25% of all New Mexican neotropical birds
that breed there are considered high conservation priority due to habitat disturbance. 40% of riparian birds
have been negatively impacted by grazing in riparian areas in New Mexico. These birds also depend on
young cottonwood trees (U.S. Geologic Survey. ).
Texas birds listed include five water birds, eight raptors, three shorebirds, and twelve songbirds. The
Federally listed endangered birds include eastern brown pelican, a water bird and the interior least tern, a
raptor, the southwestern willow flycatcher, black-capped vireo, and the golden-cheeked warbler, all songbirds.
64 Draft
Figure 37. Percent Exotic Species in Two Measurement Scales.
The Federal threatened species are the Mexican spotted owl and the piping plover ((Table 4) Gil, K. and
Wilkins. 2003).
For Mexican birds, little data is available. However, Mexican highland birds know to be at risk in
Texas include the thick-billed parrot, ferrunginous pygmy-owl, violet-crowned hummingbird, elegant trogon,
and the varied bunting. The aplomado falcon has now disappeared from Texas (U.S. Geologic Survey. ).
Mammals
Of all western hemisphere mammals, 15% are considered vulnerable and 5% are endangered. In the southwest, only 10% are at risk and 4% are endangered. Overall, New Mexico has 139 species and Texas has
120 species of mammals (U.S. Geologic Survey. ).
The beaver is nearly extinct in Colorado and New Mexico due to trapping and livestock grazing. They are keystone members of the riparian community as their natural dams shape riparian and pond habitats. They
provide opportunity for new plant growth and they increase diversity. Their removal has contributed to the 65 Draft decline of grasses, arroyo cutting, and gullying of the landscape. As the river channel cuts down into the sediment, gradients of riparian areas increase, the water table is lowered, and the area dries out. This causes the succession of more drought tolerant plants (U.S. Geologic Survey. ). However, the beaver was not listed at all of species under threat in 2003 ((Table 4) Gil, K. and Wilkins. 2003).
Other small mammals include bats and rodents. The Mexican long-nosed bat is a Federally listed endangered species in New Mexico and Texas. The long-nosed bat and spotted bat are considered threatened in New Mexico. In Texas, the long-nosed bat is a Federally certified endangered species and both the spotted bat and southern yellow bat are listed as threatened by the state. Of rodents, two are threatened in New
Mexico and one in Texas. These species are generally thought to have narrow ranges. Overall, there is little information on populations of small mammals. The species with wider ranges, such as gophers, prarie dogs, and ferrets have decreased in number over the last century due to persecution for their disturbance effects on agricultural lands. In particular, the Mexican prairie dog had declined by 98% in 1960.
Larger mammals have also been persecuted due to their consumption of livestock and general fear of these animals by humans. Grizzly bears have disappeared from the Rio Grande watershed. Black bears are threatened in Texas. Gray wolves are listed as endangered in all three states and Federally in Texas and New
Mexico. The Mexican gray wolf is also listed as endangered in Texas. Other carnivores include the threatened American marten in New Mexico and the white-nosed coati in Texas. The ocelot and jaguarondi are Federally listed as endangered in Texas ((Table 4) Gil, K. and Wilkins. 2003). The jaguar has completely disappeared in Texas. These cats’ main range is in Mexico, though little information about their presence was found.
Deer and bighorn sheep have declined due to excessive harvest in the 1970s and overgrazing of cattle.
In addition, disease caught from imported livestock has had an impact on their populations (U.S. Geologic
Survey. ). Bighorn sheep is endangered in New Mexico and Colombian white-tailed deer are federally listed as endangered in Texas ((Table 4) Gil, K. and Wilkins. 2003).
66 Draft Pronghorns and elk have increased due to reductions in their predators and their adaptability overall.
The mountain goat and moose have been introduced to Colorado, as well as the Norway rat. Other nonnative species consist mainly of feral domesticated animals. Their effects are not well understood (U.S. Geologic
Survey. ).
Offshore mammals have also been considered. The West Indian manatee is a Federally listed marine mammal known in the Rio Grande estuary. The Atlantic spotted dolphin is listed as threatened in Texas
((Table 4) Gil, K. and Wilkins. 2003).
67 Draft Table 4.
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76 Draft
77 Draft Conclusion
Overall, fish biodiversity in the Rio Grande depends on flow rates, temperature, and hydrologic modifications to their habitat. Non-aquatic animals require healthy riparian vegetation for access to water and food along the river. They in turn provide food for larger organisms. Finally, large mammals also choose riparian areas as habitat, as has been seen in Colorado. Therefore, the health of the aquatic life is an important indictor of health of the overall ecosystem.
The degradation of river water quality and riparian vegetation of the river have changed significantly over time. Factors of change include the building of dams and channels, overgrazing throughout the basin, variable water flow rates, urbanization, fragmentation, and introduction of non-native species. The occurrence of any of these disturbances reduces viability of some species. Hydrologic modifications and exotic species have proven fatal to certain populations, even causing species extinction.
The solution to the problem is not clear. Some changes may not be possible to reverse, such as giant dam-building operations and urbanization. However, it is obvious that in order to reduce further degradation of biotic communities, land use changes must be made. Suggestions include amending grazing practices, chemical release mitigation, and land owner cooperation to reduce riparian habitat fragmentation. In addition, the federal governments must work together to improve the quality of the river. The Rio Grande is very important the those it sustains, yet responsibility for its care has not been claimed by the community as a whole.
78 Draft Bibliography
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79 Draft Williams, E. J. "The Maquiladora Industry and Environmental Degradation in the United States-Mexican Borderlands." (1995).
(Figure 1) U.S. Geologic Survey. Rio Grande Factsheet, NASQAN Sampling Sites. U.S. Geologic Survey.
(Figure 2) Colorado Division of Water Resources. "Roundtable Boundaries." 2006.
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(Figure 5) Natural Heritage Institute. "Rio Grande Basin in New Mexico." (2002).
(Figure 6) U.S. Geologic Survey. "Average Annual Precipitation (in Inches) 1961-1990."
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(Figure 8) DeLorme. "Pecos River Watershed." (2004).
(Figure 9) Texas Parks & Wildlife Department. "West Texas Waterways."
(Figure 10) Texas Commission on Environmental Quality. "Rio Grande Basin."
(Figure 11) Maps of World. "Texas Rivers USA." (2005).
(Figure 12) Instituto Nacional de Estadistica. "Precipitacion Total Anual."
(Figure 13) WaterWatch, FutureWater, and Comisión Nacional del Agua. "Actual Evapotranspiration SEBAL." (2006).
(Figure 14) Skadberg, Yongxia. "Land use Land Cover of the Rio Grande Basin."
(Figure 15) Levings,G. W., D. F. Healy, S. F. Richey, and L. F. Carter. Water Quality in the Rio Grande Valley: Colorado, New Mexico, Texas, 1992-1995. U.S. Geological Survey, 1998.
(Figure 17) Texas Parks and Wildlife Department. "Vegetation Types of Texas."
(Figure 18) Comision Nacional para el Conocimeinto y Uso de la Biodiversidad. "Uso De Suelo y Vegetacion Modificado Por CONABIO."
(Figure 19) Hayden, F.V. "Economic Map of Colorado." (1881).
(Figure 20) Wheeler, G.M. " Land Classification Map of Part of Central New Mexico." (1882).
80 Draft (Figure 21) Wheeler, G.M. "Land Classification Map of Part of Southwestern New Mexico." (1881).
(Figures 22, 23, 24) Allen, Craig D., Julio L. Betancourt, and Thomas W. Swetnam. "Landscape Changes in the Southwestern United States: Techniques, Long Term Data Sets, and Trends." In U.S. Geological Survey, Biological Resources Division, 1998.
(Figure 25). Srinivasan, R, T. Ramanarayanan, H. Wang , and R. Jayakrishnan. Hydrologic Modeling of the Rio Grande/Río Bravo Basin. 1997.
(Figure 26) Metcalfe, Charles B. "Classification of Land Uses in Texas by Counties / 1935." (1935).
(Figure 27) Garcia Cubas, A. "Carta Agricola." (1885).
(Figure 28) U.S. Fish and Wildlife Service. "Dams and Diversions Along the Rio Grande." (2004).
(Figure 29) International Boundary and Water Commission. "Project Map."
(Figure 30) Texas Water Development Board. "Water and Wastewater Needs of Texas Colonias: 1996 Update." (1997).
(Figure 31) Texas Department of State Health Services. "Border Colonias." (2005).
(Figure 32) National Colonia Clearinghouse. "Colonias Along the US-Mexico Border." (1996).
(Figure 33) National Council of Industry. "Concetracion De Maquiladoras."
(Figure 34) Instituto Nacional de Estadistica, Geografia, e Informacion. "Maquiladora Map." (1991).
(Figure 35) International Boundary and Water Commission, and et. all. International Boundary and Water Commission, 1994.
(Figure 36) Texas A&M Bioinformatics Working Group. "Texas Endemics: Distribution of all Endemics."
(Figure 37) U.S. Geologic Survey. Perspectives on the Land use History of North America: A Context for Understanding our Changing Environment.1999.
(Tables 1,2,3) Hinojosa, J. R. "The Urbanization of the U.S. - Mexico Border Region." (1998).
(Table 4) Gil, K., and N. Wilkins. Rare, Threatened and Endangered Species of the Rio Grande/ Rio Bravo Basin: A Bibliography. College Station, TX: Texas Water Resources Institute, 2003.
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