Anthropogenic Effects on the Rio Grande: A Biogeographic
Assessment
Draft Executive Summary
Lauren Bilbe, River Systems Institute, Texas State University, 2006
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, beginning in southwestern Colorado, through central New Mexico, and on to El
Paso, forming the international border between Texas and Mexico, and finally emptying into the
Gulf of Mexico. The three largest tributaries include the Pecos River in the United States and the
Rio Conchos and Rio Salado in Mexico. The watershed is a total of 335,500 square miles,
though only approximately 176,000 square miles actually drain water into the Rio Grande and its
tributaries. There are about 88,000 square miles of the contributing drainage area in each
country, with 38,000 square miles of the U.S. portion below El Paso.
Major Ecoregions
Southern Rocky Mountains Province
The headwaters region in Colorado is composed mainly of mountainous forests and
semiarid steppe. Elevation ranges from 6,000 feet below sea level to mountain peaks of 14,000
feet above sea level (Levings et al. 1998). The vegetation in the mountainous areas of Colorado
and northern New Mexico is a mix of evergreen needleleaf forest, deciduous forest, mixed forest,
and grasslands. In the San Juan basin, where the various tributaries of Colorado and northern
New Mexico join the Rio Grande, the vegetation consists of a drier scrubland, particularly
sagebrush and antelope bitterbrush, with some peripheral grasslands. The lower montane
riparian, 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.
Chihuahua Desert
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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.
The Chihuahuan Desert is characterized by scrub brush, sparse grasses, and many cacti species.
Riparian vegetation consists of non-native salt cedar, as well as cottonwood, and desert willow.
South Texas Brushlands or Tamaulipan Thorn Scrub
The brushland/scrub ecoregion characterizes nearly all of south Texas and extends down
to central Nuevo Leon and Tamaulipas. The vegetation mainly consists of thorn scrub with
neotropical biotic influences. Prior to human settlement, the brushlands were mesquite and
acacia savanna with somewhat-open scrub areas and grasslands up to three feet tall. Other
vegetation types in the brushland/scrub ecoregion consist of oak forests, including live oak
mottes, and some tall riparian forests. Along the coastal areas, native vegetation includes coastal
marsh, floodplain forests, and upland grasses.
Human Impacts to the Land and Water
Historic Land and Water Use
Human land use in the Rio Grande Valley has had significant effects on the land and
water. Practices of Native Americans, including the clearing of riparian woody vegetation for
firewood, irrigated agriculture, and intentional burning, may have had significant impacts on the
land and water, though few specifics are known. Grazing and burning may have caused a shift
from woody shrub vegetation to grasses. Impacts of European immigrants and subsequent
generations, however, are believed to have been more significant. Expanded irrigated
agriculture, large scale sheep and cattle grazing operations, fire suppression, and large game
hunting have occurred throughout most of the river basin since their arrival. Riparian areas have
been cleared of woody vegetation as a result of riparian agriculture. The common practice of
overgrazing and fire suppression has caused vegetation shifts in grass types and an increase in
woody plants throughout the arid portions of the basin, leading to highly flammable landscapes.
The combination of the decrease in grasses and increase in denuded riparian areas has caused
serious erosion of banks. Deep channels, called arroyos, result from this bank erosion. In
addition, large mammals have been hunted virtually to extinction in many parts of the basin
because they are seen as a threat to both human communities and their livestock. Finally,
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logging has been practiced in the upper half of the basin for over a century resulting in habitat
destruction and erosion.
Water Law and Hydrologic Modifications
Channelization, dams, and other human modifications have changed the course of the Rio
Grande in recent history. To administer the waters that create the boundary between Mexico and
the United States, the International Boundary and Water Commission was established in 1889.
There are two major treaties that apportion waters of the Rio Grande: the Treaty of 1906 and the
Treaty of 1944. In 1906, a binational treaty was signed that guaranteed the Juarez Valley of
Mexico 60,000 acre feet of water a year from the upper half of the river. Then, the Treaty of
1944 divided the waters in the lower half of the basin such that the United States should receive
one third of the flows from six Mexican tributaries to total no less than 350,000 acre feet
annually as an average in five-year cycles. The 1944 Treaty also requires that the waters in the
main channel of the Rio Grande below the last dam be split evenly and additional waters not
apportioned in the main river be given to the United States. In addition, under this treaty, several
major dams were constructed to help apportion the waters between the two countries. These
include Elephant Butte, American, Amistad, and Falcon Dams. The reservoirs also prevent
floods, provide regular sources of irrigation water, and provide recreational opportunities.
Current Water Use Problems
The Rio Grande basin has seen a dramatic increase in population over the past century,
particularly along the border. The growth in the number of inhabitants began with the advent of
railroads and has continued with the encouragement of trade. Most of the growth has occurred in
urban areas. The profitability for U.S. and other foreign businesses of locating in Mexico
brought industry to the border. These factories, known as maquiladoras, enjoy duty-free
exporting and less rigorous environmental regulations in Mexico. The opportunities provided by
these industries and border growth have attracted numerous workers and individuals to the
region, which in turn has created a high demand for public services. Many cities have not been
able to develop their basic infrastructure, such as water and waste water services, adequately
enough for their growth. In some cases, villages have been established without any
infrastructure at all. These villages are known as colonias. Inadequate basic services and relaxed
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environmental laws has led to the degradation and over-allocation of the Rio Grande and its
natural resources.
Water Quality
Water quality in the upper half of the Rio Grande Basin was assessed in a study in 1995.
The lower half of the basin has been assessed in three studies in 1970, 1995, and in 2003. In
these studies, water quality generally deteriorated as the river flowed southeast.
Below most major cities nutrients and bacteria counts were high due to sewage effluent and nonpoint source pollution.
Dissolved solids, including salt content, increased with the downstream course of the Rio
Grande. The tributaries with particularly high counts of dissolved solids include the Trinchera
Creek in Colorado, the Rio Puerco in New Mexico, the Pecos River in Texas, and Rio Salado in
Mexico. The tributaries were tested just upstream of their confluence with the main stem of the
Rio Grande. Areas of the main stem of the river that tested high for dissolved solids were El
Paso, Presidio/Ojinaga before the confluence of the Rio Conchos in Mexico, and Brownsville.
Factors contributing to large amounts of dissolved solids include returned irrigation flows,
evaporation, and ocean water near the mouth at the Gulf of Mexico.
Pesticides were found in 94% of the surface waters, tributaries, and drains of the upper
half of the Rio Grande in the 1995 USGS study. However, standards were only exceeded in the
lower half of the river. In the 1995 study of the lower Rio Grande, four out of five sites either
exceeded standards or were in the US Fish and Wildlife Service’s 85th percentile for the U.S..
Volatile organic chemicals exceeded standards in the areas south of Laredo and from El
Paso to just before the confluence with the Rio Conchos. Semivolatile organic chemical were in
excess of standards near a number of cities and tributaries throughout the basin. Trace elements
were found near agricultural and mining sites in the northern half of the Rio Grande. In the
southern half, every site tested in the main portion of the river exceeded health standards for a
least one metal in the 1990s study.
Biological Effects
Plant Community
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Riparian vegetation depends on an intact hydrological regime, including natural surface
water flows and floods. The altering of flood regimes, the increase in levels of salt caused by
irrigation agriculture, construction of roads and other projects, and overgrazing have changed the
dynamics and communities of sensitive riparian vegetation overall. As a result, tree communities
are not reproducing, and existing communities are dying due to old age and fires. In response,
saltcedar, Russian-olive, and Siberian elm have taken over many riparian areas. Overall, a total
of 16 plants native to the river basin appeared on the Federal list of endangered species in 2003.
Six more were listed as threatened.
Fish Community
In Colorado, the population of Rio Grande cutthroat trout declined to the point of needing
reintroduction to the river in Colorado. Other Rio Grande species of special concern in Colorado
include the Rio Grande chub and sucker.
The native fish community in the New Mexico reaches of the river includes
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. The population of these fishes mainly consisted of isolated outlier populations that
remained viable via emigration. Their decline is attributable to dams built in the early to mid
20th century which inhibited migration between communities. The silvery minnow formerly
inhabited all of the river basin except in Colorado. Today the silvery minnow is only found in
New Mexico. In the 1995 USGS study, “environmental perturbation” in 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. This study also found a predominance of introduced fish likely from
stocking practices. Four fish are listed on the New Mexico endangered species list and six are
threatened.
The 1990s 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 fish species were counted throughout
the Texas-Mexico portion. At El Paso study sites, species were widely distributed and salt-
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tolerant. The confluence of the Rio Conchos showed an influx of fish species. Amistad Dam
proved to be a distinct boundary for fish influenced by turbidity upstream caused by
modifications, irrigation return flows, runoff, and the Rio Conchos. Downstream of the dam, 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 reach of the Rio Grande between Amistad and Falcon Dams.
Fish found upstream and downstream of these dams were missing, and a few species were
unique to this reach only. Additionally, several stream minnows were found in the main river
channel. Below Falcon Dam, the water had significantly higher salinity than the middle reach.
This resulted in a high percentage of estuarine and marine species found at all sites in this area,
though more so as the river came closer to the mouth at the Gulf. The lower the flow, the higher
upstream these species were found, due to the higher percentage of salinity.
Of the Rio Grande states in the U.S., Texas has the longest list of threatened and
endangered fish species with five fish, mainly endemic, that live in spring fed tributaries. Their
decline is caused mainly by over-pumping of groundwater.
General causes of fish decline in the river include the building of canals and dams, loss or
degradation of habitat, chemical pollution, over-exploitation, and introduction of non-native
species to this region. The factor of greatest effect is the construction of dams, which causes a
physical barrier and unnatural regulation of water.
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 considered extinct 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 limited to specific geographic
ranges.
Invertebrates
In 1994, only 13% of federally listed threatened and endangered species were
invertebrates. However, invertebrates make up 90% of all species on earth. This is evidence that
more research into invertebrates must be conducted. 21 species were on the U.S. federal list of
endangered species. Threats to these invertebrates include overgrazing, urbanization, natural
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water cycle changes, alteration to habitat, pollution, urbanization, and the introduction of
nonnative species to that environment.
Amphibians and Reptiles
Worldwide, amphibian success is declining due to habitat lass, predation of introduced
fish, logging, and other factors. Only three species of sea turtles with ranges in the Rio Grande
basin and its release area in the Gulf of Mexico were federally listed as endangered in 2003.
Nine species were on the state list of endangered species and many more are listed as threatened.
Birds
The Rio Grande provides important habitat for migratory birds of North America. Nearly
half of all birds whose habitat is deciduous vegetation near water will inhabit these areas. The
federal list of endangered species includes six birds in the Rio Grande basin, half of them
migratory songbirds, half are large birds of prey. More research is necessary to understand the
connection between riparian habitat destruction and the effects on migratory birds. Other
endangered and threatened birds consist mainly of raptor species.
Mammals
Large mammal populations have been greatly reduced over the last 150 years mainly due
to hunting for protection and profit. 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. Bears, wolves, and cats, such as the jaguar,
jaguarondi, and the ocelot, have nearly or completely disappeared from the range of the Rio
Grande. Four of these large mammals are on the federal endangered list, and the rest are
considered extinct. White-tailed deer and pronghorn sheep numbers are also declining due to
overgrazing, the introduction of non-native species, and excessive hunting. Pronghorns and elk
populations have increased due to reduction of predator numbers.
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
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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 indicator of the condition of
the overall ecosystem.
The degradation of river water quality and riparian vegetation of the Rio Grande has
increased significantly over time. Factors that cause these changes 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.
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