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

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

Hi storic 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, 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 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 non-point 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

Plan t Community

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-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.