The Lower Rio Grande Valley National Wildlife Refuge, in Starr, Hidalgo, Cameron, and Willacy counties, extends along the Rio Grande from Falcon Dam to the Gulf of Mexico. It is anticipated to include 107,500 acres when complete. By 1989 40,000 acres was administered by the United States Fish and Wildlife Service. The refuge was authorized by Congress in 1980 in an effort to restore and preserve animal and plant species in an area where more than 90 percent of natural vegetation had been cleared for agriculture or destroyed by rapid urbanization. Sanctuary is given to 115 species of wildlife and to some of the rarest animals and plants native to the United States. Endangered animals include the ocelot and jaguarundi. Exotic birds include the green jay and the chachalaca. Threatened plants include barreta and Esenbeckia runyonii.
The refuge lies 2½ degrees above the Tropic of Cancer and is in an area of contrasting climatic influences. Temperate and tropic climates meet, and warm, humid air from the Gulf of Mexico to the east meets dry air from the Chihuahuan Desert to the west. Annual rainfall within the refuge ranges from twenty-six inches on the Gulf shore to seventeen inches at the western refuge boundary. The mean annual temperature is 72.35 degrees. The elevation ranges from sea level to more than 300 feet. The upriver sections of the refuge lie in the Rio Grande valley, which ends near the Bordas Escarpment. From there to the Gulf of Mexico the refuge is in the Rio Grande delta. The Mississippi and Central flyways converge over the refuge, where more than 400 bird species congregate during migration.
Although the entire refuge is in the Tamaulipan Biotic Province, topological and climatic influences have been identified as eleven different biotic communities. The Chihuahuan Thorn Forest contains the largest remaining stand of subtropical thorn forest in the United States. The Ramadero has periodically wet arroyos that act as protected corridors from the Rio Grande to adjacent highlands. Thick thorn forest offers safe passage to ocelots, jaguarundis, and other endangered species. The Upper Valley Flood Forest supports riverine species, including the Moctezuma bald cypress. The Barretal, on the Bordas Escarpment, supports groves of barreta, a native citrus relative, and preserves fossil oyster beds. The Upland Thorn Scrub displays a dense growth of trees, shrubs, and cacti adapted to arid conditions. The Woodland Potholes and Basins contains ephemeral wetlands with many species of seasonal waterfowl. The Mid Delta Thorn Forest supports a jungle-like growth of trees and shrubs, including brasil, Texas ebony, and anacua. The Mid Valley Riparian Woodland is an area of lush growth and large trees, including cedar elm, Rio Grande ash, and Texas palm. Formerly watered by annual overflows of the Rio Grande, the area has been drying, with the loss of many trees, since the upriver construction of Falcon Dam. The Coastal Brushland Potholes are depressions of wind formation, rich in waterfowl after rainy seasons. The Sabal Palm Forest contains jungle-like stands of Sabal texana, the palm for which the Rio de las Palmas (Rio Grande) was named in 1519. Justicia runyonii, Trixis radialis, and Passiflora filipes carpet the forest floor. The Loma/Tidal Flats are tidal areas surrounding hills of eolian formation covered by a dense thicket of acacias, granjeno, and fiddlewood that protects many species of cacti and offers cover for ocelots, jaguarundi, and tortoises. Many animal species, including parrots and the large cats, have found sanctuary in this wildlife region on the United States side of the Rio Grande. Completion of the refuge will prevent the almost certain extinction of many species of animals and plants in the wild.