Amistad National Recreation Area: A Reservoir of Friendship on Cretaceous Limestone

Between the dry plains of western Texas and northern Mexico, yet within reach of the tropical Gulf of Mexico, the Amistad region swings between drought and flood. To control floods, secure water and create places to play, the U.S. International Boundary and Water Commission and its Mexican counterpart, the Comisión Internacional de Límites y Aguas (CILA), harnessed the Rio Grande with Amistad Dam — named in 1969 for the Spanish word for "friendship."

Image from the National Park Service's Geodiversity Atlas page for Amistad National Recreation Area.

The reservoir

The dam's reservoir sits where three rivers meet:

Water flows in from tributaries in both countries, and each country's contribution sets its water rights. The national recreation area's boundary is fixed at an elevation of 1,144.3 feet, whatever the water level, and most of the 23,675 hectares (58,500 acres) the Park Service manages lie underwater.

Governors Landing Campground from the air during low water in 2012. Image from the National Park Service's Geodiversity Atlas page for the park.

Clear water and year-round visitors

About 1.7 million people visit each year, many of them "winter Texans," for boating, fishing, camping and picnicking. The limestone beneath the reservoir keeps its water clear, making it a good place to swim and scuba dive.

Ancient art

Amistad also protects some of the world's finest prehistoric pictographs: costumed figures, animals, geometric designs and harder-to-read images painted in rock shelters along the Pecos, Rio Grande and Devils rivers. Carbon-14 dating puts the oldest, in the Lower Pecos River style, at 4,000 years old, and archaeologists believe people lived in the region as long as 10,000 years ago.

Rocks and fossils

Steep canyons of Cretaceous Salmon Peak Limestone ring the reservoir. The Devils River Limestone, Del Rio Clay, Buda Limestone and Boquillas Formation are also exposed in the park.

The fossil record here is rich and still largely unexplored. Of the Cretaceous sea creatures preserved, the most intriguing may be the rudists — reef builders quite unlike the corals that build reefs in today's oceans.

Amistad belongs to the Great Plains physiographic province. Its Geologic Resources Inventory products and a completed Soil Resources Inventory are on the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Amistad National Recreation Area, Texas," National Park Service; a work of the United States government in the public domain.