Explore Geology
geology fieldnotes title


National Recreation Area


cover of park brochure

park geology subheading
Aerial view of river at Amistad Rec Area
Amistad National Recreation Area, Texas

The land throughout most of southern Val Verde County is undulating to nearly level, but with steeper slopes and eroded canyon walls having ten to several hundred foot elevation differentials close to the rivers.The subsurface geology surrounding Amistad NRA is one primarily of limestone in the Edwards Plateau. Several excellent papers and bulletins describe the stratigraphy, materials, setting, history, and presence of fossils and are briefly summarized below. Many geologic studies have been performed in the region for several reasons: easily accessible formation exposures in eroded river channels and road cuts; geotechnical studies for dam and petroleum studies; and the presence and great interest in large subsurface springs in this dry region. The long stretch of US 90 north-northeast of the reservoir lies on a thick gray lower cretaceous limestone (the very same limestone that makes up the upper part of the cliffs in Santa Elena Canyon on the west side of Big Bend National Park). It is seen in numerous eroded and exposed locations in Amistad NRA including bluffs and cliffs west of the US 90 bridge over the reservoir and on the west side of the Pecos River canyon (US 90 bridge). The lower cretaceous limestones are overlain by upper cretaceous limestones nearby of the Austin chalk, Boquilas formation, the Buda limestone, and the Del Rio Clay in descending order. Numerous Buda limestone outcrop locations contain clam fossils (Spearing, 1991). Excavations for the Amistad Dam construction and nearby drill cores gathered by the IBWC yielded detailed limestone bedrock definition. Nearly 450 feet (137 m) of the Salmon Peak Formation, consisting of lime mudstone overlays about 300 feet (91 meter) of limestone shales, anhydrite grainstones, and lime mudstones of the McKnight Formation (Smith et al, 1983). The geologic history and hydrogeologic setting of the more than 42,000 square mile Edwards-Trinity aquifer system is described in a 1994 publication by the U.S. Geological Survey (USGS). Amistad NRA receives major groundwater flow through springs and partially spring-fed rivers that tap the Edwards-Trinity aquifer. Extensive fractures, joint cavities, and porosity caused by the dissolution of unstable carbonates and evaporates provide the conduit for the aquifer to Amistad Reservoir (Barker et al, 1994).


Soils along the United States side of Amistad Reservoir were derived from the parent limestone rock and formed through weathering and biological processes over thousands of years. The soils are almost entirely classified as Langtry-Rock outcrop Zorra. Most of these shallow, loamy soils are moderately alkaline, cobbly or stoney, about 8 inches deep, and usually found over fractured limestone bedrock or strongly cemented caliche; with exposed limestone outcrops commonly found on uplands (Golden et al, 1981). Suitability of these soils lies primarily for wildlife habitat or range, while urban and recreational uses are poor, because of depth to bedrock and slope considerations. None of the soils surrounding NRA or nearby are prime farmland soil types, but southeast of Amistad NRA and Del Rio some would be classified as such, but only if irrigation water was employed. On the United States side of the Rio Grande, Amistad NRA lies within Val Verde County, Texas. On the Mexico side, the reservoir is located entirely within the state of Coahuila. The entire Val Verde county and State of Coahuila, in proximity to Amistad NRA, is rangeland for sheep, goats, and cattle (even with the increasing development of United States lands surrounding the reservoir for residential use). Rangeland use, while a suitable use of the soils types and vegetation surrounding Amistad Reservoir, can also be a cause of erosion, non-native plant/animal influx, and potential water quality impacts.

Springs and Seeps

Springs, the discharge of groundwater at the surface, have been very important to inhabitants of the border area between Texas and Mexico. The springs' significance precedes even the arrival of the first humans in the area, as formative agents of hydrogeological processes and important factors for vegetative and wildlife habitat developments. Early human discoveries of springs in more arid regions or dry seasons would have established potable water sources. They also would have laid the patterns for early hunting sites, trails for communication and commerce, settlements, and some agriculture by irrigation.

In a study published in 1975, Texas was found to originally have had more than 280 major and historically significant springs, with more than half of those significantly decreased in flow or having ceased to flow entirely. More than half of the significant springs were found to emanate from the Edwards and the Edwards-Trinity Aquifers, with the larger springs in the Amistad NRA originating from the latter. Known springs at the Amistad NRA and the large San Felipe spring in Del Rio flow from the Georgetown Formation Limestones of the Edwards-Trinity Aquifer (Brune 1975). The Georgetown Formation Limestones, known locally as the McKnight and associated limestones, actively transmit much of the ground water and spring water to the south-southwest in Val Verde County (Armstrong 1995). Major springs of Amistad NRA are described in Table 2.

Historical Spring Flow

Historically, springs such as San Felipe Spring, Goodenough Spring, and many others throughout the state, flowed under large amounts of pressure and produced fountains at the surface. Pressure release due to well drilling, and head decrease due to long years of pumping for drinking supply and agricultural use have reduced the flows significantly at these and many other springs. Some spring flow may also have decreased due to reduced recharge over the watershed due to a shift from grass to shrub cover, and the resulting loss of infiltration capacity, resulting from grazing over the past 100 years. The completion of the Amistad Reservoir in 1969 covered many springs in the area and increased the flow of others downstream. The Cantu or Cienaga Spring, located just downstream of the Amistad Reservoir dam on the Rio Grande also has had an increased flow since the reservoir construction.

Metals and Trace Elements

The two binational toxic studies, the USGS NASQAN stations and several studies, which analyzed sediment, have detected a variety of metals and trace elements in Amistad Reservoir and it's tributaries. Table 10 lists all the substances detected. Two studies have looked at trends in metals in sediments. A USGS study analyzed trends using data collected by TNRCC sampling and other assessments between 1970 and 1994 (Lee and Wilson, 1997).. This analysis indicates that most trace elements levels appear to be steady, although mercury concentrations are increasing in both the Pecos River, the Rio Grande above Amistad and in Amistad Reservoir. Selenium is also increasing in Amistad Reservoir. The only trace elements with a decreasing trend are copper in Amistad Reservoir and silver in the Pecos River (Lee and Wilson, 1997). In another study, sediment cores were sampled from both the Rio Grande Arm and the Devils River Arm (Van Metre, et al, 1997). Eight metals (arsenic, chromium, copper, lead, mercury, nickel, vanadium and zinc) have statistically significant increasing trends in the Rio Grande Arm. Both mercury and nickel more than doubled between 1969 and 1995. All but lead and chromium were found to be increasing in the Devils River Arm of the reservoir. All of these metals are associated with atmospheric sources such as burning of fossil fuels and incineration of solid waste (Van Metre, et al, 1997). Elevated levels of arsenic have been detected in water samples and mercury in fish (TNRCC, 1997). Arsenic occurs naturally in the Rio Grande Basin and is especially high in West Texas due to the mineralization of volcanic rock.

Effects on Archeological Sites

It is estimated that there are more than 1,500 cultural resource sites adjacent to the more than 540 mile (870 kilometer) shoreline on the United States side of Amistad Reservoir. These resources span more than 10,000 years of Native American prehistory and include historic remains associated with the 19th century Southern Transcontinental Railroad and early 20th century ranching. Prehistoric archeological sites are the most common, with an estimated 900 sites within the immediate flood pool of the reservoir (Labadie, 1994). Prior to the impoundment of waters behind Amistad Reservoir in 1969, there had been over 10 years of archeological fieldwork, which inventoried and documented sites that would be inundated by the future reservoir. Over 300 major prehistoric sites were documented. Twenty-two sites, mostly caves and dry rockshelters were excavated; several sites had more than 20 feet (6 m) of cultural deposits (Anderson, 1974). The preinundation research generated more than 4,000 photographs, 65 linear feet (20 m) of documents, and produced a museum collection estimated to contain more than 1,000,000 artifacts, all of which are now managed by the park.

In the Spring of 1994, lake levels began dropping in response to what would become a multi-year, regional drought affecting most of southwest Texas and northeastern Mexico. By the Summer of 1998, Amistad Reservoir had dropped 56 vertical feet (17 m) and covered less than 20% of the area it did at normal operating levels. Since 1994, Archeological surveys in drawdown zones have documented over 150 new archeological sites and re-documented nearly 50 sites identified by the pre-inundation research. Many of the recently discovered sites initially appeared as silt-covered mounds of fistsized rocks rising above the unvegetated mud flats; some were unapproachable because of the quicksand-like nature of the mud. These new sites range from the isolated remains of a single prehistoric campfire pit to sites covering over 5 acres (2 hectares) that have several hundred campfire pits, tipi rings, and grinding holes (Labadie, 1999). The overall shape of these features, circular concentrations of tightly packed fire-cracked rock, are intermixed with darker soils, suggesting that many may still have intact archeological deposits despite nearly 30 years of inundation. Intermixed within these soils are small, modern Asiatic clam shells (Corbicula fluminea). Because of its widespread distribution throughout the reservoir, continuous burrowing behavior, and high population densities, the Asiatic clam is adversely affecting most submerged archeological sites throughout the entire reservoir basin (Shafer et al, 1997) More than 1500 individual fire-cracked rock features have been documented since 1994, nearly all of which have been significantly affected by wave-action from high winds, passing boats, and fluctuations in reservoir levels. It is now believed that the modern ground slope of exposed terraces is a basic determinant of the severity of wave-action damage to all archeological sites or individual features. An optimum ground slope angle appears to exist where wave action effects are negligible; above or below this angle, wave action is intensified resulting in somewhat predictable dispersal patterns across recently exposed ground surfaces (Collins and Labadie, 1999). Typically, sites with ground slopes above eight degrees will have a series of individual cutbanks often resembling stair-steps with each step representing a different lake elevation. Sites with low ground slope angles usually have a parallel series of drift lines or windrows (similar to high-tide lines at the beach) composed of corbicula shells, chert flakes, and small fraction fire-cracked rocks (Labadie, 1999). In either setting, horizontal relationships among artifacts or feature specific lithic associations are highly suspect given the number of times most sites have been subjected to the cycle of inundation, exposure, and reinundation. It is also becoming clear that wave-action differentially affects the various classes of archeological materials at recently exposed archeological sites (Gustavson and Collins, 2000). Small items, such as flint flakes and arrowheads, bone or shell fragments and organic materials are the first to be relocated as wave passes across the site. Larger items, such as metates or rock-lined cooking pits, require greater amounts of wave-energy to move individual items or before the waves systematically dissemble a fire-cracked rock feature. It has also been demonstrated that as a wave sweeps across an ancient campfire or cooking pit, it is capable of dislodging the associated soil matrix and, over time, can fill these voids between the rocks entirely with modern lacustrine deposits. The result of this process is that although a fire-cracked rock feature may look intact, the interior deposits can be entirely modern (Collins et al, 2000). The regional drought that has gripped the Amistad NRA has been making national headlines for several years now. Visitation has dropped substantially and the reservoir has receded to its lowest levels since it began filing in the late 1960s. On the brighter side of things, the drought has provided archeologists and other earth scientists an unprecedented opportunity to study a portion of the prehistoric landscape thought to have been long lost under the waters of Amistad Reservoir.

Shoreline Erosion Related to Water Level Fluctuations

Rockfalls are common in the limestone cliffs surrounding the reservoir. It is likely that the rock is degrading more rapidly due to the repeated inundations. This could be degrading rock shelters and other archeological remains, as well as accelerating a natural geologic process.

Adjacent Oil & Gas Operations

Few economic subsurface mineral deposits are present in the area surrounding the Amistad Reservoir. Exploring for oil and gas has resulted in dry holes, and there appears to be very little potential for successful production locally. Even the production of coal bed methane gas, which has some regional prospects, appears only minimally possible surrounding Amistad and Del Rio, because of the lack of substantial coal beds. Some of the nearest productive coal bed methane deposits along the Rio Grande River appear in the Eagle Pass area 100 miles (161 km) downstream of Amistad Reservoir. Edwards carbonaceous deposits are common to the Amistad area. Although these deposits can yield substantial oil deposits elsewhere, they are not present locally due to the extensive water presence in the Edwards in this area. Oil and gas are found further north of Amistad Reservoir, where portions of the Edwards deposits lack water (pers. communication with Lisa Norby, NPS Geologic Resource Division, November 15, 2000).


Thirty-two cave and sinkhole features have been documented at Amistad NRA. Many of these cave structures have been confirmed to be directly connected to the Amistad reservoir. Of the thirty-two known karst features, 18 have been surveyed. Most areas of the park need a baseline survey to confirm the presence of karst formations.

Source National Park Service, Water Resources Division


Anderson , B. 1974. An Archeological Assessment of Amistad Recreation Area Texas . Division of Archeology, Southwest Region, National Park Service. Sante Fe , New Mexico

Armstrong, A. W. 1995. The Use of Stable Isotope Ratios to Investigate the Relative Importance of Amistad Reservoir to Recharge of the McKnight and other associated '' Limestones, Southwestern Val Verde County, Texas, Master of Science, The University of Texas at San Antonio, Texas, 11/95 95pp.

Barker, R. A., P. W. Bush, and E. T. Baker, Jr.. 1994. Geologic History and Hydrogeologic Setting of the Edwards-Trinity Aquifer System, West Central Texas , U.S. Geological

' Survey, WRI Report 94-4039, Austin Texas, 51 pp

Brune, Gunnar. 1975. Major and Historical Springs of Texas , Texas Water Development Board - Report 189, 94pp.

Collins, M. B. and J. Labadie. 1999. The 1999 Texas Archeological Society Field School : Excavation, Rock Art Recordation, Surface Feature Documentation, and Survey at Amistad National Recreation Area. Texas Archeological Society Newsletter 43 (1):3­' 7. San Antonio , Texas.

Golden, M. L., W. J. Gabriel, and J. W. Stevens, 1981. Soil Survey of Val Verde County, Texas, U.S. Department of Agriculture, Soil Conservation Service in cooperation with • the Texas Agricultural Station and Val Verde County Commissioners Court , 64pp.

Gustavson, T. C. and M. B. Collins. 2000. An Assessment of Flood Damage to Archeological Sites in the San Pedro Drainage, Amistad National Recreation Area, August 1998. University of Texas at Austin , Texas Archeological Research Laboratory. Austin , Texas.

Labadie, J. H. 1994. Amistad National Recreation Area: A Cultural Resources Study. National Park Service, Southwest Cultural Resources Center . Santa Fe , New Mexico .

Labadie, J. H. 1999. Cultural Resources Management at the Amistad National Recreation Area, Del Rio , Texas . La Tierra, Journal of the South Texas Archeological Association, Vol. 26 (1): 18-23. San Antonio , Texas .

Lee, R.W. and J.T. Wilson. 1997. Trace Elements and Organic Compounds Associated with Riverbed Sediments in the Rio Grande/Rio Bravo Basin, Mexico and Texas . U.S. Geological Survey Fact Sheet FS-098-97, 6 pp.

Shaffer, B. S, J. P. Dering, J. Labadie, and F. B. Pearl. 1997. Bioturbation at Submerged Cultural Sites by the Asiatic Clam: A Case Study from Amistad Reservoir , SW Texas . Journal of Field Archeology 24 (1): 135-138. Boston , MA.

Texas Natural Resource Conservation Commission. 1997. Second Phase of the Binational Study Regarding The Presence of Toxic Substances in the Rio Grande/Rio Bravo and its Tributaries Along its Boundary Portions Between the United States and Mexico: Texas Natural Resource Conservation Commission, 246 pp.

Spearing, Darwin . 1991. Roadside Geology of Texas, Mountain Press Publishing Company, Missoula, Montana, pp. 137-144.

Smith, C.L, G. P. Bolden, R. E. Webster, and J. B. Brown. 1983. Structure and Stratigraphy of the VaIVerde Basin- Devils River Uplift, Texas , West Texas Geological ~Society Publication #83-77, 26pp.

Van Metre, P.C., B.J. Mahler, and E. Calendar. 1997. Water-quality trends in the Rio Grande/Rio Bravo Basin using Sediment Cores from Reservoirs: U.S. Geological Survey Fact Sheet FS-221-96, 8 pp.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album has not been prepared for this park.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscinece book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading


For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

General information about the park's education and intrepretive programs is available on the park's education webpage.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://www2.nature.nps.gov/geology/parks/amis/index.cfm   I  Email: Webmaster
This site is best viewed in Internet Explorer 6.0 or Netscape 7.0