U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3045

June 2014

Prepared in cooperation with the U.S. Army Corps of Engineers

Hydrogeologic Aspects of the Knippa Gap Area in Eastern 

Uvalde and Western Medina Counties, Texas

Printed on recycled paper

The Knippa Gap

The Edwards aquifer is the primary source of 

water for a large area of south-central Texas (fig. 1). 

The geologic structure, stratigraphy, and hydrogeologic 

characteristics of rocks that make up the Edwards aquifer 

and its confining units control the amount and direction 

of groundwater flowing through the aquifer. Groundwater 

flow pathways in the western part of the Edwards aquifer 

are complex, and this complexity is associated with the 

Balcones Fault Zone, a prominent structural feature in 

south-central Texas, and other related geologic structures. 

The existence of a geologic structure known as the 

“Knippa Gap” first was postulated by Maclay and Land 

(1988) as a subsurface structural low or “gap” located 

in an area beneath Knippa and Sabinal, Tex. (fig. 1), that 

restricts regional groundwater movement from the west 

to the east. The existence of the gap was postulated on the 

Figure 1. Location of the Knippa Gap area in south-central Texas. 

D'Hanis

Utopia

Laguna

Brackettville

Uvalde

Sabinal

San Antonio

Knippa

Leakey

Nueces River

River

San Antonio River

Nueces River

West Nueces River

Leona River

Dry Frio River

Frio River

Seco Creek

Sabinal River

Hondo Creek

Cibolo Creek

Frio River

San Migue l Creek

EDWARDS

COUNTY

REAL

COUNTY

BANDERA

COUNTY

KINNEY COUNTY

MAVERICK COUNTY ZAVALA COUNTY UVALDE COUNTY

MEDINA COUNTY

FRIO COUNTY

DIMMIT COUNTY

LA SALLE COUNTY

McMULLEN COUNTY

ATASCOSA COUNTY

BEXAR

COUNTY

COMAL

COUNTY

KARNES COUNTY

WILSON

COUNTY

GONZALES 

COUNTY

LIVE OAK COUNTY

Leona Springs 1

Leona Springs 2

Knippa Gap area

Gulf of Mexico

29°30'

29°00'

99°45' 99°00' 98°15'

Base modified from U.S. Geological Survey 1:250,000-scale digital data

Universal Transverse Mercator projection, zone 14

North American Datum of 1983

T E X A S

Study area

UVALDE

COUNTY

MEDINA

COUNTY

Trinity 

aquifer

San Antonio 

segment of the 

Edwards aquifer BALCONES FAULT 

ZONE

BEXAR

COUNTY

EXPLANATION

Edwards aquifer (George and others, 2011)

 Outcrop

 Subcrop

Study boundary

Approximate location of the Uvalde Salient

Spring and name Leona Springs 1

0 10 20 30 40 MILES

0 10 20 30 40 KILOMETERS
basis of a series of numerical simulations of groundwater 

flow made by Maclay and Land (1988) in an attempt to 

synthesize the geologic, structural, and hydrogeologic 

characteristics of the aquifer. 

Although the presence of the Knippa Gap near the 

boundary of the Devils River Trend and Maverick Basin 

depositional provinces (fig. 2) in eastern Uvalde and 

western Medina Counties, Tex., had been supported by 

Outcrop of eroded and dissected extinct volcano located 

adjacent to the Frio River and east of the city of Uvalde in Uvalde 

County, Texas. 

Central

Texas

Platform

Ancestral Gulf of Mexico

Present-day

Gulf of Mexico

Stuart City Reef Ouachita Structural Belt

Devils River Trend

Maverick

Basin

San Marcos

Arch

San Marcos

Llano

Uplift

San Marcos

Platform

Tectonic Hinge Line

0 50 100 150 200 MILES

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Depositional province

 Devils River Trend

 Maverick Basin

 San Marcos Platform

Boundary of Balcones Fault Zone

Balcones escarpment

EXPLANATION

N

Figure 2. Regional depositional provinces, boundary of the Balcones Fault Zone, and the Balcones escarpment in south-central Texas. 

Cave in fractured Austin Group rock located 

approximately 9.3 miles northeast of city of Uvalde in 

Uvalde County, Texas.
numerical simulations, efforts to map it in detail had not 

been previously attempted. In cooperation with the U.S. 

Army Corps of Engineers, the U.S. Geological Survey 

developed the first detailed surficial geologic map of the 

Knippa Gap area with data and information obtained from 

previous investigations and field observations (Clark and 

others, 2013, plate 1). A simplified version of the detailed 

geologic map depicting the hydrologic units, faulting, 

and structural dips of the Knippa Gap area is provided in 

figure 3.

Surficial geologic units and surficial expression 

of fractures, faults, and lineaments were identified and 

mapped with the aid of well log data, aerial imagery, rock 

and soil exposures, and observed changes in vegetation 

0 2.5 5 7.5 10 KILOMETERS

0 2.5 5 7.5 10 MILES

Structural trough of the

Knippa Gap area

Approximate width of

 the Knippa Gap structural trough

UVALDE

COUNTY

MEDINA 

COUNTY

A

A'

B' B

99°16'15"

99°24'

99°31'45"

99°39'30"

99°47'15"

29°28'45"

29°21'

29°13'15"

Base modified from U.S. Geological Survey 1:24,000-scale digital data

Universal Transverse Mercator projection, zone 14

North American Datum of 1983

A A'

Hydrologic unit

 Upper confining unit to

 Edwards aquifer

 Edwards aquifer

Study boundary

Fault—Dashed where

 inferred

Line of section

Inferred structural dip

EXPLANATION

Figure 3. Hydrologic units, faulting, and structural dips of the Knippa Gap area in south-central Texas. Full version of geologic plate is 

available in Clark and others (2013). 

and drainage patterns. In eastern Uvalde and western 

Medina Counties, sedimentary rocks from the Lower 

Cretaceous to the Pleistocene along with igneous rocks 

from the Upper Cretaceous are exposed at land surface 

and in roadside outcrops (Clark and others, 2013). These 

rocks include limestone, chalk, gravel, sand, silt, shale, 

clay, and basalt. Igneous rocks also were mapped to show 

the spatial distribution of the igneous bodies and their 

relation to the geologic structure of the area. Borehole 

geophysical data were used to determine formational 

contacts in the subsurface and regional dips in each of 

the geologic units. This information was used to interpret 

structural dip directions and the approximate location of 

the structural trough in the Knippa Gap (fig. 4).
Sources of Groundwater in the Knippa Gap 

Area

The Edwards aquifer, a karst aquifer with large 

porosity and permeability that includes geologic units 

from the Devils River Trend and the Maverick Basin 

depositional provinces, is the primary source of water 

in the Knippa Gap area (fig. 3). The Edwards aquifer 

in the Devils River Trend is one of the most porous 

and permeable units in the area, with permeable zones BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION

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BEND IN

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NAVD 88

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200

300

400

500

600

700

800

900

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FEET

SOUTHWEST NORTHEAST

A A'

Approximate width of 

the Knippa Gap

structural trough

SECTION

B–B'

NAVD 88, North American Vertical Datum of 1988

Hydrologic unit

 Upper confining

 unit to Edwards

 aquifer

 Edwards aquifer

 Trinity aquifer

Land surface

Fault—Dashed 

 where inferred. 

 Arrow shows

 relative

 movement

EXPLANATION

0 3,000 6,000 FEET

VERTICAL SCALE GREATLY EXAGGERATED

Figure 4. Hydrologic section A-A’ showing approximate width of the structural trough in the Knippa 

Gap area. Location of trace A-A’ shown on figure 3.

that generally are associated with solutionally enlarged 

bedding planes and fractures. Although most of the 

permeability probably results from fractures, the aquifer 

also contains bedding plane, fracture, breccia, vug, 

channel, and cave porosity. The vertical fractures near the 

top of the Edwards aquifer in the Devils River Formation 

provide the most effective paths for recharge, and where 

subaerially exposed, these fractures greatly increase the 

rate of recharge when precipitation occurs in the area 

(fig. 3). 

Channel porosity developed along a bedding plane in the Austin Group located near the intermittent Leona Springs 2 on the 

Leona River in Uvalde County, Texas.
Additional minor sources of water in the Knippa Gap 

area include the Leona aquifer, a minor aquifer associated 

with alluvium of the Pleistocene Leona Formation that 

crops out along stream channels; the Uvalde Gravel; the 

Indio Formation; the igneous basalts and serpentines; 

and other Edwards aquifer confining units (Del Rio 

Clay, Buda Limestone, Eagle Ford Group, Austin Group, 

Anacacho Limestone, and Escondido Formation) (Clark 

and others, 2013). 

Complex Structure and Groundwater Flow 

Paths

The Balcones Fault Zone is a structurally 

complicated, Miocene-aged fault complex that trends 

approximately west to east across Uvalde and Medina 

Counties and is composed of high-angle, en echelon, 

vertical or nearly vertical normal faults that are 

downthrown to the southeast (fig. 5). The Balcones 

Fault Zone contains relay ramps that commonly form Figure 5. Hydrologic section B-B’ showing vertical or nearly 

vertical, en echelon normal faults that are downthrown to the 

southeast. Location of trace B-B’ shown on figure 3.

Sandstone ledges in the Escondido Formation located 

approximately 7 miles south-southeast of the community of 

D’Hanis in Medina County, Texas.

Exposed Austin Group in road cut excavation located 

on south side of U.S. Highway 90 and approximately 

midway between Sabinal and Knippa, eastern Uvalde 

County, Texas. Photograph taken May 29, 2012.

in extensional fault systems to allow for deformation 

changes along the fault block (fig. 6). Relay ramps 

connect the foot wall of a fault segment to the hanging 

wall of the overhanging fault (Clark and Journey, 

2006). Groundwater flow paths in the Edwards aquifer 

are influenced by this complex faulting in that some 

faults may function as barriers to groundwater flow 

where the total thickness of the aquifer is offset by at 

least 50 percent. In these locations, groundwater flow 

moves parallel to the strike of the fault. The amount of 

displacement along a particular fault tends to vary; thus, 

the effectiveness of a fault as a barrier to flow probably 

changes along the fault plane. BEND IN SECTION BEND IN SECTION BEND IN SECTION BEND IN SECTION

BEND IN

SECTION

-1,100

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-850

-800

-750

-700

-650

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-550

-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050

1,100

1,150

1,200

NAVD 88

FEET

NORTHWEST SOUTHEAST

B B' SECTION

A–A'

NAVD 88, North American Vertical Datum of 1988

Hydrologic unit

 Upper confining

 unit to Edwards

 aquifer

 Edwards aquifer

 Trinity aquifer

Land surface

Fault—Dashed 

 where inferred. 

 Arrow shows

 relative

 movement

EXPLANATION

0 3,000 6,000 FEET

VERTICAL SCALE GREATLY EXAGGERATED
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143045

Publishing support provided by

Lafayette Publishing Service Center

In western Medina County, faulting within the 

Balcones Fault Zone has produced a relay-ramp structure 

that dips to the southwest from the Edwards aquifer 

recharge zone and extends westward (figs. 3, 6). The faults 

in this area function as barriers to flow, and groundwater 

originating in northern Medina County moves down the 

relay ramp from as many as four separate, fault-bounded 

flow paths (Clark and Journey, 2006) downgradient 

(downdip) to the southwest towards the structural low 

(trough) in the Knippa Gap area (figs. 4, 6). 

In Uvalde County, the rocks that make up the 

Edwards aquifer dip down from a structural high known 

as the Uvalde Salient towards the structural low (trough) 

(figs. 4, 6). Groundwater moving downgradient from 

near the end of a fault in the Uvalde Salient from the 

western part of the Edwards aquifer in the Maverick Basin 

is funneled down a relay ramp by faults that function 

as barriers to flow east-northeast towards the structural 

low (trough) in the Knippa Gap area where it joins 

groundwater flowing in from Medina County (figs. 4, 6). 

The combined flow from these two flow paths then flows 

through an area with minimal fault displacement in the 

Knippa Gap.

This fact sheet is based on the following report:

Clark, A.K., Pedraza, D.E., and Morris, R.R., 2013, Geologic 

framework, structure, and hydrogeologic characteristics 

of the Knippa Gap area in eastern Uvalde and western 

Medina Counties, Texas: U.S. Geological Survey Scientific 

Investigations Report 2013–5149, 35 p., 1 pl., http://pubs.

usgs.gov/sir/2013/5149/.

HW

FW

HW

Uvalde

Salient

Knippa Gap

relay ramp relay ramp

HW

FW

FW

Edwards

aquifer recharge zone

EXPLANATION

Inferred structural feature

Inferred groundwater flow

 direction

Fault

Hanging wall of a fault

Footwall of a fault

HW

FW Modified from Clark and Journey, 2006, fig. 5

SOUTHWEST NORTHEAST

UVALDE COUNTY MEDINA COUNTY

Figure 6. Idealized view of the relay-ramp structure that dips to the northeast from the Uvalde Salient in Uvalde County, 

the relay-ramp structure that dips to the southwest from Medina County, and the inferred direction of groundwater flow 

paths towards the Knippa Gap structural low (trough).

References Cited

Clark, A.K., and Journey, C.A., 2006, Flow paths in the 

Edwards aquifer, northern Medina and northeastern Uvalde 

Counties, Texas, based on hydrologic identification and 

geochemical characterization and simulation: U.S. Geological 

Survey Scientific Investigations Report 2006–5200, 48 p.

George, P.G., Mace, R.E., and Petrossian, Rima, 2011, Aquifers 

of Texas: Texas Water Development Board Report 380, 

172 p.

Maclay, R.W., and Land, L.F., 1988, Simulation of flow in the 

Edwards aquifer, San Antonio region, Texas, and refinement 

of storage and flow concepts: U.S. Geological Survey WaterSupply Paper 2336-A, 48 p.

—By Rebecca B. Lambert, Allan K. Clark, Diana E. Pedraza, and 

Robert R. Morris

For additional information, contact

Director, USGS Texas Water Science Center

http://tx.usgs.gov/

gs-w-txpublic-info@usgs.gov