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Figure 1 : Generalized locations of field-trip stops.
Figure 2 : Topographic and geologic maps of Santa Clara and Dry Comal Creek basins (modified from Woodruff, 1977).
Table 1 : General stratigraphic sections of Cretaceous units, Balcones Escarpment.
Depart San Antonio Convention Center (see fig. 1 for generalized stop locations).
Turn right onto East Commerce Street; proceed east.
From right lane of Commerce Street, enter southbound access road to Interstate Highway (IH) 37.
Merge with IH-37 South.
Depart IH-37 via exit 138-A--Southcross Boulevard.
Turn right off of access road.
Turn right onto East Southcross Boulevard.
Turn left onto South Presa Street; proceed south.
STOP 1--HOT WELLS HOTEL (no longer a stop, owing to problematical access)
This is the site of an abandoned resort hotel, which was built during the 1930's as a spa. Water for the hot baths came from a well that penetrates the bad-water zone of the Edwards Aquifer. This well, completed in 1933 to a depth of 572 m (1,876 ft), discharged artesian water at a temperature of 39.4 oC (approximately 103 oF). Dissolved solids total 4289 mg/L (predominantly calcium sulfate, as reported by Woodruff and others, 1982). The well still flowed as late as 1981. At that time, hydrogen sulfide odor was prevalent, and the well head was coated with a dense growth of algae. In June, 1988, lightning struck the old resort building, which was consumed by fire. The current condition of the old well is unknown. For a further discussion of geothermal resources along the Balcones Escarpment, see Foley and Woodruff (this volume).
Turn around and proceed north on South Presa Street.
At East Southcross Boulevard, turn right.
Turn left onto New Braunfels Avenue.
Turn left on the northbound access road to IH-37.
Merge onto IH-37; proceed north.
Cross IH-10 overpass.
On the left is the San Antonio skyline. The city was founded in 1718 when Spanish friars established the first of several missions along the San Antonio River, which is fed by springs issuing along faults in the north-central part of the city. Since the time of earliest occupation by Europeans, the discharge from the Edwards Aquifer has sustained the inhabitants of San Antonio (see Palmer, this volume). Today, San Antonio is the largest city in the United States that is dependent entirely on groundwater for its public water supply.
Merge right with IH-35; proceed north.
Proceed past Loop 410; continue north.
Exit onto Loop 1604 (under construction when this log was prepared); mileage is approximate. Cross IH-35 overpass, and proceed west on Loop 1604.
We are approaching the main fault line and escarpment of the Balcones system. On the east side of the fault, bedrock consists of claystones and marls of the Taylor Group (Upper Cretaceous). Deep clayey soils derived from the claystones and marls support facies of Blackland Prairie (mixed to tall-grass vegetative stands), now mostly gone. The mesquite savannah seen here represents some natural vegetative elements, but mostly this assemblage is anthropogenic (that is, man-influenced). Most deep soils on the east side of the fault line have been cultivated for small grains or tame pasture grasses.
When we cross the main fault line, substrate will be Buda Limestone (see stratigraphic section, Table 1). Vegetation changes across the fault line from disturbed prairie to a mixed low woodland dominated by live oak (Quercus virginiana). The oak woodlands occurring on the dissected but gently rolling terrain seen here are also disturbed. The potential natural vegetation adapted to local edaphic (influenced by soil rather than by climate) factors is a live-oak woodland with abundant (that is, solid) ground cover of grasses such as little bluestem, Indiangrass, and the like, as will be seen at Stop 3.
Cross Missouri-Kansas-Texas (Katy) Railroad tracks.
Cross Farm-to-Market (FM) Road 2252.
Cross Missouri-Pacific Railroad tracks.
Note the concentration of major rail lines and their proximity to the Interstate Highway. The land along the Balcones Escarpment is an important transportation corridor. Early on, part of the Spanish Camino Real (Royal Highway) lay adjacent to the escarpment. Later, the early cattle drives used the natural topographic break, and the Chisholm Trail extended along this line. As public transportation developed, this corridor became the site of stage-coach routes, and later the early state and federal highways were sited here. Today, the Interstate Highway runs along the escarpment, and there is talk about constructing a high-speed commuter rail system.
Turn right onto Green Mountain Road.
STOP 2--HIGH GRAVELS ON CIBOLO CREEK DIVIDE
At this site, we see a section of alluvial gravels perched on the divide between Cibolo Creek and Salado Creek. The gravels consist mostly of limestone fragments, but some chert clasts are also evident. The material was all locally derived; that is, the source of the sediment was the Hill Country. Fine-grained fractions of the alluvium have been calichified. The age of this material is unknown. Likewise, the stream that deposited the gravels is unknown. It may have been a former high level of Cibolo Creek, but regional geometrical evidence suggests that Cibolo Creek once flowed to the northeast (Woodruff and Abbott, this volume). Hence, this material may have been deposited either by an ancestral Salado Creek or a larger through-flowing stream, such as the Medina River, which is proposed by Woodruff and Abbott (1979) to have had a former, more eastward-directed course.
Proceed straight on Green Mountain Road.
Turn right onto Evans Road.
Cross Missouri-Pacific Railroad tracks again.
Turn left onto Nacogdoches Road (FM 2252).
As we cross Cibolo Creek, note that it has no well-defined riparian zone. This is typical of recharge reaches within the Balcones fault zone, because the recurrent high discharge of streams debauching from the Hill Country does not allow establishment of typical riparian assemblages. Vegetation common to the dry creek reaches include Mexican walnut (Juglans microcarpa), sycamore (Plantanus occidentalis), and buttonbush (Cephalanthus occidentalis). Vegetation occurring away from the channel reaches consists of scrub woodland typical of the Edwards uplands (that is, more typical of the western part of the Balcones fault zone), even though we are on the downthrown side of the fault (Pecan Gap Marl of the Taylor Group). The 37-year average discharge for Cibolo Creek where it is crossed by IH-35 is 14.8 cfs, but it is dry most of the time. Catchment area is 274 square miles. Maximum recorded discharge was 65,000 cfs on 16 July 1973; gage height was 26 .2 ft (U.S. Geological Survey, 1984).
Cross Missouri-Pacific Railroad tracks again, and proceed past Bracken Road.
We are at the edge of Cibolo Creek's Quaternary terrace. The bedrock composing the uplands is Austin Chalk.
Turn left onto Bat Cave Road.
As we proceed north, we are crossing a structurally and stratigraphically complex area of the Balcones fault zone. We are within an area bounded by the Edwards Limestone, but grabens are mapped that result in sections of Buda Limestone and Del Rio Clay being exposed. Vegetation along this part of the route is degraded oak woodlands. This land has been subjected to unwise range management: overgrazing, too many goats, ill-advised burning, and the like.
Entrance to Bracken Bat Cave.
This cave is the site of the largest concentration of mammals in the world (Texas Nature Conservancy, 1985). Two caves at this locality house 20 to 30 million Mexican free-tailed bats during summer months. These animals eat more than 150 tons of insects (mostly mosquitoes) every night. Bat guano has been mined for fertilizer and as raw material for gunpowder, and man-made bat shelters were maintained by early German farmers to tap this renewable source of nitrates.
During late summer, when migrant bats are present, the population of animals in the cave reaches a yearly maximum. They emerge at dusk and, because of their vast numbers, their exit from the cave may take as long as six hours. The photograph on the frontispiece records this dramatic evening exodus. The huge flights of bats have posed problems for airplanes from nearby Air Force bases; the animals may be sucked into the intake of a jet engine and then clog the vanes causing a crash. It is reported that the Air Force proposed bombing the caves to alleviate this problem; instead, they altered flight paths to avoid the bats.
Note catclaw acacia and kidneywood; these are typical South Texas thorn scrub/Tamaulipan thorn woodland species.
Turn left on FM 3009.
As we proceed northwest, we have moved structurally upward (onto the upthrown side of the graben system) and are now traversing Edwards Limestone. This is typical uplands of the Edwards Group within the Balcones fault zone. Most recharge, however, occurs within the major stream reaches. Studies of the water budget in the Barton Springs (Travis/Hays Counties) segment of the aquifer show that approximately 85 percent of incident rainfall is cycled through the biological pump of evapotranspiration (Woodruff, 1984). Approximately 9 percent runs off, and the remaining 6 percent recharges the aquifer. Of this fraction, 85 percent recharges via the major streams, and 15 percent (1 % of total rainfall) recharges on the uplands and in tributary drainage-ways.
Entrance to Natural Bridge Caverns; proceed straight.
Note the overgrazed area inhabited by exotic animals. These exotic species are especially abusive of native range (see Palmer, this volume). On the left, note the browse line in trees created by goats.
Turn left; continue toward Natural Bridge Caverns.
Note relatively undisturbed native grass cover. This is probably a reasonable facsimile of what the live oak woodland looked like to original settlers. Such range, with its native grass stands of little bluestem (Schizachinum coparis) and Indiangrass (Sorgastam nutans), contrasts markedly to the range grazed by the exotlc animals.
Note contrast on two sides of the road: to the left is a native plant assemblage with clumps of oak trees, open grassland, and occasional junipers; to the right is disturbed range--an anthropogenic landscape. Such a disturbed area will have increased erosion and sediment yield. Too, the water budget will be disturbed, with more incident rainfall being allocated to runoff and less to evapotranspiration. Downstream effects on water quality and on periodic flash floods is to be expected but remains unquantified. Disturbances of this type are common across the Hill Country/Edwards Plateau uplands. Also common is the spread of exotic herbivores, which are brought in as tourist attractions (as at the last stop) but are also being introduced as game (trophy) animals. The ecological consequences of this practice are unknown, but as pointed out by Palmer (this volume), Russian boars have become a threatening predator in certain areas.
STOP 3--NATURAL BRIDGE CAVERNS
Natural Bridge Caverns is the eleventh longest surveyed cave in Texas (Kastning, 1978). It was discovered in 1960 and was opened to the public in 1964.
The cave entrance is within the Edwards Limestone, but most of the passages lie within the Walnut and Glen Rose Formations (Abbott, 1973). Large caverns within the alternating strata of the Glen Rose Limestone are unusual. As suggested by Kastning (1974), the cave may be "inherited" from a previously developed cavern system that occurred along similar trends in the overlying (now removed) parts of the Edwards Limestone. As area-wide base-level changed, the water table was lowered and the cave was thus "incised" into the older, less soluble units.
Passageways within Natural Bridge Caverns are strongly controlled by joint trends. The cave lies along the Bat Cave fault, but the main passages lies at roughly right angles to the strike of this fault. Secondary passages lie parallel to the fault trend (Abbott, 1973; Kastning, 1978). As will be seen as we tour the cave, there are extensive "primary" porosity zones within the Glen Rose strata.
Turn around; proceed back to FM 3009.
Turn left on FM 3009; proceed north.
Note that the new road right-of-way is graded fencepost-to-fencepost. This is very bad for the maintenance of native plant species, as seed stock is being lost. Commonly, native seed sources along road rights-of-way contrast markedly with the surrounding vegetation. Roadways are thus important genetic ecobanks. In addition, such indiscriminate clearing and grading exacerbate erosion and downstream siltation.
Intersect Ranch Road (RR) 1863; turn right and proceed east. We are still traversing Edwards Limestone.
This area is close enough to San Antonio that many of the old ranches have been subdivided into "ranchettes"--weekend homes for urban dwellers. Such use may have a salutary impact, because the lessening of grazing pressure allows recovery of native plant communities (especially the turf-maintaining grasses).
Note dense stands of juniper (Juniperus ashei). This is commonly a sign of past abuse of the range. Juniper is very "plastic"; it is able to establish itself in shade or in full sun. Potential habitat for juniper is thus maximized owing to human activities. Range fires have been curtailed and removal of thick grasses have combined to abet the increased stands of juniper. Junipers have stomata that are open all the time (unlike those of most broadleaf plants), thus they consume an extraordinary amount of moisture. Because of this, few plants can become established in the understory beneath the junipers. Hence, erosion takes place creating ever more habitat for more junipers; in this way, problems beget problems.
A fine discourse on junipers, land use and abuse in Central Texas, and the rise of a distinct breed of Hill Country folk--the "cedar choppers"--is presented in John Graves's Hard Scrabble.
Intersect Texas State Highway 46; turn right.
Intersect Loop 336; proceed straight ahead into New Braunfels.
As we proceed down the Balcones Escarpment, we pass an excellent exposure of Edwards Limestone.
Turn left, continuing on Texas Highway 46.
Turn left into Landa Park; cross Comal River.
Cross Comal River again. Except during periods of rainfall, the entire discharge of this river is from Comal Springs. The average discharge of this river over 51 years is 298 cfs (U.S. Geological Survey, 1984).
STOP 4--COMAL SPRINGS
Comal Springs comprise the largest spring system in Texas and the fifth largest in the U.S. (Meinzer, 1927). Long-term average discharge from these springs is 300 cfs; maximum recorded discharge was 534 cfs on 16 October 1973. The springs ceased flowing for a period in 1957 after 7 years of drought (Brune, 1975). Ogden (this volume) has studied the hydrogeochemistry of discharge from individual spring orifices from Comal, Hueco, and San Marcos Springs. This research demonstrated that different discharge sites within a single spring system may have a markedly different catchment area.
The springs provided water for Indians long before their "discovery" by the French explorer, St. Denis, in 1764. The town of New Braunfels was established in 1845 by German settlers led by Prince Carl of Solms-Braunfels. The springs were a prime reason for their selection of this locality as a settlement site.
Proceed straight; depart Landa Park.
Ascend the abrupt fault-line scarp.
Turn left onto Loop 337.
Proceed beneath Texas Highway 46 overpass.
On the left is the contact between the Georgetown Limestone and the underlying Edwards Limestone. Both rock units are considered part the Edwards Aquifer, although, of the two, the Edwards Limestone is the main water-bearing unit. The lumping of these two units within the aquifer is probably as much a function of the difficulty in areal mapping as it is in the hydrologic properties of the two formations.
Here we see the woody overstory of live oak and juniper trees that are so typical of sloping terrain within the Hill Country/ Edwards Plateau.
STOP 5--LOOP 337 ROAD CUT
This stop affords an excellent view into the karstified Edwards Limestone along the Balcones Escarpment. Several generations of solution activity are evident here, including Cretaceous collapse and subsequent deposition of undeformed strata, early (perhaps Cretaceous) leaching of selected parts of certain strata producing "boxwork" and honeycomb aspects, and the evident large cavern development including collapses and infilling with terra rossa (see Young, this volume).
Proceed straight on Loop 337.
As we descend the escarpment, we have a dramatic overview of the Blackland Prairie in the distance. We are crossing two grand divisions of North America (Fenneman, 1931): from the Great Plains (of which the Hill Country/Edwards Plateau is a part) to the Coastal Plains (of which the Blackland Prairie is one part). There are probably few places in the country where physiographic breaks are so abrupt. The fault-based juxtaposition has ramifications on terrain (as seen so dramatically here), on weather and climate (the Balcones Escarpment is the locus of the largest flood-producing storms in the conterminous U.S.), on water regimes (as evidenced by Comal Springs issuing from the base of the escarpment), on soils (Aandahl, 1972), on plants (Riskind and Diamond, this volume), and on animals (Neck, this volume).
Cross Dry Comal Creek; this drainage system has its headwaters in the Hill Country, but at the escarpment it abruptly changes direction from a prevailing southeastward trend to one of a northeast direction. The implication of this is discussed at the next Stop.
Stop sign; proceed across IH-35.
Stop again; proceed straight. Note washboard road. This is an indication of insufficient road-base materials being emplaced on the high-plasticity Blackland (vertic) clay soils.
Turn left on Santa Clara Road. Note severely overgrazed land on right--a "goatscape."
STOP 6--SANTA CLARA CREEK
Here we are viewing a beheaded stream valley. We are near the head of drainage of the southeast-flowing Santa Clara Creek. Note the broad alluvial valley and its proximity to the Balcones Escarpment to the west. This abandoned stream valley and the geometrical relations within the Dry Comal Creek basin (figure 2), suggest that Dry Comal Creek has eroded headwardly along the Balcones Escarpment, progressively capturing the headwater tributaries to Santa Clara Creek (see Woodruff, 1977). If such processes continue, Dry Comal Creek will eventually capture Cibolo Creek, diverting it from the San Antonio River watershed into the Guadalupe watershed.
Turn left onto Marion Road.
At this high topographic level we are crossing other high alluvial gravels shed off the Balcones Escarpment. This indicates that piracy events were episodic and recurrent.
Turn right onto Weil Road; the condition of this road clearly indicates the swelling clay substrate.
LUNCH STOP--GOERKE'S COUNTRY TAVERN
Right-angle turn (to left) on Weyel Road.
Intersect Tolle Road (FM 1103), turn right.
Brushy areas represent thin soil zones on Pecan Gap Marl. Once this land was open woodland, but it has been disturbed by overgrazing and attempts at plowing and planting. Now a mosaic of brush species occurs across the uplands, and willows, hackberry, and elm trees grow along the drainage ways.
Note the roof tops of a subdivision; this is what is happening along the IH-35 "growth corridor" between San Antonio and Austin. This suburbanization is taking place without regard for the constraints imposed by the bedrock, soils, and processes that occur within the fault zone. Problems include flooding, ground failure, pollution or depletion of surface water or groundwater supplies, septic tank failure, and the like.
Stop sign; turn right onto IH-35 access road.
Merge onto IH-35; proceed north. On the left are large quarries that extract stone from the Edwards Limestone. The quarries are of sufficient size that they are clearly visible on Landsat images (sensed from an altitude of approximately 500 miles).
Cross Guadalupe River; note cypress gallery along river.
Leave IH-35 via exit 191--Canyon Lake Road (FM 306).
Turn left onto FM 306; proceed northwest across high terraces of the Guadalupe River.
We are crossing the main Balcones fault line, which extends along the Missouri-Pacific Railroad tracks. We are traversing a mosaic of faulted slices of Edwards, Austin, Buda, and Del Rio Formations. The Escarpment is not expressed in this area owing to the gradual changes in rock type (and hence in erodibilities) across these small-displacement faults, which are part of a ramp structure such as that described by Grimshaw and Woodruff (this volume).
Note stand of mesquite trees--an indicator of clay substrate (probably Del Rio Clay).
Note scrub brush indicating shallow soils.
STOP 7--MEGA-COLLAPSE FEATURE
At this exposure there is a complete section of strata from the upper Edwards to the Buda. The Bat Cave fault runs along the north boundary of this outcrop, but the distortion seen here is not a result of tectonic faulting. Instead, the formations have collapsed into a mega-collapse feature (such as those described by Abbott, 1973). Note that the upper part of the Edwards Limestone is highly fractured and exhibits intensive microkarstification. Farther up the section, the rocks are less competent, and the amount of displacement is less. The result is that the Del Rio-Buda contact is expressed as a broad syncline, that is, a sag into the solution-collapse feature.
Proceed north on FM 306.
STOP 8--RELICT STREAM CHANNEL ON DIVIDE
Here we see a plano-convex channel cut into the Edwards Limestone. Channel-lag material is clearly evident, although the fine-grained fraction has been calichified. The deposit, as mapped by Bills (1957), extends for about one mile and is as much as 90 ft thick. Clearly, we have part of a major fluvial deposit situated on what is now a drainage divide.
This site is situated on the divide the between Guadalupe River and Purgatory Creek approximately 330 ft above the present course of Guadalupe River. It is the hypothesis of Woodruff and Abbott (this volume) that this material represents a relict channel of the Guadalupe River, before scarp-normal streams captured and diverted the river to its present course.
These relict alluvial deposits support a distinctive vegetative assemblage of post oaks and cedar elms, which contrasts with the prevailing live oak-juniper assemblage that usually dominates the limestone uplands.
Note terra rossa soil.
Bear left; stay on FM 306.
Note collapse feature in Edwards Limestone outcrop.
Here we are descending rapidly off the Edwards uplands onto the dissected Hill Country below. Bedrock underlying this dissected terrain is Glen Rose Limestone, which consists of a mosaic of lithofacies representing subtidal, intertidal, and supratidal environments. The alternating hard and soft strata of limestone, dolomite, and marl result in the "stair-step hills" characteristic of the Central Texas Hill Country. The basal member of the Glen Rose in this area consists of a massive rudist reef deposit. In that basal part of the section, karst features are common, and the Glen Rose behaves hydrologically similar to the Edwards. For most of the Glen Rose section, however, the limestone and dolomite beds act as small, discrete aquifers separated by marly aquitards. Many wells tap the Glen Rose across the Hill Country, but--except for the lower member (which is localized in geographic extent)--these wells produce water of erratic quality and quantity.
As we proceed across this dissected landscape, note that the north-facing hills support stands of Spanish oak (quercus texana); the southwest-facing slopes support more junipers . This is a function of soil microclimate, with the north-facing slopes maintaining cooler temperatures and more soil moisture. The greater insolation on the southern slopes produces a warmer, more arid microclimate that favors junipers with scattered live oak.
Cross Guadalupe River--the first of many times as we proceed down the "canyon" of the Guadalupe.
Turn left onto FM 2673; enter the town of Sattler.
Turn left onto River Road.
Cross the Guadalupe River. Note how the river valley narrows as we proceed downstream. The incised reaches are presumed to be a response to stream piracy. The relict channel viewed at Stop 8 may be a pre-piracy channel of Guadalupe River. For further discussion of this piracy hypothesis, see papers by Woodruff (1977) and Woodruff and Abbott (this volume).
Note the classic "cedar breaks" on the west-facing slope here. Note the dissected, broken, exposed soil. Junipers comprise approximately 70 percent of woody vegetation at this locality; most of the remainder consists of various oak species.
As we drive along the river, the mesic microclimate along the well-watered alluvium supports bald cypress, pecan, ash, box elder, willow, and sycamore. The tributaries have larger bald cypress trees than the main course of the Guadalupe River. This may be because of logging practices as well as periodic destruction of large trees during the occasional catastrophic floods (see Slade, this volume, and Baker, 1975). Clearly, the large trees seen here survived both floods and logging.
Note the sheer bluffs formed on Glen Rose Limestone. Cedar breaks extend up these dry, steep slopes (a xeric microclimate). Here, we have compressed vegetative assemblages, with the riparian woodlands juxtaposed with the xeric associations on the steep slopes. Farther upslope is the gently sloping terrain of the Edwards Plateau.
Cross the Guadalupe River; note the bald cypress community, all approximately the same age with a few survivors of past floods interspersed. Bald cypress tends to reproduce in stands of equal age. Large floods destroy large stands of adult trees, producing open areas along the floodways. Seedlings can then become established only if there are subsequent periods of low water (droughts even). By looking at the age of stands of trees, one might date major fluctuations in climate, from catastrophic floods to persistent dry periods, during which a "cohort" stand of trees is established.
Note bluff ahead. There, a complete section of local Cretaceous strata is exposed--Glen Rose Formation in the lower part; Walnut Formation in the middle; and Edwards Limestone at the top.
The upper alluvial terraces have deep, well-drained soils and support large stands of live oak trees. The telescoped plant assemblages that reflect underlying substrate are related to the following geomorphic features: plateau tops and rolling limestone uplands support live oak/juniper; steep slopes and bluffs support xeric assemblages (such as cedar breaks); high terraces support live oak savannah; colluvial slopes/fans support mixed hardwoods with local cedar breaks; and floodplain/channel areas support riparian woodlands.
Cross the Guadalupe River.
Cross the Guadalupe River again.
ROLLING STOP--HUECO SPRINGS (also spelled "Waco Springs," as by Bills )
Two springs issue forth from alluvial gravels along the floodplain of the Guadalupe River. The source of the groundwater is the Edwards Aquifer, and the water rises along the Hueco Springs fault. Maximum recorded discharge from these springs was 131 cfs in 1968 (Brune, 1975). The springs are fed by a limited catchment area, and they commonly stop flowing during dry periods. The springs were dry on 22 April 1986, when this road log was prepared.
The limited catchment area for these springs is illustrated by an observation of Bills (1957). An isolated rainfall during the drought of 1956 produced approximately 5 inches of rain near Smithson Valley, about 12 miles to the west. All surface runoff entered the aquifer where tributaries to Dry Comal Creek crossed the Bear Creek fault. Within 24 hours, Hueco Springs began discharging turbid water at a rate of about 2 cfs.
As we ascend from the river bottom onto the Edwards Limestone uplands, note the abrupt change to a xeric assemblage of plants. This indicates a warm, dry microclimate owing to thin soil cover, and well-drained bedrock.
Turn left at the stop sign. We are still traversing Edwards Limestone and typical uplands of the recharge zone.
Cross Blieders Creek.
Blieders Creek provides an excellent case study of an extraordinary rainfall event and the flooding that occurred as a consequence (this case study is abstracted from Baker, 1975). During the night of 11 May 1972, intense thunderstorms occurred along the Balcones Escarpment near New Braunfels. The center of this storm produced about 16 inches of rain within a 4-hour period, with approximately 75 percent of the rainfall occurring between 2040 and 2140 that night.
The most intense rain fell within the Blieders Creek basin, a 15-square-mile watershed, which is a tributary to Comal River, which in turn flows into the Guadalupe River. The resulting flood crest (gaged on the Comal River) rose 7.5 ft in 15 minutes and 30 ft in 1 hour 45 minutes. The flood that resulted on the Guadalupe River is estimated to have a recurrence interval of about 40 years. But this recurrence interval is computed assuming runoff from the entire 1,518-square-mile watershed. In fact, most of the runoff was generated within the 15-square-mile Blieders Creek basin. Canyon Dam was designed to protect residents along the downstream reaches of the Guadalupe River from floods of this magnitude, but the dam had no effect on this flood event because the runoff was entirely generated within the 86 square miles downstream from the dam.
Intersect Texas Highway 46 (Loop 337); turn left.
Cross Blieders Creek again.
We are crossing the main Balcones fault line and descending onto Quaternary terraces.
Cross the Guadalupe River.
The average discharge of this river, which drains approximately 1,518 square miles, was 372 cfs between 1929 and 1962 (before the completion of Canyon Dam). Maximum discharge was 101,000 cfs on 15 June 1935; the river ceased flowing several times during 1956 (U.S. Geological Survey, 1984).
Proceed east on alluvial terrace deposits.
Intersect IH-35; proceed under freeway.
Turn left onto north-bound access road.
Merge with IH-35; proceed north.
We are now crossing the drainage divide between the Guadalupe River and Blanco River. This flat-topped hill is capped with alluvial gravels cemented by caliche (abandoned gravel pits are visible from the highway). Such areas of inverted topography provide evidence for ongoing landscape evolution along the Balcones Escarpment, which is clearly visible on the left.
Cross York Creek. Here, we see a mixture of vegetative assemblages. South Texas brush species, which are limited in their northern extent by winter temperature, are seen here. This indicates that we are in a broad north-south ecotone, just as we have been crossing back and forth across a more abrupt east-west ecological boundary.
Take exit 204-A to San Marcos.
San Marcos, the county seat of Hays County, was first settled by Europeans when two Spanish missions were relocated here, after abandonment of sites established earlier in East Texas owing to difficulties with the Indians and the French. This site was chosen because of the perennial flow of the spring-fed San Marcos River. The town was organized by Anglo-Americans in 1851.
Turn left onto Loop 82.
Cross south-bound access road; proceed straight.
Bear right on LBJ Parkway.
Turn right on Cheatham Street.
At stop sign, turn left onto C.M. Allen Parkway.
Cross Purgatory Creek. The Old Main Building of Southwest Texas State University (SWTSU) occupies the crest of the Balcones Escarpment ahead. SWTSU is the alma mater of former U.S. President Lyndon B. Johnson.
Cross Hopkins Street; proceed straight.
Merge with Loop 82 again (now called University Drive); proceed straight.
As we enter the campus area, note the "waterscape" on both sides of the road. The San Marcos River is on the right, and a public park occupies the floodplain here. On the left is the SWTSU Drama Department theater. The ponds that grace the grounds are part of a now-discontinued Federal Fish Hatchery. The old hatchery grounds contain 26.6 acres with 12 ponds, 2 long raceways, an artesian well, and various laboratory facilities.
This waterscape owes its presence to the aquifer. The San Marcos River is sustained by discharge from San Marcos Springs, and the hatchery ponds are fed by an artesian well on the grounds of the Edwards Aquifer Research and Data Center, a research branch of SWTSU. This well has been the site of much of the zoological collecting that has allowed the characterization of fauna living within the aquifer waters (see Longley, this volume).
Intersection with Sessoms Street; the building on the left is the H.M. Freeman Aquatic Biology Building, which houses the Edwards Aquifer Research and Data Center.
Cross the San Marcos River; note the water discharging from the weirs on the left. This provides a good visualization of the flow from San Marcos Springs. Mean discharge from the springs is 161 cfs.
The upper San Marcos River contains an unusual assemblage of plants and animals. Many are extremely range-restricted, and some are endemic to this locality. They occur here because of the nutrient availability, constant water flow, and thermal stability of the spring discharge. These factors reflect the vast and intricate underground catchment area of the springs.
Examples of unique plants and animals include Texas wild rice (Zizania texana), and the San Marcos dwarf salamander (Eurycea nana). The river also contains rare finfish, caddisflies, and a giant fresh-water prawn, which can weigh as much as 3 pounds and attain a body length of 12 inches (with antennae extending twice this length). Nearby, from the waters of Ezell's Cave, is the world's only occurrence of the San Marcos blind salamander (Eurycea rathbuni).
Proceed straight; note good expression of the Balcones Escarpment on the left.
Pass Aquarena Springs entrance and bear left onto Post Road (immediately before the Missouri-Pacific Railroad tracks).
Turn left onto Lime Kiln Road. We are crossing the trace of the main scarp-forming fault, but the topographic break is not evident here, as we are traversing the alluvial valley of Sink Creek and Blanco River. This topography is a response to the step faults that compose the northeast block of the ramp structure described by Grimshaw and Woodruff (this volume).
Cross Sink Creek.
Note the abandoned lime kiln on left.
Cross Sink Creek again; note poor expression of its channel. During most rainfall events, flow will be consumed by recharge. But during heavy rains, these channels become the sites of dangerous torrents.
On right is a quarry, from which stone is extracted from the Edwards Limestone. Also exposed are Georgetown Limestone and Del Rio Clay.
Turn left onto Hays County Road 222. Note overgrazed range.
Turn left onto dirt road; proceed to recharge structure.
STOP 9--SINK CREEK RECHARGE STRUCTURE
This dam was constructed by the U.S. Soil Conservation Service (SCS), as one of a series of similar structures in Hays County designed to enhance recharge into the Edwards Aquifer. It rains about 2 percent of the time in the Austin area (Raymond Slade, oral comm., 1985). Most low-order Hill Country streams have very modest base flows and, where they cross the permeable Edwards Limestone, they are dry most of the time. Yet, as already pointed out in this volume, the Balcones Escarpment region has a history of recurrent catastrophic rainfall events and consequent floods. During periods of heavy rains, runoff occurs so rapidly that a relatively small fraction recharges the aquifer. Recharge dams such as the one seen here are designed to catch part of this peak flow. In so doing, more water is recharged, while at the same time a fraction of the surface runoff is detained and does not contribute to the downstream flood hydrograph.
Similar recharge structures are presently in place in the western part of the Edwards Aquifer recharge zone, in Uvalde County especially. Some of those structures use sinkholes as actual drains into the aquifer. Medina Lake, northwest of San Antonio, was designed as a hydroelectric power source, but a large fraction of its design storage volume infiltrates into the Edwards aquifer.
Presently, a major recharge dam is proposed on Onion Creek to sustain the discharge of Barton Springs in the face of increasing groundwater development in that segment of the aquifer. That proposed structure is designed not to lie directly over the Edwards Limestone but a short distance upstream from the recharge zone. The reservoir thus created would act as a buffer, storing water and meting out a continuous volume of flow that would be within the assimilative capacities of the recharging reaches of Onion Creek.
Turn around; proceed back to San Marcos.
Turn right onto Post Street.
Turn right onto Loop 82, and immediately turn right again onto grounds of Aquarena Springs.
Turn left and proceed to visitor's center.
STOP 10--SAN MARCOS SPRINGS
Five large fissures and several smaller orifices compose the San Marcos Springs system, second in volume of discharge only to Comal Springs among Texas springs. Spanish explorers located these springs in 1743. The springs provided water supply for the missions sited here and were an important stop on the Camino Real. Later, after settlement by Anglo-Americans, the springs continued to provide water and power for various uses and were a major stop along the Chisholm Trail (Brune, 1975). Today, the springs are the site of an amusement park and hotel.
These discharge points rise along the main Balcones fault line. These springs have never gone dry during historic times, but discharge decreased to a minimum of about 54 cfs in 1956 (William F. Guyton and Associates, 1979). Maximum recorded discharge was 300 cfs on 5 November 1973 (Brune, 1975). Studies by Ogden (this volume) show that the various spring orifices have distinctively different catchment areas. Some drain the area to the northeast near the Blanco River, whereas others draw on water far to the west.
The spring orifices are now inundated beneath the impounded Aquarena Lake. We will conclude the field trip with a ride on glass-bottom boats in order to view the springs.
Turn around; proceed back to Loop 82.
Turn left; cross Missouri-Pacific Railroad tracks, and proceed east to IH-35.
Turn right onto south-bound access road of IH-35.
Merge onto IH-35; proceed south.
Cross the San Marcos River.
Note the nice expression of the Balcones Escarpment on the right. There, Edwards is juxtaposed against Austin Chalk. The Blacklands in this area are so-called chalk prairies (mollisols) instead of being the true blacklands, which are vertisols formed on claystone substrate.
As we descend from the Guadalupe/Blanco drainage divide, note the excellent view of the Balcones Escarpment behind New Braunfels. As already mentioned, this high divide is underlain by alluvial gravels that are "on grade" with the locus of piracy seen at Stop 8.
Cross the Guadalupe River.
Cross the drainage divide between Cibolo Creek and Guadalupe River. On the right is the rapidly eroding headwater course of Dry Comal Creek, a possible future locus of stream capture.
Cross Cibolo Creek.
Pass beneath Loop 1604 overpass.
Longhorn Portland Cement quarry and plant are on the right; continue straight on IH-35 past Loop 410 interchange.
Cross Salado Creek.
Depart IH-35, proceed south on IH-37.
Leave IH-37 via Commerce Street exit.
Turn right onto Commerce Street.
Turn left; proceed to San Antonio Convention Center.
END OF TRIP.
Abbott, P.L., 1973, The Edwards Limestone in the Balcones Fault Zone, south-central Texas: The University of Texas at Austin, Ph.D. dissertation, 122 p.
Aandahl, A.R., 1972, Soils of the Great Plains: Map, copyright Andrew R. Aandahl, Lincoln, Nebr. scale 1:2,500,000.
Baker, V.R., 1975, Flood hazards along the Balcones Escarpment in Central Texas--alternative approaches to their recognition, mapping, and management: The University of Texas at Austin, Bureau of Economic Geology Geologic Circular 75-5, 22 P.
Bills, T.V., 1957, Geology of the Waco Springs Quadrangle, Comal County, Texas: The University of Texas (Austin), Master's thesis, 110 p.
Brune, Gunnar, 1975, Major and historical springs of Texas: Texas Water Development Board Report 189, 94 P.
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Kastning, E.H., 1978, Caves and karst hydrogeology of the southeastern Edwards Plateau, Texas: Guidebook, Geology Field Excursion, National Speleological Society Annual Convention, 46 p.
Meinzer, O.E., 1927, Large springs in the United States: U.S. Geological Survey Water-Supply Paper 557, 71 p.
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U.S. Geological Survey, 1984, Water Resources Data Texas, Water Year 1983: Water-data Report Tx-83-3, 451 p.
Woodruff, C.M., Jr., 1977, Stream piracy along the Balcones Escarpment, Central Texas: Journal of Geology, v. 85, no.4, p. 483-490.
Woodruff, C.M., Jr., 1984, Water budget analysis for the area contributing recharge to the Edwards aquifer, Barton Springs segment, in Woodruff, C.M., Jr., and Slade, R.M., Jr., coordinators, Hydrogeology of the Edwards aquifer--Barton Springs segment: Austin Geological Society Guidebook 6, 96 p.
Woodruff, C.M., Jr., and Abbott, P.L., 1979, Drainage-basin evolution and aquifer development in a karstic limestone terrain, south-central Texas, USA: Earth-Surface Processes, v. 4, no. 4, p. 319-334.
Woodruff, C.M., Jr., Dwyer, L.C., and Gever, C., 1982, Geothermal Resources of Texas: map prepared by the National Geophysical Data Center, National Oceanic and Atmospheric Administration, for the Geothermal and Hydropower Technologies Division, U.S. Department of Energy, scale 1:1,000,000.
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