June 2008

Birth of a Gorge

The floods of 2002 unearthed a geologic wonder in Central Texas.

By E. Dan Klepper

Once the clouds opened up above the plateaus and canyons of the central Texas Hill Country, the rain began to fall in unrelenting torrents. The deluge swiftly overwhelmed the voids and pockets of the Hill Country's subterranean limestone, filling the organic maze of crevices, cracks and tunnels until the groundwater rose to the surface like an angry sprite, once bone dead and dry, now awake, alive and spitting up mad.

Creeks and arroyos already saturated by early summer storms roiled in the excess. The rain continued unabated, its clatter on tin roofs rising to a ting-ting-ting before the clouds unleashed full bore, shattering the steady rhythm with a jackhammer pounding. The flotsam of rocky draws – bits of tortoise shell, sun-bleached mandibles, drift logs warted with bug pits and pecker holes – swirled around in the floodwaters before being flushed downstream. Intermittent streambeds running scattershot across the cattle country swelled up until their fern-laden grottos, muddy burrows, fishy underbanks and lastly the banks themselves completely disappeared beneath a wave of turgid flow. The rain refused to stop, sopping the meager topsoil and pushing debris across the ranchland shallows, hanging the duff and detritus against the barbed-wire fencerows. Rain fell against rain day after day, rumbling down the terra-formed watershed until it merged with the swollen Guadalupe River, the riparian tumbleway that drains over 1,146 square miles of the Hill Country heartland.

The Guadalupe transformed is a rain-besotted steamroller born from a placid unraveling of emerald-tinted riffles. It cartwheels across the landscape, upending the shallow-rooted junipers and oak mottes that line its floodbanks, swamping anything in its path. The river corridor makes swift drops of 11 feet every mile before it reaches the Balcones Escarpment, a geological demarcation separating the Hill Country's Edwards Plateau from the Blackland Prairies. Here, the landscape's declination eases to 6 feet per mile, slowing the flow of water with it where, during past episodes of abundant rainfall, the river routinely inundated the countryside in floodwaters. But now, just as the river water leaves the limestone plateau to cross the rolling plains and prairies that will carry it all the way to the Gulf of Mexico, the surge gathers up instead against a massive earthen dam.

Canyon Dam was built to hold back an average of 378,852 acre-feet of Guadalupe River water where the contents are supposed to rest, becalmed, at 909 feet above sea level before they are released, slowly and safely, downstream. So on July 1, 2002, after three days of rain across the Upper Guadalupe River Basin, Canyon Dam's reservoir waters began to rise. Constructed to provide flood control and water conservation, the Canyon Dam and Reservoir Project was designed to mitigate the damage from flooding events that once plagued the Lower Guadalupe River Basin. The project has successfully brought relief to the communities downstream since its completion in the 1960s.

But by 4 p.m. on July 1, reservoir levels had risen 5 feet. Previous rain events had already tested the flood control system, and a mere 5-foot rise was not a particular cause for alarm. However, Canyon Lake was busy. It is a recreational parkland favored by campers, anglers and boaters, and by July 1 the Independence Day holidays had just begun. With more rain in the forecast, authorities made the decision to evacuate the lake's lower campsites. The rain, meanwhile, continued to fall across the Upper Guadalupe River Basin. Some areas reported soaking up as much as 16 inches of rain in one day. Forty-eight hours later, on July 3, reservoir levels had risen to just 10 feet below the spillway. All of the lake's picnic and camping areas were closed and emergency personnel were alerted to prepare for an evacuation of downstream homes. They were also told to anticipate what is known as a "spill."

Canyon Dam rises to 974 feet above sea level, a height established based on studies resulting from flood events of the early 1900s. It is an earthen dam, creating a relatively secure impoundment as long as water neither breaches the top nor undercuts the base. To ensure this doesn't happen, a 10-foot release tunnel below the dam allows up to a maximum of 5,000 cubic feet per second (cfs), or 2,244,000 gallons per minute, to be released downstream. The Canyon Dam tunnel typically releases a constant 300 to 500 cfs downriver, providing an ideal current for tubing, trout fishing and canoeing.

In addition, a spillway was created alongside the dam. The constructed section of the spillway, a swath of excavated limestone more than 1,000 feet wide, parallels the reservoir's shoreline just south of the dam and forms a broad, flat slab that stops abruptly where it joins a natural declination in the landscape. The spillway route continues naturally from here, following the downward elevation of a small wooded valley for about a mile and a half before it crosses under the South Access Road. The spillway route continues for another mile beyond this road as a wider, alluvial drainage before it finally ends at its confluence with the Guadalupe River below the dam. This spillway allows lake water to "spill" out of the reservoir should water levels exceed 943 feet above sea level, a difference of 31 feet between the spillway and the top of the dam. In the entire course of the dam's 40-year history there had never been a spill.

As the upper regions of the Guadalupe River basin continued to rain and drain, the reservoir level reached 938 feet in the early morning hours of July 4, just 5 feet below the spillway. Officials proceeded to evacuate homes and businesses along the River Road, a scenic, winding stretch of pavement that follows the Guadalupe River as it continues be-yond the dam. By 3:30 p.m. the reservoir water levels exceeded 943 feet and the spillway began to perform just as it had been designed to do. Water inundated its limestone slab, ran the 2.5-mile course down the natural spillway and joined the Guadalupe River below. The skies, however, weren't as cooperative and proceeded to release another 5 inches of rain. It was the catalyst that elevated an already damaging storm event to a catastrophe.

Reservoir levels continued to rise, exceeding 950 feet, while the spillway did its best to swallow 66,800 cubic feet of water per second. The sheer force and volume of water scoured the spillway route, carting off and dumping 300,000 cubic yards of rock and soil 2.5 miles down to the spillway's confluence with the Guadalupe River. The excess material then disrupted the spillway flow as well as the steady release of water from the tunnel beneath the dam, causing much of the river to flood its banks and creating severe flood damage downstream. Fortunately the area had been evacuated and there was no loss of life. The spillway, however, continued to flow.

Water spilled from the lake for another day, then another, until the spill finally ended an astounding six weeks later. It was estimated that the final amount of water that flowed over the spillway would have filled Canyon Lake no less than twice. It was an incredible statistic considering it took four years, between the start of impoundment in late 1963 and the point at which the reservoir reached 909 feet in 1968, to fill it just once. But while the extensive flood damage was regrettable, the record rain event of 2002 also resulted in one of the most remarkable and unexpected consequences – the formation of a pristine, whistle-clean limestone canyon christened, appropriately, the Canyon Lake Gorge.

The Canyon Lake Gorge is an ideal study in the phenomenal forces of nature that were at work over the six-week spill. The hydrodynamics in action during its creation were formidable, shoving boulders aside by the ton and removing foundation-size slabs of limestone as easily as if peeling onion skin. For almost a mile and a half, from the edge of the concrete spillway construction down to the spillway's crossing beneath South Access Road, the surge of floodwaters blasted away the earth's superficial building blocks to a depth of 40 feet, scouring the spillway valley until what remained laid bare the story of the earth itself, revealing a geological history tens of millions of years in the making.

The gorge represents a descent through time, beginning with its terraced layering of ancient mud flats, once covered in soil and vegetation, now wiped clean and exposed. These are silent shorelines trampled by the likes of the carnivorous Acrocanthosaurus, a mini Tyrannosaurus rex around 30 feet long that walked on its hind legs, and a massive sauropod - the Pleurocoelus (the official dinosaur of Texas). As these creatures crossed the hot, sloppy flats, their feet sucked up the mud like suction cups, leaving perfectly defined tracks that baked and dried in the sun. At different points in time other layers of sediment inundated the mud flats, sealing the hardened track layer beneath them for eons. Then, during the spillway's scouring, the creatures' footprints were uncovered when the layers above them simply eroded away.

Below the tracks the gorge begins to unravel a far more compelling account of the region's geology. The gorge's location, by chance on the northwestern edge of the Balcones Fault Zone, reveals fracturing that occurred during a more seismically violent time. Once hidden beneath layers of soil and vegetation, this evidence of activity is apparent in all kinds of newly exposed sedimentation layers. As the floodwater's hydrology worked against the layers, much of the soft layer beneath the rocks eroded, causing them to break apart. But as the force of the spill cut deep into the surface and carried away layer upon layer, it also provided a way to view how these layers are structured. The flow created a visual cross-section of the land beneath our feet, much like the way the sides of our road cuts along the highway illustrate the internal structure of the hills they carve through. Some of the rock layers are extremely porous and riddled with voids. Other layers are thick and hard; some are soft and crumbly. Still others are studded with geodes.

Most remarkable of all is the diversity of fossils uncovered in the water's excavating action. Each of the gorge's sedimentary layers has its own unique set of characteristics and correspondingly contains different fossils, some quite revealing, that reflect its specific geologic time. Giant gastropods, algae, clams, snails and tiny shells are em-bedded in rock layers throughout the gorge. In fact, an entire fossil ridge that demarcates the Upper Glen Rose and the Lower Glen Rose can clearly be seen while descending the gorge. Re-markable as well is the clearcut evidence of the Hidden Valley Fault, a fracture that geologists were certain existed beneath the surface but were never able to examine directly until the creation of the gorge.

Above and beyond the myriad fossils or the crush and expansion of seismic movement, the Canyon Lake Gorge reveals something far more fundamental. For Texans who depend upon groundwater and its natural system of capture, filtration, storage and release, the gorge is an opportunity to understand this system in a way that perhaps has never been made available before.

Water is still present in the gorge, continuing its descent in a routine manner rather than an overwhelming one. Groundwater is flowing into the canyon along fractures in the harder layers and is seeping from the walls where fractured, permeable layers crop out. Groundwater moves down through the permeable layers until it comes to impermeable layers, which commonly are the softer, marly layers. Then it flows laterally at the base of the permeable layers. Here it slowly works across the broad rock slab in a cross-hatch of channels before disappearing into gravel beds or crevices. It reappears again to collect in translucent pools or gurgle along rivets of stone, an intricate subterranean network puzzling itself out before the eyes in the clear light of day. It is an amazing illustration of the state's precious karst aquifer system and an experience that every Hill Country Texan who draws a glass of tap water should take time to see.

Time, it appears, is the gorge's nemesis as well as its vainglory. Nature abhors stasis, constantly soldiering her forces forward in a never-ending process of transformation. The gorge was created in a brief, apocalyptic period of six weeks. Gentler times, but those with equal force, have also begun to recreate it, drawing soil into the crevices, catching and sprouting seeds, spreading roots, and slowly sealing the limestone and all its secrets under a layer of green.

It is a cycle even more ancient than the geology revealed and illustrates the profound but simple truth that nothing on this bright blue planet ever quite remains the same.

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