ROCK OUT: A severe Texas flood in 2002 pulled meter-size boulders from the ground and carried them away to etch a canyon several meters deep in a matter of days. Image: COURTESY OF MICHAEL LAMB/CALTECH
A geologic scar left by a catastrophic Texas flood in 2002 is providing an unexpected scientific benefit. A new study demonstrates how researchers can use a channel carved by floodwaters pouring over the dam of a flooded reservoir as a laboratory to test scientific theories of how such canyons are formed. The research could help to inform the hydrological histories of Earth and Mars by indicating the kind of imprints large, sudden floods leave on a planet's surface.
The storm that struck central Texas eight years ago wreaked havoc on the region, which President George W. Bush declared a disaster area. Floodwaters killed 12 people and damaged 48,000 homes in dozens of counties, according to a report from the U.S. Geological Survey (USGS). At Canyon Lake, a reservoir north of San Antonio, water rushed over the dam's spillway, pouring into the valley below. Within days a 50-meter-wide channel now known as the Canyon Lake Gorge had been carved into the soil and bedrock, drastically transforming the landscape on a short timescale.
Thanks to the recent nature of the event, along with USGS monitoring, extensive topographic information and eyewitness reports, researchers can explore the geologic aftermath of a large flood whose discharge and duration are well constrained. That is a rare if not unprecedented opportunity, says California Institute of Technology geologist Michael Lamb, a co-author of the study published online June 20 by Nature Geoscience describing the gorge's formation. (Scientific American is part of Nature Publishing Group.)
He became interested in the gorge while studying canyon formation on Earth and Mars during a 2008–2009 postdoctoral fellowship at the University of Texas at Austin, about 100 kilometers away. Along with his colleague Mark Fonstad, a geographer at Texas State University–San Marcos, Lamb found that the landscape below Canyon Lake had been remodeled in just three days or so, during which hundreds of thousands of cubic meters of rock and sediment were flushed downstream.
Mars's surface is dotted with deltas, floodplains and gullies that indicate a rich hydrological history for the Red Planet, even though conditions today preclude the appearance of liquid water there. Some researchers believe that conditions were once much different and that Mars could have once had vast oceans of standing water, which would bode well for the past development of life on the planet, but others have come to believe that the water activity was confined to short-lived bursts of rainfall or catastrophic flooding.
Most large floods on Earth, Lamb notes, occur in rivers or other areas where water has been carving its way through the land over long timescales. The uniqueness of the Texas site is that the valley below the dam had historically been essentially dry land, making the changes attributable to the 2002 flood plainly visible. Victor Baker, a hydrologist, planetary scientist and geologist at the University of Arizona in Tucson, says that even with the ubiquity of sensing and imaging data, and despite the heavy rains endemic to Texas, it may be a while before researchers are able to observe the fresh carving of another brand-new channel.
With the amount of flood information available, Lamb says, Canyon Lake Gorge can provide a field experiment to test theories of how water flows form channels and canyons. That would come in handy for a place like Mars, where ancient channels are all that remain; planetary scientists must infer the Red Planet's hydrologic history from the way water shaped the surface. "It's important to reaffirm these reconstructions to understand the geologic history of Earth as well as Mars," Lamb says.
The 2002 Texas flood was powerful, plucking meter-size limestone boulders out of the bedrock and carrying them away to leave a channel that in places exceeds 12 meters in depth. It still does not hold a candle to the ancient megafloods that shaped Earth and Mars, Baker notes, but it is closer than most lab research comes to replicating those conditions. "An experiment like this one, which is intermediate between what we can do in a lab and what's on Mars, is relevant because it shows us how these physical principles interact to create outcomes that we can see," he says.