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(Credit: Comal County, Texas)
In the summer of 2002, a week of heavy rains in Central Texas caused Canyon Lake -- the reservoir of the Canyon Dam -- to flood over its spillway and down the Guadalupe River Valley in a planned diversion to save the dam from catastrophic failure. The flood, which continued for six weeks, stripped the valley of mesquite, oak trees, and soil; destroyed a bridge; and plucked meter-wide boulders from the ground. And, in a remarkable demonstration of the power of raging waters, the flood excavated a 2.2-kilometer-long, 7-meter-deep canyon in the bedrock.
According to a new analysis of the flood and its aftermath -- performed by Michael Lamb, assistant professor of geology at the California Institute of Technology (Caltech), and Mark Fonstad of Texas State University -- the canyon formed in just three days.
A paper about the research appears in the June 20 advance online edition of the journal Nature Geoscience.
Our traditional view of deep river canyons, such as the Grand Canyon, is that they are carved slowly, as the regular flow and occasionally moderate rushing of rivers erodes rock over periods of millions of years. Such is not always the case, however. "We know that some big canyons have been cut by large catastrophic flood events during Earth's history," Lamb says.
Unfortunately, these catastrophic megafloods -- which also may have chiseled out spectacular canyons on Mars -- generally leave few telltale signs to distinguish them from slower events. "There are very few modern examples of megafloods," Lamb says, "and these events are not normally witnessed, so the process by which such erosion happens is not well understood." Nevertheless, he adds, "the evidence that is left behind, like boulders and streamlined sediment islands, suggests the presence of fast water" -- although it reveals nothing about the time frame over which the water flowed.
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