Monday, September 20, 2010

One flood. Three days -> One canyon. We often get glimpses into the overwhelming power of nature. One such was in June-July 2002 in Texas: Fearing the worst during torrential rains, the waters of Canyon Lake (a reservoir on the Guadalupe River) were diverted into an emergency spillway. The consequential flood event, with a flow rate of over 1450 cubic metres per second and duration of six weeks, destroyed a bridge and trees, and carved a 2.2 km long, 7 m (average) deep canyon into the limestone bedrock. This new Canyon Lake Gorge is highlighted by Michael Lamb (Caltech) and Mark Fonstad (Texas State University) in their paper published in Nature Geosciences on the ‘Rapid formation of a modern bedrock canyon by a single flood event’.

Deep river canyons are considered to have formed gradually over the millions of years as a result of moderate and recurring flow of water. Megaflood events too results in similar canyons and channels, except that these are formed more rapidly, as in the case of the Canyon Lake Gorge. In this event, it is undeniably known that the erosion happened during the flood and the more destructive episode happened over a time span of just three days.

Using pre and post data and techniques (aerial photographs/images, models, flood measurements) Lamb and Fonstad analysed the formation of the canyon, identified the characteristics, and reconstructed the hydraulics of the flood. The important findings are:

- The rate of gorge erosion/formation was rapid, limited only by the amount of sediments carried by the floods. About 460,000 cubic metres of materials were dislodged, almost half of which were rocks. Even boulders with a size of 1m were plucked and moved downstream.

- The very creation of the canyon was dependent on the nature/characteristics of the bedrock. For instance, the aforementioned plucking of the 1m limestone boulders was facilitated by the fractures (horizontal bedding planes and vertical cracks probably due to tectonic movements) which were already present in the layered limestone bedrock.

- The rock type determined the resultant morphology of the canyon. The first part features 10-12m high waterfalls, channels, and terraces due to the plucking of the boulders. The latter part features sediment islands which are considered to be characteristic of large flood events.

Caveat: Is this a really megaflood (floods with a flow of1 million cubic metres per second) per se?

Lamb, M., & Fonstad, M. (2010). Rapid formation of a modern bedrock canyon by a single flood event Nature Geoscience, 3 (7), 477-481 DOI: 10.1038/ngeo894

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