To determine the rock avalanche\u2019s volume of 286 million cubic meters, Moore and colleagues used a computer and \u201cour best geological judgment to recreate what the canyon looked like before the slide.\u201d In a similar way, they then reconstructed the top surface of the landslide before it began to erode.<\/p>\n

The landslide dammed the Virgin River, and \u201cthere was a lake in Zion Canyon for approximately 700 years,\u201d Moore says.<\/p>\n

He calculates that if the river\u2019s discharge was similar to today\u2019s, it would have taken five to 10 years for the lake to fill. The long, narrow lake covered 2.4 square miles and extended from the north end of the rock avalanche deposit \u2013 which is less than a mile south of present-day Zion Lodge \u2013 northward almost to The Narrows. With the initial lake surface at 4,658 feet elevation, the site of the lodge was under 380 feet of water.<\/p>\n

Then the lake breached the top of the landslide dam, the lake surface fell to 4,413 feet in elevation and the lake\u2019s area shrank to about 1.2 square miles, no longer extending to The Narrows but only 4.3 miles to the Temple of Sinawava.<\/p>\n

Sediment amounts produced today by a nearby fork of the Virgin River suggest the lake filled with sediment in 700 years, give or take a century. By about 4,100 years ago, the lake had completely filled with sediment, forming the flat floor of Zion Canyon.<\/p>\n

Moore calculates the Virgin River has eroded away about 45 percent of the original landslide deposit during the past 4,800 years. At that rate, in several thousand years Zion \u201cwill again be a steep, rocky, narrow canyon,\u201d he says.<\/p>\n

So Moore calls the landslide \u201ca minute with up to 10,000 years of consequences.\u201d<\/p>\n

Moore notes that large landslides \u201ccreate habitable land in otherwise steep landscapes around the world. People use flatter areas behind large landslide deposits for farming or to build a village\u201d \u2013 although they sometimes are at risk from future slides.<\/p>\n

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Funding and co-authors<\/strong><\/p>\n

The study was funded partly by the David and Inga Chapman Fund, established by students of longtime University of Utah geophysicist David Chapman and his wife.<\/p>\n

Moore conducted the study with Jessica Castleton, who earned her master\u2019s degree at the University of Utah last year and works for the Utah Geological Survey; Jordan Aaron, who wrote the computer code to simulate the landslide\u2019s distance and speed and who is a geological engineering doctoral student at the University of British Columbia; and Marcus Christl and Susan Ivy-Ochs, rock-dating experts at ETH Zurich, the Swiss Federal Institute of Technology.<\/p>\n

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