Over the past several issues of this journal, we have presented evidence we’ve found of extensive, big floods in western Grand Canyon that resulted from the cataclysmic failure of at least five separate lava dams. In this, our final chapter on our studies of western Grand Canyon lava dams, we present to you our pièce de résistance. We have estimated the magnitude of one of the floods, the Hyaloclastite Dam flood, that occurred 165,000 years ago upon failure of the Hyaloclastite Dam, the remnants of which are found on river left at river mile 188.5. What we have called the Qfd4 deposits provide the evidence of the flood following this failure. The lifespan of the Hyaloclastite Dam could be as little as one and a half years, assuming that the dam failed through piping (i.e. “leakage”) before the impounded river could spill over the top of the dam. Assuming the maximum height of this dam was about 980 feet, the reservoir created by this dam would have held 390 billion cubic feet of water. Using the average discharge of 18,500 cfs for the Colorado River over the past 450 years (Stockton and Jacoby, 1976), this reservoir could have filled in approximately 35 weeks.
Using one-dimensional hydraulic modeling, we calculated the height and discharge of the flood wave that left behind the Qfd4 deposits between river mile 189 and 209 (Figure 1; Fenton et al., in preparation). Our modeling indicates that the reservoir would have drained in less than 35 hours. This “rapid release” produced a flood with a maximum peak discharge ranging from nine to fourteen million cfs (Figure 1). This range in discharge results in part from uncertainties in channel geometry and degradation of the high-water marks (the Qfd4 deposits), which, after all, have been present for 165,000 years. We produced two flood waves whose elevations bracket the elevations of the Qfd4 deposits. It is possible that the maximum elevations preserved in flood deposits at river mile 189.3 and 193.5 might be achieved by failure of a 685-foot dam if we assume that debris from the failed lava dam raised river level immediately downstream from the dam, but no evidence remains to evaluate that assumption. A minimum steady flow of 4.2 million cfs is required to match the water-surface elevation of Hyaloclastite Dam outburst-flood deposits at river mile 204–205, seventeen miles downstream of the dam failure, where the Hyaloclastite Dam flood waters approached steady, uniform flow.
We don’t know exactly how long the Hyaloclastite Dam flood lasted, but dam-break floods typically have short duration times. The failure of Teton Dam (Idaho) in 1976 drained a reservoir holding ten billion cubic feet of water in less than ten hours (Fread et al., 1998). Likewise, Russell Lake, a modern lake formed by a glacial dam in Alaska, released 1.1 trillion cubic feet of water over 36 hours following a dam failure in August 2002 (Trabant et al., 2003).
The peak discharge of the Hyaloclastite Dam flood dwarfs all known Holocene and historic floods produced by seasonal and weather conditions in the Colorado River basin (Figure 2). Our upper- and lower-bound discharge estimates of nine to sixteen million cfs are ten to a hundred times larger than both the largest Holocene runoff flood (500,000 cfs; O’Connor et al., 1994) for the Colorado River. The Hyaloclastite Dam flood was anywhere from ninety to three hundred times bigger than the 1996 flood in Grand Canyon (45,900 cfs), fifty to seventy times bigger than the 1884 flood (300,000 cfs, usgs website; 220,000 cfs; Topping et al., 2003). The Hyaloclastite Dam was basically equivalent to sending the 1964 Amazon River flood through the Grand Canyon, or thirty of the 1993 Mississippi River floods through the Grand Canyon at once.
When people think of floods, they normally think of floods brought on by changes in the weather, such as the onset of spring runoff, or flash floods brought on by summer monsoons. Most big floods in the historical record are these types of floods, but the largest floods that have ever occurred in Earth history resulted from the rapid release of stored water during high runoff events and dam failures (O’Connor et al., 2002). For most rivers, the peak discharge of a dam-break flood is usually much larger than runoff floods (Fread et al., 1998).
Worldwide, the largest floods have been caused by the failure of ice dams, whereas comparatively few have been attributed to failure of lava dams (O’Connor et al., 2002; Walder and Costa, 1996; Baker and Nummedal, 1978; Jackon et al., 2001). All known floods with discharges greater than eighteen million cfs resulted from the rapid release of water stored behind natural dams or within glaciers (O’Connor et al., 2002). The best-known example of these is the Spokane Flood (six- hundred million cfs) in the Pacific Northwest, which resulted from the failure of a glacial-ice dam (Baker and Nummedal, 1978). Outburst floods also result from rapid melting of glaciers, failure of landslide dams, lake overflows or breaches. For example, overflow of Lake Bonneville initiated a 35 million cfs flood on the Snake River in Idaho. The 1976 failure of the Teton Dam in Idaho produced the largest flood (2.3 million cfs) resulting from the failure of a human-made dam. In comparison, the Hyaloclastite Dam flood in Grand Canyon had a higher peak discharge than the 1976 Teton Dam failure, is the third-largest flood known in the continental United States (Figure 2) and is possibly the fourrth-largest flood known worldwide (O’Connor et al., 2002). Now, that’s some big water!
The Colorado River and lava from the Uinkaret volcanic field had many explosive, conflicting, and complicated interactions over the past 630,000 years, and those lava dams did little to stop the flowing of the river (Hamblin, 1994; Lucchitta et al., 2000; Fenton et al., 2001; 2002; 2004; Pederson et al., 2002). In the end, the river “won,” eventually removing the lava plugging its downstream progress.
Cassie Fenton and Bob Webb
We would like to take this opportunity to thank all of the many river guides who helped us over the years during our research trips, namely, Kirk Burnett, Tillie Klearman, Sam Walton, and many others. Thanks for getting us safely from points A to B and making it fun all the while! And thanks for helping us with field work and contributing your seemingly endless knowledge of the Big Ditch.
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Fenton, C.R., Poreda, R.J., Nash, B.P., Webb, R.H., and Cerling, T.E., 2004, Geochemical discrimination of five Pleistocene lava-dam outburst-flood deposits, western Grand Canyon, AZ, Journal of Geology, v. 112, p. 91–110.
Fenton, C.R., Webb, R.H., Cerling, T.E., in preparation, Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA.
Fenton, C.R., Webb, R.H., Cerling, T.E., Poreda, R.J., and Nash, B.P., 2002, Cosmogenic 3He Ages and Geochemical Discrimination of Lava-Dam Out-burst-Flood Deposits in Western Grand Canyon, Arizona, in House, K. et al., eds., Paleoflood Hydrology, American Geophysical Union, p. 191–215.
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Trabant, D.C., March, R.S., and Thomas, D.S., 2003, Hubbard Glacier, Alaska: Growing and advancing in spite of global climate change and the 1986 and 2002 Russell Lake outburst floods: U.S. Geological Survey Fact Sheet 001-03, 4 p.
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