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Water Science and Applications

 

Keywords

  • Paleohydrology
  • Floods

Index Terms

  • 1821 Hydrology: Floods
  • 1824 Hydrology: Geomorphology
  • 1827 Hydrology: Glaciology

Article

WATER SCIENCE AND APPLICATION, VOL. 5, PP. 217-235, 2002

Evaluation of glacial outburst flood hypothesis for the Big Lost River, Idaho

K. L. Knudsen, J. M. Sowers, D. A. Ostenaa, and D. R. Levish

Geomorphic evidence along the Big Lost River in Idaho suggests that a catastrophic flood likely occurred earlier than previously thought. Previous researchers described flood-related features in the Box Canyon area and suggested that a large flood might have originated about 20 to 30 ka from Glacial Lake East Fork, about 100 km upstream of Box Canyon. Geomorphic mapping of glacial till and outwash terraces near the location of Pleistocene Glacial Lake East Fork reveals problems with this model and raises new questions about the viability of 20 to 30 ka glacial outburst flooding as the mechanism responsible for forming the features in Box Canyon. Smooth low hills of older till downstream from the former lake show no evidence of overtopping or scouring. Outwash terraces estimated to be 20 to 30 ka also show no evidence of having been overtopped by a large flood. The incised valley between these terraces has insufficient channel capacity to pass a large flood. Between the glaciated headwaters and Box Canyon, geomorphic features suggestive of outburst flooding are few and most are explicable by other processes. A 5-m granitic boulder on an outwash terrace about 15 km downstream of Glacial Lake East Fork may have been ice rafted in an earlier outburst flood. Limestone walls of narrow canyons, through which the Big Lost River flows, show no evidence of abrasion, and their abundant ledges and hollows contain no fluvial sediment or exotic clasts. Scarps cut into the toes of middle to late Pleistocene alluvial fans near Mackay could have been produced by a large flood or by a laterally migrating stream. Upstream of Box Canyon, near Arco, fluvial terraces estimated to be at least 50 ka appear not to have been overtopped since their formation, providing a minimum age for large floods on this reach of the Big Lost River. Downstream of Box Canyon, soil development on alluvium that should have been modified substantially or may have been deposited by an outburst flood is similar to or greater than soil development on late Pleistocene terrace surfaces near Arco, suggesting a minimum age of 50 to 90 ka for any large floods. Thus, the features near Box Canyon must be significantly older than previously suggested.

Citation: Knudsen, K. L., J. M. Sowers, D. A. Ostenaa, and D. R. Levish (2002), Evaluation of glacial outburst flood hypothesis for the Big Lost River, Idaho, in Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology, Water Sci. Appl., vol. 5, edited by P. K. House et al., pp. 217–235, AGU, Washington, D. C., doi:10.1029/WS005p0217.

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