MI-98 Catastrophic Glacial Outburst Floods on the Upper Arkansas River, Colorado


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Citation: Lee, Keenan. “MI-98 Catastrophic Glacial Outburst Floods on the Upper Arkansas River, Colorado.” Miscellaneous Investigation. Golden, CO: Colorado Geological Survey, November 8, 2019.


This publication from Mines Geology Professor Emeritus Keenan Lee is an in-depth exploration that illustrates one facet of the dramatic sweep of Colorado’s glacial past. With their high altitude, the central Rockies of Colorado saw numerous waves of intense glaciation, giving broad form to the mountains we see today. In particular, the Sawatch Range, with its Collegiate Peaks area to the west of the Upper Arkansas River valley was the site of glaciation several times in the last 700,000 years. Glacial terrains often feature dramatic ice- and land-forms. Lesser known are the occasional catastrophic events — also known as jökulhlaups — as are detailed in this report. Digital PDF download. MI-98D

From the Overview:

Three catastrophic floods swept the upper Arkansas River valley and deposited flood gravel at least as far downstream as the town of Salida. The oldest flood took place about 640 thousand years ago (ka), and the two younger floods were comparatively recent and closer together in time, about 130 ka and 19 ka.

These floods occurred during three clearly differentiated glaciations in this area. On the basis of the Rocky Mountain terminology of Richmond (1986), these glacial intervals were, listed oldest to youngest, as follows:

  • Sacagawea Ridge glaciation, Marine Isotope Stage (MIS) 16, about 675–621ka;
  • Bull Lake glaciation, MIS 6, about 190–130 ka; and
  • Pinedale glaciation, MIS 2, about 35–14 ka

The two younger floods originated as glacial outburst floods at the horseshoe-shaped narrows here called the “Damsite”, and the oldest flood likely had the same origin. The glaciers that dammed the Arkansas River flowed down the east flank of the Sawatch Range in the contiguous drainages of Clear Creek and Pine Creek. The glaciers advanced into steep granite walls on the east side of the Arkansas River valley, where the ice thickened and spread laterally, merging into a compound ice dam. The dammed Arkansas River formed a glacial lake, Three Glaciers Lake, which extended about 12 miles (mi.) upstream and was more than 600 feet (ft) deep at the dam. When the dam failed, the lake drained catastrophically. Floodwaters swept away the end moraines at the damsite and transported the debris down valley. Floodwaters spread out in the valley floor below the damsite, until they were funneled into and deepened in the narrow granite reach of Wild Horse Canyon. Bursting out of Wild Horse Canyon, floodwaters again spread out in the wider valley at the town of Buena Vista before being confined in Browns Canyon and spreading out again in the Sand Park area.

Each of the three outburst floods deposited sheets of unsorted flood gravel on the valley bottom. Little remains of the oldest flood gravel, flood gravel 1, except in one area just upstream of Buena Vista and in narrow valley-margin terraces, but the gravels extend as far downstream as Sand Park. Flood gravel 2, the most extensive flood gravel, is more-or-less continuous from the damsite south to Salida, except in two narrow canyons (Wild Horse and Browns) where it was swept downstream by Flood 3. Flood gravel 3, which is more restricted, lies just below the damsite and below the mouths of the two canyons.