 Glacial Lake Columbia (west) and Glacial Lake Missoula (east) are shown south of the Cordilleran ice sheet. The areas inundated in the Columbia and Missoula floods are shown in red. | |
| Cause | Ice dam ruptures |
|---|---|
| Meteorological history | |
| Duration | Between 15,000 and 13,000 years ago |
| Flood | |
| Overall effects | |
| Areas affected | The current states of: Idaho, Washington, and Oregon |
 | This article needs additional citations for verification. (August 2024) |
The Missoula floods (also known as the Spokane floods, the Bretz floods, or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. These floods were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the lake drained, the ice would reform, creating Glacial Lake Missoula again.
These floods have been researched since the 1920s. During the last deglaciation that followed the end of the Last Glacial Maximum, geologists estimate that a cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred several times over the 2,000 years between 15,000 and 13,000 years ago. U.S. Geological Survey hydrologist Jim O'Connor and Spain's Museo Nacional de Ciencias Naturales scientist Gerardo Benito have found evidence of at least twenty-five massive floods, the largest discharging about 10 cubic kilometers per hour (2.7 million m³/s, 13 times that of the Amazon River).[1][2] Alternate estimates for the peak flow rate of the largest flood range up to 17 cubic kilometers per hour.[3] The maximum flow speed approached 36 meters/second (130 km/h or 80 mph).[3]
Within the Columbia River drainage basin, detailed investigation of the Missoula floods' glaciofluvial deposits, informally known as the Hanford formation, has documented the presence of Middle and Early Pleistocene Missoula flood deposits within the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and the Walla Walla Valley. Based on the presence of multiple interglacial calcretes interbedded with flood deposits, magnetostratigraphy, optically stimulated luminescence dating, and unconformity truncated clastic dikes, it has been estimated that the oldest of the Pleistocene Missoula floods happened before 1.5 million years ago. Because of the fragmentary nature of older glaciofluvial deposits, which have been largely removed by subsequent Missoula floods, within the Hanford formation, the exact number of older Missoula floods, which are known as ancient cataclysmic floods, that occurred during the Pleistocene cannot be estimated with any confidence.[4][5]
- ^ "Science writer Richard Hill gives a brief geologic history of the Columbia River Gorge". The Oregonian. Archived from the original on 2008-08-04. Retrieved 2008-06-15.
- ^ Lehnigk, KE; Larsen, IJ (2022). "Pleistocene megaflood discharge in Grand Coulee, Channeled Scabland, USA". Journal of Geophysical Research: Earth Surface. 127 (1): e2021JF006135. Bibcode:2022JGRF..12706135L. doi:10.1029/2021JF006135. S2CID 245545657.
- ^ a b Bjornstad, Bruce N. (c. 2006). On the trail of the Ice Age floods: a geological field guide to the mid-Columbia basin / Bruce Bjornstad. Sandpoint, Idaho: Keokee Books. p. 2. ISBN 978-1-879628-27-4.
- ^ Medley, E. (2012) Ancient Cataclysmic Floods in the Pacific Northwest: Ancestors to the Missoula Floods. Unpublished Masters thesis, Portland State University, Portland, Oregon. 174 pp.
- ^ Spencer, P. K., and M. A. Jaffee (2002) Pre-Late Wisconsinan Glacial Outburst Floods in Southeastern Washington—The Indirect Record. Washington Geology. vol. 30, no. 1/2, pp. 9–16.