Bonneville flood

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Red Rock Pass in Bannock County , Idaho, site of the Lake Bonneville Dam breach

The Bonneville Flood was a catastrophic flood event during the last Ice Age about 14,500 years ago. The natural dam of prehistoric Lake Bonneville , a forerunner of today's Great Salt Lake in Utah , broke at Red Rock Pass in southeast Idaho . Large amounts of water flooded parts of southern Idaho and eastern Washington when the dam broke . The tide penetrated the Snake River and inundated the banks, flushing deep channels, leaving extensive high sand and gravel ripples and boulders. The tidal wave lasted for several weeks and reached a height of about 125 meters at the site of today's Brownlee Dam in Hells Canyon near Homestead in Oregon . The Bonneville Flood is one of the greatest catastrophic floods in known geological history.

Research history

Grove Karl Gilbert (1891)

Grove Karl Gilbert , an American geologist, examined Lake Bonneville from 1872. He named the lake after Benjamin Bonneville (1796–1878), a United States Army officer and fur hunter who, in the 1830s, included the Great Basin , the Snake River along Hells Canyon, the Wallowa Mountains and the Upper Bear area River had explored.

According to Gilbert, the inflow of the Bear River caused the overflow of Lake Bonneville at the Red Rock Pass. The alluvial cone consisted of poorly consolidated, heavily weathered rubble and mud . The overflow took away about 114 meters of rubble before it hit bedrock on which the lake with the so-called Provo shore rested. Gilbert estimated that about 2,000 cubic kilometers of water leaked out of the lake, and he estimated the subsidence lasted about 25 years.

The lake, about 250 kilometers wide, was located in the Basin and Range Province (German: basin and mountain range ). Each of these ridges jutted out of the lake like an island, and each was surrounded by the same succession of coastlines . Gilbert determined the height of the various coastlines, noting that the coastlines in the center of the former Lake Bonneville were higher than the corresponding coastlines on the edges. He explained the higher lying coastlines in the middle of the lake by the rise of the pool floor due to the removal of the water load by the Bonneville tide. A review of Gilbert's data using photogeology and geodesy confirmed that the earth reacted isostatically to the relief from the Bonneville Flood . In the central part of the basin, the earth has lifted at least 64 meters from the southern end of the lake and the former northern outlet in Red Rock Pass.

Gilbert's work fell into oblivion and was only taken up again in the 1950s by Harold Edwin Malde (1923–2007). He found evidence that the Bonneville Flood flooded the Snake River Plains along the course of the Snake River in much less time than Gilbert assumed. His examination of this basalt plain revealed a system of canals and cataracts comparable in size, structure, and composition to those of the Channeled Scablands that formed along the route of the Missoula Floods , a series of catastrophic flood events caused by ice dam breaks.

flood

Map of the northwestern United States during the Pleistocene , some 14,500 years before our time

Lake Bonneville was located on the present basin of the Great Salt Lake, but was much larger than this with an area of ​​about 52,000 square kilometers and a depth of 300 meters. It is believed that during the last Ice Age, lava flows near Pocatello began to form a barrier in the Gem Valley in southeast Idaho, diverting the Bear River through Lake Thatcher and then through the Oneida Narrows Gorge into Lake Bonneville. The Gem Valley takes its name from the gemstone used as obsidian , a naturally occurring volcanic rock glass that is commonly found there. Due to the influx of the Bear River, the lake reached about 14,500 years ago at 1,552 meters, the highest water level since its formation.

At the Red Rock Pass, the deepest point on the edge of Lake Bonneville, alluvial cones of the adjacent Bannock and Portneuf mountain ranges had connected with the sediments of the Tertiary and the limestone of the Cambrian . An overflow at this point in connection with the runoff of water through the alluvial gravel and through channels in the limestone karst controlled the so-called Bonneville level of the lake for about 500 years. This runoff may have created the basis for the catastrophic failure of the alluvial cone. Earthquakes may also have contributed to the dam breach.

About 4750 cubic kilometers of water flowed out of the lake through the dam breach  , the water level of the lake decreased by about 110 meters. This led to the largest known freshwater flood in the history of the earth. The flood lasted for about eight weeks and followed the course of today's Marsh Creek , a 90-kilometer tributary of the Portneuf River , and the Portneuf River itself.

Then the tide reached the Snake River Plains at Pocatello and poured into American Falls Lake . The flood left large amounts of debris at the bottom of the lake. In the vicinity of Pocatello the rubble had a diameter of up to 2.5 meters, 20 kilometers further rubble the size of gravel was deposited. At American Falls Lake, the flood generated a system of canals that ultimately led to the dam breach and drained the lake before flooding about 750  square kilometers along the Snake River to the Rupert Basin . Most of the water then drained over a basalt ridge at the head of the Snake River Canyon and a channel in the highlands. At the confluence of the two rivers, about 50 kilometers downstream at Twin Falls , the flood created a canyon about 16 kilometers long, 1.5 kilometers wide, and about 500 meters deep.

Flood features

The mega-flood overflowed the Red Rock Pass with a maximum discharge of around 1,000,000  cubic meters per second. Downstream in Snake River Canyon at Sinker Creek , the discharge was 935,000 cubic meters per second, and at Lewiston , about 1,100  river kilometers from Red Rock Pass, the maximum discharge had dropped to about 0.57 to 0.62 million cubic meters per second. The speed of the tidal wave is estimated at 80 to 100 kilometers per hour.

Along its outflow the Bonneville flood left a number of characteristic features such as flood scour , sediment transport and hydraulic jumps that enabled a reconstruction and quantitative evaluation of many aspects of the flood.

Melon Gravel

Melon Gravel in Celebration Park, Idaho

In Celebration Park , an archaeological park in southwest Idaho, large camps of so-called melon gravel can be found. The name was coined by Harold E. Malde and HA Powers after they saw a sign on a scree field, on which the scree was being offered as Petrified Watermelons (petrified watermelons). The Melon Gravel are large, rounded blocks of basalt that were swept away by the Bonneville Flood and rounded off by transport. Melon Gravel has a diameter of about one to three meters.

American Falls Lake

American Falls Lake was north of Lake Bonneville and was formed about 72,000 years ago when the Snake River was partially blocked by a lava flow in the eastern Snake River plain. American Falls Lake was approximately 30 to 60 meters deep. Over time, a drainage channel had formed in the basalt that was about 8 to 15 meters below the maximum water level. The Bonneville Flood inundated the lake, forming large cataracts along the drainage canal and ultimately draining the lake. However, the additional water masses were small compared to the runoff from Lake Bonneville.

Shoshone Falls

Shoshone Falls near Twin Falls, Idaho

The Bonneville Flood created the 64-meter-deep Shoshone Falls at Snake River Canyon, one of the largest waterfalls in the northwestern United States. In this gorge the Snake River flows over massive and resilient rocks made of silicate volcanic rocks. The flood formed the falls in Snake River Canyon in a matter of weeks. The water deepened the Snake River to the rocky bottom and softer stones were torn from the canyon walls and scattered across the Snake River plain. Shoshone Falls is bounded by a 24-meter-high landscape formed by erosion.

literature

  • Charles G. Oviatt, John F. Shroder: Lake Bonneville: A Scientific Update. Elsevier Verlag, Amsterdam 2016, ISBN 978-0-444-63590-7 .
  • Jim E. O'Connor: Hydrology, Hydraulics, and Geomorphology of the Bonneville Flood. (= GSA Special Papers. Volume 274). Geological Society of America, 1993, ISBN 0-8137-2274-8 , doi: 10.1130 / SPE274 .
  • Harold Edwin Malde: The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain. (= Geological Survey Professional Paper. Edition 596). Washington 1968.

Web links

Commons : Lake Bonneville  - Collection of Images, Videos, and Audio Files

Individual evidence

  1. ↑ Harold T. Stearns: Evidence of Lake Bonneville Flood along Snake River below King Hill, Idaho. In: Geological Society of America Bulletin. 73, 1962, pp. 385-388, doi : 10.1130 / 0016-7606 (1962) 73 [385: EOLBFA] 2.0.CO; 2 .
  2. ^ GK Gilbert: Lake Bonneville , In: Monogr. US Geol. Surv. , Vol. 1, US Geol. Surv., Washington, DC, 1890, doi: 10.3133 / m1 .
  3. ^ Grove Karl Gilbert: Lake Bonneville . United States Geological Survey, Washington DC 1890, p. 126 ( archive.org ).
  4. ↑ a b J Harlen Bretz : The Lake Missoula Floods and the Channeled Scabland. In: The Journal of Geology , 77, 1969, pp. 505-543, doi : 10.1086 / 627452 .
  5. ^ Reflections on the Legacy of Grove Karl Gilbert, 1843–1918 at eos.org. Retrieved August 18, 2019.
  6. ↑ Max D. Crittenden Jr .: New data on the isostatic deformation of Lake Bonneville. In: Shorter Contributions to General Geology , Geological Survey Professional Paper, No. 454-E. US Govt. Print. Off., 1963.
  7. ^ A b Robert D. Jarrett, Harold E. Malde: Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence . In: Geological Society of America Bulletin , 99, 1987, pp. 127-134, doi : 10.1130 / 0016-7606 (1987) 99 <127: POTLPB> 2.0.CO; 2 .
  8. ^ Genevieve Atwood, Don R. Mabey: Flooding Hazards Associated with Great Salt Lake. In: Environmental and Engineering Geology of the Wasatch Front Region , 1995, pp. 483-494.
  9. ↑ Charles G. Oviatt, John F. Shroder: Lake Bonneville: A Scientific Update. Elsevier Verlag, Amsterdam, 2016, ISBN 978-0-444-63590-7 , p. 29.
  10. ↑ Keenan Lee: Bonneville-Flood, 2004.
  11. ^ SU Janecke, RQ Oaks Jr .: Reinterpreted history of latest Pleistocene Lake Bonneville: Geologic setting of threshold failure, Bonneville flood, deltas of the Bear River, and outlets for two Provo shorelines, southeastern Idaho, USA. In: J. Lee, JP Evans: Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the US Cordillera. Geological Society of America Field Guide 21, pp. 195-222, doi: 10.1130 / 2011.0021 (09) .
  12. ↑ a b c Devon M. Burr, Paul A. Carling , Victor R. Baker : Megaflooding on Earth and Mars , Cambridge University Press, 2009, ISBN 978-0-521-86852-5 , p. 132.
  13. ↑ a b c Harold Edwin Malde: The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain. Geological Survey Professional Paper, Issue 596, 1968, pp. 6-7.
  14. ^ A b c Jim E. O'Connor: Hydrology, Hydraulics, and Geomorphology of the Bonneville Flood . GSA Special Papers, Volume 274, 1993, Geological Society of America, doi: 10.1130 / SPE274 , ISBN 978-0-813-72274-0 , pp. 1-5.
  15. ↑ Harold Edwin Malde: The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain. Geological Survey Professional Paper, Issue 596, 1968, p. 46.
  16. ^ Keith Heyer Meldahl: Rough-Hewn Land: A Geologic Journey from California to the Rocky Mountains. University of California Press, 2013, ISBN 978-0-520-27577-5 , p. 135.
  17. ↑ Harold Edwin Malde: The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain. Geological Survey Professional Paper, Issue 596, 1968, p. 25.