Red Rock Pass is a low mountain pass in the

western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...

United States in southeastern

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...

, located in southern Bannock County, south of

Downey Downey may refer to: People * Downey (surname) *Robert Downey Jr. Places *Downey, California, US * Downey, Idaho, US * Downey, Iowa, US Businesses * W. & D. Downey, photographic studio * Downey Studios, created out of a former Boeing plant Scho ...

. It is geologically significant as the

spillway A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure th ...

of ancient Lake Bonneville. It is traversed by

U.S. Route 91 U.S. Route 91 or U.S. Highway 91 (US-91) is a north–south United States highway running from Brigham City, Utah, to Idaho Falls, Idaho, in the U.S. states of Idaho and Utah. Despite the "1" as the last digit in the number, US-91 is no lo ...

at an elevation of above sea level, bounded by two mountain ranges; the Portneuf to the east and the

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to the west. The pass was cut through resistant Paleozoic shale, limestone, and

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, and forms a narrow gap in length. At one time the pass was higher, where the shoreline of Pleistocene Lake Bonneville stood. The pass takes its name from the red limestone cliffs which border it. Red Rock Pass has a surface deposit of

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

silty

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

with topsoil of dark grayish brown silt loam.

Bonneville flood

It is believed that during the last ice age lava flows in the vicinity of

Pocatello Pocatello () is the county seat of and largest city in Bannock County, with a small portion on the Fort Hall Indian Reservation in neighboring Power County, in the southeastern part of the U.S. state of Idaho. It is the principal city of the ...

began to divert the Bear River through Lake Thatcher and then into Lake Bonneville. This sudden influx caused Bonneville to overflow at Red Rock about 14,500 years ago. This overflow caused a sudden erosion of unconsolidated material on the northern shoreline near Red Rock Pass. As the material gave way, Marsh Creek Valley, immediately downstream, was flooded from wall to wall, and the rapid discharge eroded the pass to its present level. The flood then flowed into the Snake River Plain, generally following the path of the present-day Snake River to its outlet in the Pacific Northwest.Univ. of Utah Media
– Lake Bonneville – accessed 2009-08-19 The Bonneville flood, as it is known, was a catastrophic event. The maximum discharge was about 15 million cubic feet per second (420,000 m³/s), or about three times the average flow of the

Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of ...

, the world's largest river. The speed of flow was approximately , and though peak flow lasted only a few days, voluminous discharges may have continued for at least a year.

References

External links

Huge Floods.com
– Bonneville

– Lake Bonneville {{Idaho Landforms of Bannock County, Idaho Historical geology Mountain passes of Idaho Geology of Idaho Natural history of Idaho