Red Rock Pass
Encyclopedia
Red Rock Pass is a low mountain pass
in eastern
Idaho
, south of Downey
in southern Bannock County
. It is geologically significant as the spillway
of ancient Lake Bonneville
. It is located along highway US-91
at an elevation
of 4785 feet (1458 m) above sea level, bounded by two mountain ranges; the Portneuf to the east and the Bannock to the west.
The pass was cut through resistant Paleozoic
shale
, limestone
, and dolomite
, and forms a narrow gap two miles (3 km) in length.
At one time the pass was 300 feet (100 m) higher, where the shoreline of Pleistocene
Lake Bonneville stood.
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.
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 world's largest river. The speed of flow was approximately 16 mph (7 m/s), and though peak flow lasted only a few days, voluminous discharges may have continued for at least a year.
Mountain pass
A mountain pass is a route through a mountain range or over a ridge. If following the lowest possible route, a pass is locally the highest point on that route...
in eastern
Eastern Idaho
Eastern Idaho is a generic term used to describe areas of Idaho which lie east of the Magic Valley region. It is generally understood to include: Bannock, Bear Lake, Bingham, Bonneville, Butte, Caribou, Clark, Custer, Franklin, Fremont, Jefferson, Madison, Oneida, Power and Teton...
Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
, south of Downey
Downey, Idaho
Downey is a city in Bannock County, Idaho, United States. It is part of the 'Pocatello, Idaho Metropolitan Statistical Area'. The population was 613 at the 2000 census.-Geography:...
in southern Bannock County
Bannock County, Idaho
Bannock County is a county located in the southeastern part of the U.S. state of Idaho. It was established in 1893 and named after the local Bannock tribe. It is part of the Pocatello, Idaho Metropolitan Statistical Area, which encompasses all of Bannock and Power counties. As of the 2000 Census...
. It is geologically significant as the spillway
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
of ancient Lake Bonneville
Lake Bonneville
Lake Bonneville was a prehistoric pluvial lake that covered much of North America's Great Basin region. Most of the territory it covered was in present-day Utah, though parts of the lake extended into present-day Idaho and Nevada. Formed about 32,000 years ago, it existed until about 14,500 years...
. It is located along highway US-91
U.S. Route 91
U.S. Route 91 is a north–south United States highway. The highway currently serves as a connection between the Cache Valley area of Utah and Idaho to the Salt Lake and Idaho Falls population centers. Prior the mid-1970s, U.S. 91 was an international commerce route from Long Beach, California...
at an elevation
Elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
of 4785 feet (1458 m) above sea level, bounded by two mountain ranges; the Portneuf to the east and the Bannock to the west.
The pass was cut through resistant Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, and dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, and forms a narrow gap two miles (3 km) in length.
At one time the pass was 300 feet (100 m) higher, where the shoreline of Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
Lake Bonneville stood.
The Bonneville flood
It is believed that lava flows in the vicinity of PocatelloPocatello, Idaho
Pocatello is the county seat and largest city of 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 Pocatello metropolitan area, which encompasses all of Bannock...
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
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
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
Snake River Plain
The Snake River Plain is a geologic feature located primarily within the state of Idaho in the United States of America. It stretches about westward from northwest of the state of Wyoming to the Idaho-Oregon border. The plain is a wide flat bow-shaped depression, and covers about a quarter of Idaho...
, generally following the path of the present-day Snake River
Snake River
The Snake is a major river of the greater Pacific Northwest in the United States. At long, it is the largest tributary of the Columbia River, the largest North American river that empties into the Pacific Ocean...
to its outlet in the Pacific Northwest.
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
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...
, the world's largest river. The speed of flow was approximately 16 mph (7 m/s), and though peak flow lasted only a few days, voluminous discharges may have continued for at least a year.
External links
- Huge Floods.com – Bonneville
- Idaho State Univ. – Digital Geology of Idaho – Lake Bonneville