Bonneville Flood
Encyclopedia
The Bonneville Flood or Lake Bonneville Flood was a catastrophic flood
ing event in the previous ice age
, which involved massive amounts of water inundating parts of southern Idaho
and eastern
Washington along the course of the Snake River
. Unlike the Missoula Floods
, which also occurred during the same period in the Pacific Northwest
, the Bonneville Flood only happened once. The flood is believed to be the second largest in known geologic history.
Lake Bonneville
in northern Utah
reached its highest water level since its formation. The lake occupied the present-day basin of the Great Salt Lake
, except it was far larger, covering about 32000 mi2. As it rose the lake level caused seepage at, then breached, the ancient level of Red Rock Pass
, a mountain pass at the headwaters of the Portneuf River
, a tributary of the Snake River above present-day American Falls Reservoir. Ancient Red Rock Pass was the site of two alluvial fan
s descending from opposite sides of the notch, forming a natural dam. The roughly 410 feet (125 m)-high flood crest raced down the Portneuf River valley, and eroded away a lava dam that previously was at the site of the present-day American Falls, releasing a 40 miles (64.4 km)-long lake, American Falls Lake, that had formed behind the natural dam.
The peak of the flood, approximately 33000000 cuft/s, poured over the Snake River Plain
at a speed of 70 miles (112.7 km) per hour and deposited hundreds of square miles of sediments eroded from upstream. The flood scoured the 600 feet (182.9 m)-deep Snake River Canyon
through the underlying basalt
and loess
soil, creating Shoshone Falls
and several other waterfalls along the Snake River. It also carved and increased in size many other tributary canyons, including those of the Bruneau River
and Salmon Falls Creek
. Eventually, water also spilled down the Bear River into the plain, raising the water flow significantly. The flood then entered Hells Canyon
, significantly widening the gorge. Its waters eventually reached the Pacific Ocean
via the Columbia River
.
resembling the Columbia Plateau
, although the scale of the flood was much smaller than the Missoula Floods that swept over the Columbia Plateau. Also left by the flood were the many "watermelon" rocks distributed throughout the canyons in the Snake River Plain. According to some geologists, the total volume of the Bonneville flood was actually greater than any individual one of the Missoula Floods
, although the Missoula floods released more water as a whole.
Much of the sediment scoured by the flood was deposited near the mouth of the Snake River, on top of about 20 layers of Missoula Floods deposits.
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
ing event in the previous ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
, which involved massive amounts of water inundating parts of southern 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....
and eastern
Eastern Washington
Eastern Washington is the portion of the U.S. state of Washington east of the Cascade Range. The region contains the city of Spokane , the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Hanford Nuclear Reservation and the fertile farmlands of the Yakima Valley and the...
Washington along the course of the 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...
. Unlike the Missoula Floods
Missoula Floods
The Missoula Floods refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. The glacial flood events have been researched since the 1920s...
, which also occurred during the same period in the Pacific Northwest
Pacific Northwest
The Pacific Northwest is a region in northwestern North America, bounded by the Pacific Ocean to the west and, loosely, by the Rocky Mountains on the east. Definitions of the region vary and there is no commonly agreed upon boundary, even among Pacific Northwesterners. A common concept of the...
, the Bonneville Flood only happened once. The flood is believed to be the second largest in known geologic history.
Cause and events
About 14,500 years ago, pluvialPluvial
In geology and climatology, a pluvial was an extended period of abundant rainfall lasting many thousands of years. Pluvial is also applied to the sediments of these periods . The term is especially applied to such periods during the Pleistocene Epoch...
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...
in northern Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
reached its highest water level since its formation. The lake occupied the present-day basin of the Great Salt Lake
Great Salt Lake
The Great Salt Lake, located in the northern part of the U.S. state of Utah, is the largest salt water lake in the western hemisphere, the fourth-largest terminal lake in the world. In an average year the lake covers an area of around , but the lake's size fluctuates substantially due to its...
, except it was far larger, covering about 32000 mi2. As it rose the lake level caused seepage at, then breached, the ancient level of Red Rock Pass
Red Rock Pass
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...
, a mountain pass at the headwaters of the Portneuf River
Portneuf River (Idaho)
The Portneuf River is a tributary of the Snake River in southeastern Idaho in the United States. It drains a ranching and farming valley in the mountains southeast of the Snake River Plain...
, a tributary of the Snake River above present-day American Falls Reservoir. Ancient Red Rock Pass was the site of two alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
s descending from opposite sides of the notch, forming a natural dam. The roughly 410 feet (125 m)-high flood crest raced down the Portneuf River valley, and eroded away a lava dam that previously was at the site of the present-day American Falls, releasing a 40 miles (64.4 km)-long lake, American Falls Lake, that had formed behind the natural dam.
The peak of the flood, approximately 33000000 cuft/s, poured over 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...
at a speed of 70 miles (112.7 km) per hour and deposited hundreds of square miles of sediments eroded from upstream. The flood scoured the 600 feet (182.9 m)-deep Snake River Canyon
Snake River Canyon (Idaho)
Snake River Canyon is a large canyon formed by the Snake River in the Magic Valley region of southern Idaho. It is well known as the site of an unsuccessful 1974 attempt by Evel Knievel to jump it in the Skycycle X-2....
through the underlying basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
and loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
soil, creating Shoshone Falls
Shoshone Falls
Shoshone Falls is a waterfall on the Snake River located approximately five miles east of Twin Falls, Idaho. Sometimes called the "Niagara of the West," Shoshone Falls is 212 feet high—45 feet higher than Niagara Falls—and flows over a rim 1,000 feet wide.A park overlooking the waterfall is...
and several other waterfalls along the Snake River. It also carved and increased in size many other tributary canyons, including those of the Bruneau River
Bruneau River
The Bruneau River is a tributary of the Snake River, in the U.S. states of Idaho and Nevada. It runs through a narrow canyon cut into ancient lava flows in southwestern Idaho...
and Salmon Falls Creek
Salmon Falls Creek
Salmon Falls Creek is a tributary of the Snake River, flowing from northern Nevada into Idaho in the United States. Formed in high mountains at the northern edge of the Great Basin, Salmon Falls Creek flows northwards , draining an arid and mountainous basin of...
. Eventually, water also spilled down the Bear River into the plain, raising the water flow significantly. The flood then entered Hells Canyon
Hells Canyon
Hells Canyon is a wide canyon located along the border of eastern Oregon and western Idaho in the United States. It is North America's deepest river gorge at and part of the Hells Canyon National Recreation Area....
, significantly widening the gorge. Its waters eventually reached the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
via the Columbia River
Columbia River
The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada, flows northwest and then south into the U.S. state of Washington, then turns west to form most of the border between Washington and the state...
.
Legacy
Although the peak of the flood lasted a few weeks at most, erosion at Red Rock Pass continued for a few years before water ceased to spill over. The flood removed the top 351 feet (107 m) of Lake Bonneville, which constituted about 1200 cubic miles (5,001.8 km³) of water, and lowered the lake level to a stage known as the Provo shoreline. The flood transformed the Snake River Plain into a series of channeled scablandsChanneled scablands
The Channeled Scablands are a unique geological erosion feature in the U.S. state of Washington. They were created by the cataclysmic Missoula Floods that swept periodically across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. Geologist J Harlen Bretz coined...
resembling the Columbia Plateau
Columbia Plateau
The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River...
, although the scale of the flood was much smaller than the Missoula Floods that swept over the Columbia Plateau. Also left by the flood were the many "watermelon" rocks distributed throughout the canyons in the Snake River Plain. According to some geologists, the total volume of the Bonneville flood was actually greater than any individual one of the Missoula Floods
Missoula Floods
The Missoula Floods refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. The glacial flood events have been researched since the 1920s...
, although the Missoula floods released more water as a whole.
Much of the sediment scoured by the flood was deposited near the mouth of the Snake River, on top of about 20 layers of Missoula Floods deposits.