The Bonneville flood was a catastrophic

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

ing event in the last

ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

, which involved massive amounts of water inundating parts of southern

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 ...

and

eastern Eastern may refer to: Transportation *China Eastern Airlines, a current Chinese airline based in Shanghai *Eastern Air, former name of Zambia Skyways *Eastern Air Lines, a defunct American airline that operated from 1926 to 1991 *Eastern Air Li ...

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

along the course of the Snake River. Unlike the

Missoula Floods The Missoula floods (also known as the Spokane floods or 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 las ...

, which also occurred during the same period in the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Tho ...

, 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 (radiocarbon dating, 17,400 years ago calendar, "calibrated" dating),

pluvial In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...

Lake Bonneville Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. It was a pluvial lake that formed in response to an increase in precipitation and a decrease in evaporation as a result of cooler temperature ...

in northern

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

reached its highest water level since its formation. The lake occupied the present-day basin of the Great Salt Lake, and was far larger, covering about . As it rose the lake level caused seepage at, then breached, the ancient level of

Red Rock Pass Red Rock Pass is a low mountain pass in the western United States in southeastern Idaho, located in southern Bannock County, south of Downey. It is geologically significant as the spillway of ancient Lake Bonneville. It is traversed by U.S ...

, a mountain pass at the headwaters of the Portneuf River, a tributary of the Snake River above present-day

American Falls Reservoir The American Falls Dam is a concrete gravity-type dam located near the town of American Falls, Idaho, on river mile 714.7 of the Snake River. The dam and reservoir are a part of the Minidoka Project on the Snake River Plain and are used primari ...

. Ancient Red Rock Pass was the site of two

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

s descending from opposite sides of the notch, forming a natural dam. When the dam collapsed, it released a flood crest down the Portneuf River valley, also spilling into the neighboring Bear River valley. When it reached the Snake River, it eroded away a

lava dam A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those creat ...

that had been at the site of the present-day American Falls, releasing a lake, American Falls Lake, that had formed behind the natural dam. At the peak of the flood, approximately poured over the Snake River Plain at speeds of up to and deposited hundreds of square miles of sediments eroded from upstream. The flood scoured the Snake River Canyon through the underlying

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

and loess soil, creating

Shoshone Falls Shoshone Falls () is a waterfall in the western United States, on the Snake River in south-central Idaho, approximately northeast of the city of Twin Falls. Sometimes called the "Niagara of the West," Shoshone Falls is in height, higher than ...

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 The Bruneau River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. 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 sout ...

and Salmon Falls Creek. The flood then entered

Hells Canyon Hells Canyon is a canyon in the Western United States, located along the border of eastern Oregon, a small section of eastern Washington and western Idaho. It is part of the Hells Canyon National Recreation Area which is also located in p ...

, significantly widening the gorge. Its waters eventually reached the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

via the Columbia River. A 2020 theory presented evidence that Lake Bonneville achieved a stable outflow for possibly a thousand years leading up to the Bonneville Flood and then a massive, multi-segment earthquake on the Wasatch Fault caused surging and tsunami in Lake Bonneville with a surge wave over 140' (~42m) high. This surge carried up into the Cache Valley and resulted in the natural dam failure at the Zenda threshold just north of Red Rock Pass.

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 drained the top of Lake Bonneville, which constituted about 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 scablands The Channeled Scablands are a relatively barren and soil-free region of interconnected relict and dry flood channels, coulees and cataracts eroded into Palouse loess and the typically flat-lying basalt flows that remain after cataclysmic floods ...

resembling the

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 Col ...

. Also left by the flood were the many "melon" boulders 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

, although the Missoula floods released more water as a whole, and at least one had a much higher peak flow rate. Much of the sediment scoured by the flood was deposited near the mouth of the Snake River. It now lies beneath about 20 layers of Missoula Floods deposits.

References

{{Reflist Floods in the United States Snake River Megafloods Pleistocene events Pleistocene United States Quaternary Idaho Quaternary Utah

Cause and events

Legacy

See also

References