Base jumping Snake River Canyon - Click to view more images Melon Red Rock Pass Shoshone Falls Bonneville Floods Lake Bonneville Historical Marker
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American white pelican near Hagerman, Idaho.
Click to view nearby Bonneville Flood feature.
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Lake Bonneville and the Bonneville Flood

The Bonneville flood released nearly 1,000 cubic miles of water. The volume of this flood was twice as large as the biggest Missoula Flood. However, unlike the Missoula Floods, the Bonneville Flood occurred over a period of several weeks approximately 17,400 years ago.

Snake River Canyon near Twin Falls, Idaho.

Snake River Canyon near Twin Falls, Idaho -Lake Bonneville's floodwaters completely filled the canyon in some locations and flowed above the canyon rim in other areas.

LAKE BONNEVILLE

Lake Bonneville Shorelines
Wave-cut platforms from Lake Bonneville preserved on Antelope Island, Great Salt Lake, Utah. (Mark A. Wilson Image)
Lake Bonneville and path of the Bonneville Flood.

Imagine an ice-age lake in Utah whose surface area roughly equaled that of Lake Michigan. Also imagine that same ice-age lake releasing a surge of floodwater whose volume was six times the flood flow of the Mississippi River. Since no humans witnessed this event we can only picture it in our minds. But the lake and its destructive outflow were not someone's imaginary fantasies. Geologists know that it was all very real.

After geologic processes millions of years ago created the unique basin-and-range topography of the western Rocky Mountains, an immense basin dominated the landscape of western Utah. Exactly when the body of water--later named Lake Bonneville--began to form is uncertain. The evidence indicates that it existed at least 600,000 years ago. As the climate became cooler and wetter, Lake Bonneville expanded in size, gradually spreading over a surface area of about 20,000 square miles and overlapping into Nevada and Idaho.

Because Lake Bonneville was in a basin, it lacked natural outlets. Once captured, water from rainfall, stream flows, and glacial melting remained in the lake, except for those amounts lost through evaporation. At its greatest extent, the lake level stood at 5,090 feet elevation--nearly 1,000 feet higher than the current level of Great Salt Lake. Distinct shorelines, including sand and gravel beaches, were formed along the 2,000 miles of Lake Bonneville's rim. Animals such as bears, camels, deer, mammoths, horses and musk oxen roamed its shores.

In addition to being confined by higher ground at the basin rim, a "natural dam" at Lake Bonneville's northern limit at Red Rock Pass prevented outflows. The pass [ south of Downey, Idaho, on U.S. 91 ] was the site of overlapping alluvial fans--huge deposits of rock, sand, and soil that had eroded during the weathering-down of nearby mountains.



About 17,400 years ago, this natural --but unstable--dam gave way.

Play Bonneville Flood Slideshow

Click above to view story of Lake Bonneville Flood. Photos and illustrations show many amazing features created when 1,000 cubic miles of water raced down the course of the Snake River.

Red Rock Pass, Idaho

Red Rock Pass, Idaho.
View south through Red Rock Pass - Into basin that held Lake Bonneville.


THE CATACLYSMIC BONNEVILLE FLOOD

The breaching of the alluvial deposits at Red Rock Pass unleashed a torrent that ran about 300 feet in depth northward through the valleys of Marsh Creek and the Portneuf River and joined the Snake River near Pocatello, Idaho. The floodwaters followed the route of the Snake River and its existing canyons across southern Idaho, then veered north through Hell's Canyon, and finally poured into the Columbia River near Pasco, Washington. South of Pasco the flow passed through Wallula Gap and on to the Pacific through the Columbia Gorge.

It is estimated that the initial flood volume, which persisted for perhaps two weeks, amounted to 15 million cubic feet per second--about 300 times the maximum flows ever recorded on the Snake River. Over a period of weeks and months, nearly 1,000 cubic miles of water were released, and Lake Bonneville dropped more than 350 feet.

The flood scoured the walls and bed of the Snake's existing canyons from Pocatello to Pasco and also inundated a wide swath on the adjacent Snake River Plain across southern Idaho. The current cut deep recesses in the canyon walls and gouged holes in the canyon floor, creating waterfalls, alcoves and gravel bars. In Hell's Canyon the floodwaters left gravel bars along the river bends more than 100 feet above the modern river level. Deposits of sand and gravel also partially blocked Hell's Canyon's side valleys. The deluge then overwhelmed the Clearwater River at Lewiston, Idaho and temporarily reversed its flow for several miles. [The flood is described more fully in David Alt and Donald W. Hyndman, Northwest Exposures: A Geologic Story of the Northwest].



Portneuf Narrows - Bonneville Flood.

Scoured and fluted basalt surface near Portneuf Narrows. Flow left to right. -Jim O'Connor photo

-Click to enlarge-

VISIBLE EVIDENCE

Lake Bonneville shorelines....The pre-flood shoreline at 5,090 feet elevation is clearly visible as a terrace-like terrain feature in nearby mountain ranges, such as the Wasatch in northeastern Utah. The post-flood shoreline at 4,700 feet elevation is less prominent but also discernible.

Scablands and sand and gravel deposits.....Authors William and Elizabeth Orr [in Geology of the Pacific Northwest] described the process as "hydraulically vacuuming up millons of tons of rock" and then depositing the materials downstream. The flood stripped surface material in the Snake River plain, creating scablands visible northeast of Twin Falls, Idaho and a region of sand and gravel deposits near Burley and Rupert, Idaho. Huge boulders ripped loose by the flood were deposited at Massacre Rocks State Park [near Exit 28 on Interstate 86 southwest of American Falls].

Snake River Canyon.....The existing canyon was deepened in places, creating the spectacular Shoshone Falls and Twin Falls northeast of the city of Twin Falls, Idaho. Northwest of the city are several examples of alcoves--or "dry falls"--at Devil's Washbowl, Devil's Corral, and Blue Lakes.


Priest Ranch Bar - Jim O'Connor photo.
YouTube video by fourshropsfarm
.

NOTE: Hover your mouse over aerial image.

Aerial image above (Jim O'Connor photo - HUGEfloods mouseover effect) - shows view to the east (upstream) of the Snake River Plain and the incised Snake River within the Murtaugh reach. At the peak discharge, only about a third of the flow followed the Snake River Canyon in this reach; the remainder flowed over an upland route to the north and reentered the Snake River Canyon near Twin Falls. The maximum extent of erosion (stripped basalt surfaces) by the flood is shown fairly precisely by the limits of cultivation on the uplands.

The large set of falls in the middle foreground is Cauldron Linn* which stopped the Hunt Party's 1811 attempt to descend the Snake River in wooden canoes. That group described this stretch of river as "The Devil's Scuttle Hole".
*A linn may refer to a waterfall or a pool at the foot of a waterfall, with the derivation a confusion of Scots Gaelic linne (pool) and Old English hlynn (torrent). - Wikipedia definition


Lake Bonneville Flood cataracts near Twin Falls, Idaho.
Snake River Canyon near Twin Falls, ID. - Jim O'Connor photo

View Larger Map Note Shoshone Falls in Google Map above (also visible in O'Connor's aerial image at left). Use map navigation tools to explore area. Six mouse clicks on "Pan right" arrow will take you past some impressive scabland features on your way to a pair of large cataracts. Click "+" for a closer look at specific features or "-" to view flood path.

Lake Bonneville Flood.

The 101 Ranch along US HWY 30 appears to be looking for help removing basalt boulders from their pasture near King Hill, ID. The material was ripped from bedrock upstream, then tumbled by floodwaters before being dropped near this location.

Lake Bonneville Flood.


Surrounding images show flood-tumbled boulders below Swan Falls Dam.

Lake Bonneville Flood.

Melon Gravels.....As the water ripped through narrow reaches of the Snake's canyon, it dislodged large basalt boulders and tumbled them downstream. Within a few miles these boulders had been rounded with diameters ranging from three feet to ten feet. The process created large bars of sand and boulders, some of them several miles long and nearly 300 feet high. These remnants were given the name melon gravels after geologists observed a billboard that advertised one patch of boulders as "petrified watermelons."

Impressive deposits of melon gravels can be found near Hagerman; in Malad Gorge; in Melon Valley south of Wendell; at Swan Falls south of Melba; and near the towns of Hammett, Glenn's Ferry and King Hill.

Lake Bonneville Flood.

RESOURCES:


Harold E. Malde- Geological Survey Professional Paper
(1968 - 73 pages)

The Catastrophic Late Pleistocene Bonneville Flood in the Snake River Plain, Idaho


"A study of colossal features of erosion and deposition produced along the Snake River by sudden overflow of Lake Bonneville"

Open Benjamin Crosby's 2011 Bonneville Flood Guidebook[20.88 MB] and page of links to Bonneveille Flood Papers.

Jim O'Connor USGS geologist leads Ice Age Floods field trip. Bruce Bjornstad photo.

Visit HUGEfloods Jim O'Connor page to view more photos of Bonneville Flood features and watch CWU's Nick Zentner interview O'Connor (USGS hydrologist). Jim describes both Bonneville and Missoula floods in the 25 minute program - Bonneville discussion begins at 14 minute mark.



Bonneville Flood deposits near Boise River. Jim O'Connor photo
Slackwater deposits near Boise River. Jim O'Connor photo
Massacre Rocks State Park, Idaho.
Many interesting Oregon Trail exhibits are found along the Bonneville Flood path.


Register Rock Oregon Trail interpretive site, Idaho.
Register Rock - A huge flood-tumbled boulder that holds the signatures of Oregon Trail emigrants.


Lake Bonneville Flood boulders, Massacre Rocks State Park, Idaho.
Basalt Boulders at Massacre Rocks State Park.
Lake Bonneville Flood boulders on Snake River Canyon floor.
Boulders tumbled by the Lake Bonneville Flood on Snake River Canyon floor.


Snake River and Bonneville Flood gravel in Hells Canyon, Idaho.
Snake River between huge Bonneville Flood gravel terraces.


Trip Report


May 2010 hike to view Bonneville Flood features in Hells Canyon:

Click to View

Area covered: Pittsburg Landing, Kirkwood, Suicide Point and Big Bar

At Tammany Bar south of Lewiston, Idaho - Bonneville Flood gravels (B) are capped with Lake Missoula Floods rhythmites (M) 2-20 feet thick.

The Missoula sand and silt deposits were backflooded to this location when Ice Age floodwaters temporarily reversed the Snake River's course.

Idaho Geological Survey -Surficial Geologic Map of Lewiston Area (pdf)
Multiple Lake Missoula deposits cap Bonneville Flood material at Tammany Bar. Lewiston, Idaho.

THE AFTERMATH

The most intense flooding occurred over a period of several days--at most, several weeks, but over spilling at Red Rock Pass may have continued for as long as a year before the lake stabilized at 4,700 feet--the so-called "Provo shoreline"--and remained at that level for at least 600 years.

As the Ice Age ended the climate became warmer and drier. With less rainfall and glacial melting to sustain Lake Bonneville, coupled with increased evaporation, the vast lake began to retreat. The current Great Salt Lake's drainage area is roughly that of ancient Lake Bonneville. Other prominent remnants include Utah Lake, Sevier Lake, and the Great Salt Lake Desert.

BENJAMIN BONNEVILLE

Lake Bonneville was named for Benjamin L. E. Bonneville, a French-born U.S. Army officer, who in the 1830s was involved in exploring and trading ventures in the Pacific Northwest. Geologist G.K. Gilbert was the first to study these prehistoric lake features, and he decided to honor Captain Bonneville in this way.



Learn more about Lake Bonneville and the Bonneville Flood:

USGS Lake Bonneville and Bonneville Flood Page
Benjamin E. Bonneville

Benjamin L.E. Bonneville
Image-Public Domain

NASA image

Bonneville Crater

Impact crater on Mars, named after Benjamin Bonneville and Utah's anicent Lake Bonneville. [NASA Image]


EXPLORING SOUTHERN IDAHO

People who want to visit the Snake River region to see for themselves the evidence of the Bonneville flood and other geologic wonders in the area, may find David Alt and Donald Hyndman's Roadside Geology of Idaho to be a helpful guide.

Balanced Rock near Buhl, Idaho.
Balanced Rock - Buhl
Craters of the Moon
Craters of the Moon - Arco
Lake Bonneville Shorelines
Owl in Yahoo Clay deposits - Hagerman

Click images to expand


Read more about Craters of the Moon: Craters of the Moon-(Wikipedia) --- Craters of the Moon-(National Park Service)



All photos by Tom Foster unless otherwise noted. [Google]

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