DESCRIPTION:
Lake Bonneville and the Bonneville Flood

The "Bonneville flood" occurred bout 15,000 years ago when the great Lake Bonneville, which covered much of Utah (Great Salt Lake is the minuscule remnant) overtopped Red Rock Pass causing the lake surface to drop 300 feet to the Provo shoreline. The Bonneville flood released 1,000 cubic miles of water. The volume of this flood was twice as large as the biggest Missoula flood. However, unlike the Ice Age Floods, the Bonneville Flood occurred over a period of a couple of weeks.

Ice Age Lake Bonneville, which existed around 14,500 years ago, covered more than 20,000 square miles in Utah and parts of Idaho and Nevada. For hundreds of years, the water level of Lake Bonneville maintained a fairly constant level. The water level dropped almost 400 feet when part of Red Rock Pass, which was holding back the water, eroded. The floodwaters flowed down the Snake River and joined the Columbia River near the Tri-Cities. For a short period of time, the resulting floodwaters from Lake Bonneville increased the size of the Snake River and the Columbia River by more than 20 times their normal flow. After the flood occurred, the water levels of the Great Salt Lake eventually subsided close to what they are now. Lake Bonneville drained only once, with catastrophic results.

As glaciers receded during the last ice age, the inland basin of central Utah slowly filled with meltwater, creating Lake Bonneville. This lake covered approximately 20,000 square miles. The water level rose and finally crested at the lowest point in the basin -- Red Rock Pass, Idaho. The soft soils were easily eroded as the water spilled over, creating a large outlet for the lake. Water spilled out of Lake Bonneville and flowed north into the valleys of Marsh Creek and the Portneuf River. The deluge entered the Snake River Plain just north of Pocatello, and flowed west across southern Idaho before turning back north into the Hell's Canyon region. This flood took place about 14,000 years ago.

The Snake River and its canyons were the major geographic features across the volcanic plain and became the main conduit for the Bonneville flood. The varying topographic features of the Snake River produced distinct types of hydraulics. In places where the canyon is deep and constricted, the velocity of the water increased tremendously. This increased energy allowed the water to pick up talus boulders the size of houses, turn, roll, and smooth out their rough edges, and deposit them many miles downstream. When the water entered wide, open stretches, the velocity decreased and the energy of the water could not keep the boulders suspended. The rocks settled in the bottom of the river and are now exposed on the larger bars along the river. These large, rounded boulders were nicknamed "melon gravel" due to the resemblance to big watermelons. ...

Some people think the Bonneville Flood created the Snake River Canyon. This is not completely accurate. The canyon existed prior to the flood in much the same condition as it is today. Floodwaters completely filled the canyon in some locations and flowed above the canyon rim in other areas. The force of the flood waters scoured the canyon in constricted locations. The river carved out many "box" canyons along the cliffs in places where large eddies formed.

What would it have been like to stand on the rim and watch the river flow by at that time? From geologic evidence it has been calculated that the peak discharge flow at Swan Falls was 33 million cubic feet/second. If railroad tank cars were filled with 33 million cubic feet of water, it would form a train 165 miles long. This volume of water passed by Swan Falls every second! For those who think a raft trip through the Grand Canyon would be exciting, a raft on the river during the peak discharge would have been traveling at 70 miles/hour (without rowing) on a wall of water 312 feet deep. Now that's big water rafting.

The Great Salt Lake is the remnant of ancient Lake Bonneville, which gave the Bonneville Salt Flats their name. Geologists estimate that Lake Bonneville existed between 23,000 and 12,000 years ago, during the last glacial period. Lake Bonneville's existence ended abruptly when the waters of the lake began to drain rapidly through Red Rock Pass in southern Idaho into the Snake River system. As the Earth's climate warmed and became drier, the remaining water in Lake Bonneville evaporated, leaving the highly saline waters of the Great Salt Lake. The reason for the high concentration of dissolved minerals in the Great Salt Lake is due to the fact that it is a "terminal basin" lake; water than enters the lake from streams and rivers can only leave by evaporation. As the process occurs over time, the dissolved substances in the river water become increasingly concentrated.

As you drop into the Snake River Canyon, you can trace some of the geologic history of the area. The canyon cliffs show layer upon layer of lava flows interspersed with sedimentary layers. As you reach the bottom, the canyon floor is scattered with hundreds of house-size boulders left behind from the Bonneville flood. This flood raced through the canyon 15,000 years ago with more than 100 meters (350 feet) of water rushing at 110 kilometers (70 miles) per hour.

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