Geologists know …

Shoshone Falls) at Twin Falls and the "watermelon" boulders near Hagerman.Currey, D.R., Atwood, G., and Mabey, D.R., 1984, Major levels of Great Salt Lake and Lake Bonneville: Utah Geological and Mineral Survey Map 73.Link, P.K., Kaufman, D., and Thackray, G.D., 1999, O'Connor, J.E., 1993, Hydrology, hydraulics, and geomorphology of the Bonneville flood: Geological Society of America Special Paper 274, 83 p. Imagine an ice-age lake in Utah whose surface area roughly equaled that of Lake Michigan. After the Lake Bonneville Flood, the lake fell to the Provo shoreline level. with Lake Missoula Floods rhythmites (M) 2-20 feet thick.

It also includes digital images of each stage, digita… The flood drained the top 351 feet (107 m) of Lake Bonneville, which constituted about 1,200 cubic miles (5,000 km ) of water, and lowered the lake level to a stage known as the Provo shoreline. The Bonneville Flood, which occurred about 15,000 years ago, dropped the level of Lake Bonneville more than 300 feet to the Provo Level (4,740 feet above sea level). As the Ice Age ended the climate became warmer and drier. 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.

South of Pasco the flow passed through 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. 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. The current cut deep recesses in the canyon walls and gouged holes in the canyon floor, creating waterfalls, alcoves and gravel bars. It also was relatively fresh. The peak discharge from the Red Rock Pass outlet is estimated to have lasted for about 8 weeks and to have been about 500 times that of the maximum discharge ever recorded from the Snake River at Idaho Falls.Several geological and environmental factors likely contributed to the Bonneville Flood.

The flood emptied the top 107 meters of water from Lake Bonneville , an estimated volume of 4,750 cubic km of water (Link and Phoenix, 1996). Click Hereto see the History of Lake Bonneville section, which includes other lake level stages not discussed above. Flow left to right.

During its transgressive phase in the closed basin, lake level oscillated because of changes in climate but the lake gradually rose until about 18,000 years ago when it reached its highest elevation, marked by the Bonneville shoreline. The maximum extent of erosion (stripped basalt surfaces) by the flood is shown fairly precisely by the limits of cultivation on the uplands. At the beginning of the colossal prehistoric flood from Lake Bonneville through Red Rock Pass, the spillway first formed as the water level in the lake on the south side reached the 5,100 feet level of the dam. 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. At Tammany Bar south of Lewiston, Idaho - Bonneville Flood gravels are capped with Lake Missoula Floods rhythmites 2-20 feet thick. -Jim O'Connor photoAerial 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. Lake Bonneville was the largest stage of the ancient lake that is now the Great Salt Lake. 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. The highest buildings in the valley would have been submerged by 700 feet of water if Lake Bonneville were still in existence. The Lake Bonneville flood filled the Pocatello Valley and flowed up through Marsh Valley and then out onto the Snake River Plain. Lake Bonneville began to rise from elevations similar to those of modern Great Salt Lake about 30,000 years ago. Other spectacular examples of Bonneville Flood features are the Snake River Canyon, alcoves, and waterfalls (e.g. The Missoula sand and silt deposits were backflooded to this location when Ice Age floodwaters temporarily reversed the Snake River's course. The current Great Salt Lake's drainage area is roughly that of ancient Lake Bonneville. By Laura DeGrey, Myles Miller & Paul Link, Idaho State University, Department of Geosciences Ancient Lake Bonneville was located in north-central Utah and was fed by the Bear River, until 14,500 years ago, during the Pleistocene Epoch. The deluge then overwhelmed the Clearwater River at Lewiston, Idaho and temporarily reversed its flow for several miles.

Evidence of this is seen in the Pocatello area; the removal of the Portneuf Valley basalt from the Portneuf Narrows area and the deposition of rounded basalt boulders (up to 3.5 meters in diameter) in the adjacent basin are prime examples.