Portneuf River Ecosystem - the Lake Bonneville Flood

The Land Around the River
Geology	Air Quality	Bonneville Flood

The Lake Bonneville Flood
One of the major events that shaped the lower Portneuf valley was the Bonneville flood. Lake Bonneville was a large lake that at its largest extent covered about 20,000 square miles of central Utah. Lake Bonneville was one of several large lakes that were formed in the interior regions of the Intermountain West during the Pleistocene as snow and ice melted at the end of the last ice age. About 14,500 years ago the lake broke through its shore near Red Rock Idaho, spilling a huge volume of water north through the Portneuf Gap at the southern end of Pocatello. This flood may have lasted for more than a year, and at its peak had a flow of as much as 33 million cubic feet per second.
Local effects of the flood can be seen in the way that the lava flows were exposed and shaped in the lower Portneuf valley. Excavation in the valley frequently uncovers massive rounded boulders that were carried and smoothed by the flood.
For additional information about the Bonneville flood, including maps that show the likely extent of Lake Bonneville and photographs of geological formations affected by the flood, check out these links:
Digital Atlas of Idaho http://imnh.isu.edu/digitalatlas/hydr/main/lbffr.htm Utah Geological Survey http://www.ugs.state.ut.us/online/PI-39/index.htm