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The Lake Bonneville Flood in the Pocatello Area
The flood removed the Basalt of Portneuf Valley from the Portneuf Narrows area, where the water was 300 feet deep and moving at 60 miles per hour. As the water slowed down in the Pocatello area it deposited boulders up to 12 feet in diameter in the downtown Pocatello area. A flat delta deposit known as the Michaud Gravel was laid down northwest of Pocatello, and is the present site of the city of Chubbuck and the Pocatello airport.

(left) View of Portneuf River Valley and Pocatello, Idaho, looking northwest from Portneuf Hill, south of Portneuf Narrows, (March, 1982). The Portneuf River meanders in a valley widened by the Lake Bonneville Flood about 14,500 years ago. Some of the river's meandering has been curtailed by construction of the Union Pacific Railroad. In the upper left, just west of the railroad tracks, the Portneuf enters a flood control channel built by the U.S. Army Corps of Engineers after much of downtown Pocatello was flooded in February, 1962 and 1963.
The Basalt of Portneuf Valley forms the dark escarpment northeast of the railroad. At least two lava flows can be seen. About 600,000 years ago, these flowed down the canyon of the Portneuf River from a source area near Bancroft in Gem Valley. Interstate 15 makes sweeping curves that parallel the edge of the lava flows. The dry Highway Pond can be seen in the middle distance.

(right) Basalt boulders up to 8 feet in diameter on North Main Street, downtown Pocatello, (April, 1990).
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