Bannock County Groundwater

Geologic History of Our Aquifer

Lake Bonneville

Approximately 15,000 years ago, Lake Bonneville, the ancestor of the Great Salt Lake, suddenly discharged an immense volume of water to the north through Red Rock Pass near Downey, Idaho. These flood waters continued north through Marsh Valley, turned west near Inkom, passed through the Portneuf Gap, spread out through the valley that is occupied by Pocatello and Chubbuck, and eventually flowed onto the Snake River Plain.

The enormous volume of water scoured and eroded the landscape, carrying soil and rock material that was laid down or left behind where water velocity decreased. Deposits of large rounded boulders of basalt, ranging from three feet to ten feet in diameter are characteristic of this flood event, particularly in the Lower Portneuf Valley. Plucked from nearby basalt flows and rounded during several miles of transportation by the flood, these boulders were dropped from the flood waters into deposits up to 300 feet thick.  Click here for a more complete description of the geologic history of our aquifer.

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