About 14,500 years ago the Alluvial Dam to Lake Bonneville broke at Red Rock Pass,
about 40 miles south of here. The flood that followed lasted about a year, with a peak flow
for two months at about one million cubic meters per second. This was about 500 times
the maximum discharge of the modern Snake River and 7 times the average discharge of
the Amazon River. The entire volume of the flood was 4,750 cubic kilometers.
Portneuf Narrows was a constriction for flood waters, which piled up to over 300 feet deep
here. This area is the source for boulders, up to 12 feet in diameter, strewn
through the west side of Pocatello.