Prehistoric Lake Bonneville once filled much of the Great Basin. About 14,500 years ago, a natural gravel dam at Red Rock Pass gave way, and a flow five times the volume of the Amazon River rushed down Marsh Creek, through Portneuf Narrows and the present site of Pocatello, then on down the Snake and Columbia Rivers. Within a few months the lake was lowered more than 300 feet. The ancient shorelines from before and after the flood can be seen on the mountains near Salt Lake City and Ogden, Utah. The second largest flood known to have occurred on the earth, it scoured the basalt lava flow in Marsh Valley and at Pocatello’s Ross Park. The lava was completely eroded from the Portneuf Narrows area. The flood-washed boulders in the Square were found during construction.