In basic terms, geology is the study of the earth. That is a simple definition for a complex subject that fascinates me. I enjoy hiking and have taken many walks along the Portneuf River, the lava flows from McCammon into Pocatello, and in the Portneuf Gap.

The lava flows offer stunning though treacherous scenery and originated from the Bancroft area approximately 600,000 years ago. Many petroglyphs can be found carved on the basalt from McCammon all the way to the west end of Pocatello. Exemplars of some of these carvings were placed onto the face of the petroglyph bench on the Pocatello Greenway near Taysom Rotary Park. Those images can be viewed from the highway as you drive by the bench.

Beautiful rock specimens were carried into the Portneuf Gap and Pocatello valley. These stones tell many tales, and we placed plenty of them in the Kizuna Garden at the Pocatello Airport thanks to permission from the Bureau of Land Management and Ed Quinn’s generosity.

Walking along the Portneuf River is usually peaceful. That hasn’t always been the case. There was a brief period when the largest flash flood in world history (outside of Noah’s biblical deluge) flowed through our valley. This cataclysmic event occurred from the partial draining of Lake Bonneville approximately 14,500 (radiocarbon) years ago — practically a day in geologic time for a planet that is billions of years old.

The Great Salt Lake in Utah is still the largest lake west of the Mississippi River and is all that remains of what was Lake Bonneville, a water body that once covered most of Northwestern Utah and extended into Idaho and Nevada.

The breach that resulted in this roaring flood and shrinking of Lake Bonneville occurred at Red Rock Pass in Idaho. Drive south past Downey on highway U.S. Route 91, and you will eventually reach the spot which has a historic marker. Heading into Utah, you can view the Wasatch Mountains where prior shorelines from Lake Bonneville are evident.

When Lake Bonneville breached at Red Rock, 350 feet of the lake dropped in a matter of eight weeks. The flood moved toward Downey, Marsh Valley and into the Portneuf Gap, before flowing through the Pocatello Valley. The wall of water then moved westward overpowering the Snake River. At its height, the water flowed 500 times larger than the highest recording for the Snake River at Idaho Falls.

I try to imagine what that looked like when hiking mountains in the Portneuf Gap, which is the narrowest spot on the water’s journey before reaching the Snake River. It was hundreds of feet deep — a flow that boggles the mind. The force ripped away parts of the lava ridges that originated from near Bancroft.

The Amazon River (the world’s largest by far) averages a discharge of 170,000 cubic meters per second at its outlet, while the Bonneville flood flowed at about a million cubic meters per second for several weeks. This behemoth roared through Pocatello, dwarfing the world’s largest river. That’s pretty cool stuff, although the loss of animal and plant life was likely catastrophic.

Many area residents generally know this history of when Lake Bonneville broke out toward the sea. Evidence exists throughout Pocatello in the giant boulders that were rolled into our valley. They are often unearthed when people excavate for construction projects.

My former law firm owned a building on Arthur Street that we expanded, and our contractor encountered a Bonneville “pebble” so large that the only option for its removal was using dynamite. Lawyers being risk averse, we opted to construct the corner of our building on top of the rock as the safest course of action.

Some of the boulders and gravel that flowed into Pocatello continued onwards toward Michaud Flats where a “flat-topped delta” was deposited. A quarry close to the Pocatello Airport was supposedly created when the airport was constructed. It is partially filled with gravel and boulders excavated when the airport was built. Stones were also placed in the Kizuna Garden from this quarry, courtesy of the Bonneville flood.

The information for this column was drawn from numerous sites, but primarily from the book “Rocks, Rails & Trails” by Paul Link and Chilton Phoenix. Their book offers fascinating insights into local geology, and details how Pocatello’s valley once briefly hosted the greatest “river” on earth. That had to have been quite a show.

Jesse Robison is a Pocatello native educated in Idaho. He works as a mediator and insurance claim consultant, but his passion is public art. Robison has spearheaded art improvements throughout Pocatello, and serves on the Bistline Foundation.