The Bonneville Flood and Idaho’s Red Rock Pass

September 02, 2014



Photographer: Ray Boren
Summary Author: Ray Boren

What a sight it would have been, on a late Ice Age day 14,500 or so years ago, to see nature pull the first plug on the immense Pleistocene inland sea we retroactively call Lake Bonneville. It happened at Red Rock Pass, between modern-day Downey and Preston, Idaho, as shown in the photo above, taken on August 18, 2014. At the time, the locale was the northern extent of Lake Bonneville, a 32,000 sq mi (82,879 sq km), mountain-rimmed body of water as big as one of today’s North American Great Lakes. The lake was named for a French-born American military officer and explorer, Capt. Benjamin Louis Eulalie de Bonneville, by later 19th-century topographer G.K. Gilbert, one of the first scientists to recognize the importance of the vanished lake’s still-visible prehistoric features.

Lake Bonneville, more than 1,000 ft (305 m) deep in places, covered most of the northwestern portion of today’s state of Utah, overlapping into modern-day Idaho and Nevada. Red Rock Pass was the site of a natural dam consisting of mostly unconsolidated alluvial fans amid Paleozoic shale, limestone and dolomite rock.

Geologists believe lava flows to the north re-routed the Bear River, which flows north and west from the Uinta Mountains to the east, turning it south, toward Lake Bonneville. For thousands of years, the big lake had filled, at various levels, a land-locked basin with no natural outlets to the sea. With the Bear River’s contribution, the ancient lake’s surface levels rose even more. Beginning on that epic day, water decisively topped, overflowed and eroded the Red Rock formations, turning the spot into a spillway. Most of the resulting downstream carving and erosion occurred in a matter of days or weeks, but the discharge probably continued for more than a year, investigators say. All that was left is today’s comparatively shallow Great Salt Lake.

Photo Details: Top - Camera: NIKON D3200; Lens: AF-S DX VR Zoom-Nikkor 18-55mm f/3.5-5.6G; Focal Length: 22mm (35mm equivalent: 33mm); Aperture: f/11.0; Exposure Time: 0.0020 s (1/500); ISO equiv: 220; Software: iPhoto 9.5.1. Bottom: Same except: Focal Length: 20mm (35mm equivalent: 30mm); ISO equiv: 250.