Red Rock Pass and Bonneville Floods, Idaho

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

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Red Rock Pass

	Although this highway marker tells of the outlet of Lake Bonneville, it doesn't tell of the huge flood that happened when the lake cut the outlet. The Bonneville flood carried about 1 million cubic meters of water per second, three times the largest flood of the Amazon and 14 times the largest flood on the Mississippi, but still only about 5-6% as large as the largest Scabland floods.
	The rocks here are relatively soft, so when the lake finally topped the drainage divide about 14,500 years ago, the flow rapidly incised a deep channel and sent a megaflood down the Snake River. This flood, only a single event, was coeval with the Missoula (Scablands) floods, and there are places near the Idaho-Washington border where the Bonneville flood deposits are interbedded with Missoula flood deposits.
	Looking south at the pass.
	Streamlined outcrop in the channel.

A large rock knob in the channel is shown in detail in the map above. It appears that water scoured around both sides but incised the western channel deeper. A very shallow channel, too shallow to show up even at 5-meter contour resolution, is visible on aerial photos about along the white dashed lines.

Right and below: views of the rock knob.
	Left and below: looking north up the main channel.
	Left and below: looking south from the pass. Note the scattered knobs in the channel in the picture at right below.
Below: the southern end of the pass, at the northern terminus of Lake Bonneville.

Flood Bar Near Pocatello

	This ridge obstructing a valley east of Pocatello, where I-15 turns from east-west to north-south, looks at first glance like a moraine except this area was never glaciated.

Snake River Gorge at Twin Falls

 

The valley topography at Twin Falls (above) displays some interesting small scale Scablands topography, including a couple of fairly convincing miniatures of Dry Falls.

	Above: Perrine Memorial Bridge Left and below: Looking west. The scenery is very reminiscent of the Scablands.
	Looking east. The eponymous falls are out of sight around the bend

The World's Largest Floods, Past and Present: Their Causes and Magnitudes By Jim E. O'Connor and John E. Costa U.S. Department of the Interior 2004, U.S. Geological Survey Circular 1254

O'Connor, J.E., 1993, Hydrology, hydraulics, and geomorphology of the Bonneville flood: Geological Society of America Special Paper 274, 83 p.

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Created 20 August 2007, Last Update 08 June 2020