r/MapPorn Jun 15 '21

12500 BC Lake Bonnevilke flood scours Snake River valley

Post image


u/alcesalcesg Jun 15 '21

For reference, that peak flow is 55x the total flow of the Mississippi river in Louisiana. In Alaska, the pleistocene Lake Atna existed under similar circumstances. When it flooded, peak flows are estimated to have been 3x larger than this event, or over 150x larger than the flow of the Mississippi.


u/SchizoidRainbow Jun 15 '21 edited Jun 15 '21

From the wiki:

About 14,500 years ago, pluvial Lake Bonneville in northern Utah reached its highest water level since its formation. The lake occupied the present-day basin of the Great Salt Lake, but was far larger, covering about 32,000 square miles (83,000 km2). (Surface the size of Lake Superior, but far deeper) As it rose the lake level caused seepage at, then breached, the ancient level of Red Rock Pass, a mountain pass at the headwaters of the Portneuf River, a tributary of the Snake River above present-day American Falls Reservoir. Ancient Red Rock Pass was the site of two alluvial fans descending from opposite sides of the notch, forming a natural dam. When the dam collapsed, it released a 410-foot-high (120 m) flood crest down the Portneuf River valley, also spilling into the neighboring Bear River valley. When it reached the Snake River, it eroded away a lava dam that had been at the site of the present-day American Falls, releasing a 40-mile-long (64 km) lake, American Falls Lake, that had formed behind the natural dam.

At the peak of the flood, approximately 33,000,000 cubic feet per second (930,000 m3/s) poured over the Snake River Plain at speeds of up to 70 miles per hour (110 km/h) and deposited hundreds of square miles of sediments eroded from upstream. The flood scoured the 600-foot-deep (180 m) Snake River Canyon through the underlying basalt and loess soil, creating Shoshone Falls and several other waterfalls along the Snake River. It also carved and increased in size many other tributary canyons, including those of the Bruneau River and Salmon Falls Creek. The flood then entered Hells Canyon, significantly widening the gorge. Its waters eventually reached the Pacific Ocean via the Columbia River.


u/Synensys Jun 15 '21

Can you imagine being some Native American dude just fishing away at the mouth of Columbia when that comes rolling down stream?


u/SchizoidRainbow Jun 15 '21

Odds are good there was one fishing on lake bonneville when the dam went. That’s a hell of a ride.


u/nrith Jun 15 '21

410 foot high surge?!


u/SchizoidRainbow Jun 15 '21

Yep. Did not pull the drain plug so much as break the side of the tub


u/ironic_name_here_ Jun 15 '21

Similar to the floods from Glacial Lake Missoula that created the channeled scablands of Washington State. Everything in its path wiped clean to the bedrock, carrying basalt boulders the size of 2 story houses as if they were pebbles.


u/huskiesowow Jun 15 '21

Flying to/from Seattle and Spokane is pretty cool. It's really easy to see evidence of the floods, the channeled scablands look like ripples in sand.


u/btsd_ Jun 16 '21

Potholes, around moses lake