The Bonneville flood was a catastrophic event in the last ice age, which involved massive amounts of water inundating parts of southern Idaho and eastern Washington along the course of the Snake River. Unlike the Missoula Floods, which also occurred during the same period in the Pacific Northwest, the Bonneville flood happened only once.
Cause and events
About 14,500 years ago (radiocarbon dating, 17,400 years ago calendar, "calibrated" dating),
pluvial lake Lake Bonneville in northern
Utah reached its highest water level since its formation.
[Spedden, R.H., 2021, Part II - Lake Bonneville and The Wasatch Fault] The lake occupied the present-day basin of the Great Salt Lake, and was far larger, covering about . 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
descending from opposite sides of the notch, forming a natural dam.
When the dam collapsed, it released a
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 lake, American Falls Lake, that had formed behind the natural dam.
At the peak of the flood, approximately poured over the Snake River Plain at speeds of up to and deposited hundreds of cubic miles of sediments eroded from upstream.
A 2020 hypothesis presented evidence that Lake Bonneville achieved a stable outflow for possibly a thousand years leading up to the Bonneville Flood and then a massive, multi-segment earthquake on the Wasatch Fault caused surging and tsunami in Lake Bonneville with a surge wave over high. This surge carried up into the Cache Valley and resulted in the failure of the natural dam at the Zenda threshold just north of Red Rock Pass.[Spedden, R.H., 2020 Evidence of Earthquake-Induced in Pleistocene Lake Bonneville, Geological Society of America – annual meeting – Session: T166. Part I - Lake Bonneville and The Wasatch Fault]
Legacy
Although the peak of the flood lasted a few weeks at most, erosion at Red Rock Pass continued for a few years before water ceased to spill over. The flood drained the top of Lake Bonneville, which constituted about of water, and lowered the lake level to a stage known as the Provo shoreline. The flood transformed the Snake River Plain into a series of channeled scablands resembling the
Columbia Plateau. Also left by the flood were the many "melon" boulders distributed throughout the canyons in the Snake River Plain. According to some geologists, the total volume of the Bonneville flood was actually greater than any individual one of the
Missoula Floods, although the Missoula floods released more water as a whole,
and at least one had a much higher peak flow rate.
Much of the sediment scoured by the flood was deposited near the mouth of the Snake River. It now lies beneath about 20 layers of Missoula Floods deposits.
See also
