Toward the end of the Last Glacial Maximum, catastrophic flooding surged down parts of the Columbia River watershed as natural dams creating large lakes failed. Some originated from Glacial Lake Missoula, which was created by a large dam of ice in Montana and Idaho that periodically failed. These, often termed Missoula Floods are probably the most known in Eastern Washington and affected areas downstream but they likely weren’t the only ones.
Along the way, water was impeded by terrain creating several large temporary lakes such as Lake Lewis covering the Tri-Cities and another covering the Willamette Valley, including Portland. These areas would have been flooded to some extent by water originating in different spots. Upstream of the Tri-Cities, however, flood locations varied.
Since the Missoula Floods are reasonably well known, I won’t cover them extensively here. Instead, I’ll discuss two other possible flood origins. These are in addition to the Missoula Floods, not instead of the Missoula Floods.
Before diving in, an important note: A few YouTube videos are cited here. These are videos produced by geologists themselves. As always, use caution on YouTube and other social media to ensure the info you’re receiving is accurate.
BRITISH COLUMBIA
Some flooding may have originated from the Okanagan (using the Canadian spelling here because it’s in Canada) and Rocky Mountain regions of interior British Columbia. The mechanism of how this water drained into the Columbia Basin is disputed, but the evidence was noted in a 1999 paper as having been seen in flood deposits near Touchet between the Tri-Cities and Walla Walla.
That paper describes water from an unknown large subglacial lake surging into Glacial Lake Missoula, possibly adding the water needed to cause the ice failure to create the flood. This is just one possible source of what appears to be sediment originating from British Columbia in the Touchet beds.
Nick Zentner, a geology professor at Central Washington University, detailed another theory in a recent podcast episode of his. This one focuses more on bringing water down through the Okanogan (US spelling) Valley to meet the Columbia River near Brewster north of Wenatchee among other locations along the edge of the ice sheet.
Similar to the BC to Lake Missoula theory, this is believed to have traveled under the ice sheet from British Columbia but unlike that one, this would deliver the water directly to the Columbia Basin. Robert Young, professor at University of British Columbia – Okanagan, described in 2007 one possible source as being meltwater created during subglacial volcanic eruptions in the British Columbia interior.
Landforms likely created by significant amounts of flowing water as well as subglacial volcanic eruptions can be seen in and near the British Columbia Okanagan.
LAKE BONNEVILLE (UTAH)
The other source of flooding is less disputed than the British Columbia theory. Around the same time as the glacially-sourced floods originating in Montana and possibly Canada, Lake Bonneville was covering a huge amount of northern Utah and adjacent parts of Idaho and Nevada. The Great Salt Lake is the largest remaining waterbody from Lake Bonneville in existence today.
Rather than an ice dam, a natural dam of sediment at Red Rock Pass south of Pocatello held the water from Lake Bonneville back. Approximately 15,000 years ago water overtopped the natural dam and surged into the Snake River Plain additionally causing another lake to overtop its natural barrier to add to the flood.
Surging down the Snake River Plain, the Bonneville flood created channeled scabland terrain like what’s seen in the northern half of the Columbia Basin and deepened several existing canyons – most notably the Snake River Canyon. It also widened Hells Canyon on its way to the Pacific. Nick Zentner notes in a YouTube video that a lot of the sediment from this flood can be found under Missoula Flood deposits where the Snake River meets the Columbia in the Tri-Cities area.
It is possible that Lake Bonneville just overtopped the dam at Red Rock Pass when it reached its highest point. Research presented at the 2020 Geological Society of America annual meeting (see associated YouTube video) discussed evidence that the overtopping and subsequent flood were caused by a tsunami in Lake Bonneville caused by an earthquake on the Wasatch Fault in Utah.
While the glacially sourced ice age floods may have only lasted a few days or weeks, the Lake Bonneville flood would likely have been much longer lasting (though peak flooding would have only been a few weeks). Water may have flowed through Red Rock Pass for anywhere from a few to a thousand years.
The featured image at the top of this photo is of the Lake Lewis monument on Badger Mountain in Richland, sourced from Wikimedia Commons.
