Missoula Floods
A Geologic History (~15,000 years ago)
Enormity of Floods
Imagine 520 cubic miles of turbulent water roaring down the Columbia River Gorge carrying large boulders and heavy sediment, scraping the land as it rages west. It reaches the Willamette Valley where some of it diverts south, pouring out onto what is now the known as the City of Roses, Portland, Oregon. To give context, 520 cubic miles of water is larger than Lakes Erie and Ontario combined; and in the Columbia River Gorge, that much water would reach depths of 1100 feet and speeds of 70 mph. This means that water would reach up to the summit of Castle Rock in the Columbia River Gorge!
Image showing Castle Rock in Columbia River Gorge.
If a flood this massive would happen today, even the tallest buildings downtown Portland would be almost entirely submerged underwater.
Image showing computer simulation of downtown Portland during Missoula Floods.
Significance of Floods
So why do we care about these cataclysmic floods, which happened before people started settling the lower Willamette Valley? For one, it can be helpful to understand how the landscape was formed for watershed managers to make informed decisions, especially regarding flood control. The image below shows the flood paths using light detection and ranging data (LiDAR imaging) (Burns & Coe, 2012). It has been discussed by some Portland City Engineers that the mainstem of Johnson Creek could have been pushed south from fluvial scouring.
LiDAR showing Missoula Flood flow path in Portland Metro Region (Burns & Coe, 2012).
Details of Floods
Cordilleran ice dam
This story describes how the landscape of the Pacific Northwest and more specifically, the Johnson Creek basin in SE Portland, was formed. These historic floods originated from a glacial lake approximately 12,000-15,000 years ago. During this period, the Cordilleran ice sheet covered Canada, and crept down into parts of Washington, Idaho, and Montana. The glacier spread its fingers down into a valley where the Clark Fork River empties into Lake Pen Oreille in Northern Idaho, there it created a mountain of glacial ice and eventually, an ice dam (Site #1 on map).
Glacial Lake Missoula
The ice dam grew to over 2,000 feet tall, backed up the Clark Fork River, and formed what was known as Glacial Lake Missoula (Site #2 on map). Over time as the lake grew, the ice dam broke from increasing pressure igniting the catastrophe.
Photo (above) showing present day Missoula and Mount Sentinel, which was almost completely underwater during the massive floods of this era.
Columbia River Gorge
The flood path traveled west through much of the Columbia River Gorge covering 16,000 square miles (Kartevold & Stanley, 2001). Some of the floodwater diverted at the Willamette Valley reaching as far south as Eugene, the rest of it continued west pouring out into the Pacific Ocean.
The catastrophic flooding that carved the landscape throughout the Pacific Northwest occurred at least 40 times, flowing 60 times the rate of the Amazon River (Allen et al., 2009).
Click here to watch a documentary on these historic floods and how they shaped the Pacific Northwest.
Image: artwork by Stev H. Ominski (site)
Flood flow paths
This map shows another image of the floods' flow path (blue arrows) and how Portland's landscape (and Johnson Creek watershed) was formed by these floods.
Several gravel bars and terraced land features were formed by the glacial floods that now shape the Johnson Creek watershed and impact its flooding characteristics (Allen et al., 2009). A kolk depression, which is a sunken landform caused by an underwater vortex (Neuendorf et al., 2005), was formed at the west end of Kelly Butte from a channel that cut into the north side of the butte. Similar activity occurred at Powell Butte, creating a kolk depression, which is now between 128th and 136th Streets (Allen et al., 2009). Drainage into this depression formed a seasonal lake, known by locals as Holgate Lake, that has caused flooding along Johnson Creek every few years (Allen et al., 2009).
