DOGAMI Interpretive Map Series

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IMS-36, Missoula floods - inundation extent and primary flood features in the Portland metropolitan area, Clark, Cowlitz, and Skamania Counties, Washington, and Clackamas, Columbia, Marion, Multnomah, Washington, and Yamhill Counties, Oregon, by William J. Burns and Daniel E Coe.

Awarded WAURISA (Washington State Chapter of The Urban & Regional Information Systems Association) 2013 Conference Best Cartographic Design Award.

View/download PDF: onscreen resolution (9 MB) | print resolution (34 MB)

50 x 38 inches; scale 1 inch = 8,200 feet (1 inch = 1.553 miles)

In the early 20th century, geologist J Harlen Bretz suggested that the large-scale fluvial features he saw in the Pacific Northwest were caused by catastrophic flooding. When Lake Missoula waters breached an ice dam, some of the largest floods known on earth discharged nearly 350 million cubic feet per second—over 1,000 times the average discharge of the current Columbia River. The dam-and-breach process was repeated at least 40 times over 3,000 years as the ice sheet advanced and retreated. With each breach, huge volumes of water raced across eastern Washington, eroding and depositing material and creating the channeled scablands noted by Bretz before converging into the Columbia River Gorge and scouring out the current shape of the gorge. As the floods rushed out of the constricted Columbia River Gorge and entered the Portland Basin, the water gushing across the landscape created much of the large-scale geomorphology that exists today. Geomorphic features include huge primary channels that are in some places scoured down to bedrock, large sand and gravel bar deposits, and extensive fluvial sculpting throughout the basin. Using new technologies like light detection and ranging (lidar), from which high-resolution land surface imagery can be made, scientific investigation of these floods and their aftermath continues today.

In 2007-2011, airborne light detection and ranging (lidar) data were collected throughout the Portland Basin. Lidar-derived imagery allows us to map the shape of the surface of the earth in resolution never before seen, especially in places that are densely vegetated, such as the Portland Basin.

IMS-36 contains the plate in onscreen and print resolutions and a readme.txt file.

Missoula floods map of Portland OR area