DescriptionThe same violent glacial floods which carved out the channeled scablands in eastern Washington helped to form the Devil’s Canyon coulee. North of Lower Monumental Dam on the Snake River, Devil’s Canyon linearly extends five miles toward the town of Kahlotus, WA. Once a floodway channel from Washtucna Coulee to the Snake River Valley, Devil’s Canyon is a recessional cataract canyon, meaning that the floodwaters over-topped Washtucna Coulee and eroded the basalt and undercut the edges of the channel, forming the canyon.
The Purcell Lobe of the Cordilleran ice sheet formed an ice dam which plugged up the Clark River Valley, forming glacial Lake Missoula. Periodically, the dam failed and the lake drained, releasing vast amounts of floodwaters to the south and southwest (Weis and Newman, 1976). Nearly four hundred feet in elevation separated the waters of the Washtucna Coulee from the Snake River, the resulting fluid pressure leading those floodwaters to preferentially erode along the fractured basalt and spillover (Bjornstad et al., 2007). The wall rock of Devil’s Canyon also resulted from flooding: the basalt flows that make up the Wanapum and Grande Ronde Basalt formations of the Columbia River Basalt Group formed 15 to 16 million years ago, flooding across much of Washington, Idaho, and Oregon (Reidel et al., 2002). It was fractures in these basalt flows that the floodwaters spilling over from Washtucna Coulee were able to exploit in their deviation toward the Snake River.
A gravel mound projects from the mouth of the canyon, extending a third of the way into the Snake Canyon (Bretz et al., 1956). Kahlotus Lake once existed within the Washtucna Coulee to the north of Devil’s Canyon; since the water table lowered, the lake has vanished (Bjornstad 2006).