Lake Lewis was a large transient lake (lasting only days to weeks) in the Pacific Northwest region of North America, formed by periodic Missoula Floods along the Columbia River paleocourse between 21,000 and 16,000 years ago.
Lake Lewis was formed when the restricted flow of waters[Flow was restricted by a hydraulic dam—a restriction to the flow rate caused by a narrowed reach in a river valley.] from periodic cataclysmic floods from Glacial Lake Missoula, pluvial Lake Bonneville, and perhaps from subglacial outbursts, backed up through the constriction formed by the Wallula Gap in the Horse Heaven Hills (southern Washington). Water also backed up further downstream on the Columbia River between Washington and Oregon, delaying the drainage of Lake Lewis. The water remained for a period of weeks before the flood waters drained through Wallula Gap, just southeast of the Tri-Cities area. Lake Lewis reached an elevation of about above sea level (today's sea level) before subsiding.
Lake Lewis also flooded the Yakima River, Walla Walla, Touchet River and Tucannon River river valleys.
Evidence for Lake Lewis
Wallula Gap
Wallula Gap is a large
water gap through basalt
in the
Columbia River basin just south of the confluence of the Walla Walla River and the Columbia River. The Wallula Gap, which has existed for many millions of years, was widened by the prehistoric flow of the Salmon-Snake and Columbia Rivers combined with the glacial waters that poured across the Channeled Scablands during the Missoula Floods. The Wallula Gap constrained the flow such that less than 1/5 of the 800 km
3 of water per day entering could be discharged. As a result, the floods filled the Pasco Basin and formed, for a short period, Lake Lewis. The large volumes of flood water passing through the gap contributed substantially to the erosion of the gap, as is evidenced by the shear walls and of scab-features such as the "Sisters."
Touchet Formation
Lake Lewis backflooded into the
Yakima River, Walla Walla,
Touchet River and
Tucannon River River Valleys. In these relatively calm arms of the lake, the slackwaters were thick with suspended materials eroded from the scablands above. Some of the suspended materials settled out, creating thick Touchet Formation layers which are found throughout these valleys.
Glacial erratics
The maximum elevation of the flood, as established by other indications, is confirmed by
, which were stranded on the slopes of the Horse Heaven Hills and other elevated regions in the mid-Columbia at elevations of up to above sea level.
[ Lewis and Clark's Columbia River - Missoula Floods ]
There were several long ridges (Saddle Mountains, Frenchman Hills, and Rattlesnake Mountain) that were above flood level. Peaks like the Badger, Candy, and Red Mountains were islands.
[ The Friends of Badger Mountain (a nonprofit organization dedicated to preservation of and access to one of the regions mountains) overview ][ The Friends of Badger Mountain have mapped the erratics found and posted them at this link. ][ Northwest Science & Technology, Spring 2004 Issue ] At this level, much of the Columbia Basin would have been submerged.
External links
