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Missoula Flood, Lake Lewis rhythmites near Zillah Washington Ice-rafted erratic cluster on Goose Hill. Ice-rafted erratic on Rattlesnake Slope. Badger living in slackwater flood deposits, Lake Lewis floor. Geologist/Author Bruce Bjornstad on Badger Coulee erratic. Home Page Not much time? ... Click here for a quick Ice Age Floods summary Ice Age Floods Feature of the month For many years one man understood the clues but no one would listen Glacial Lake Missoula Channeled Scablands of Eastern Washington Temporary Lake Lewis Columbia Gorge Explore the variety of features created by the Ice Age Floods Columbia River Basalt Group The Pleistocene Grand Coulee Dam and the Columbia Basin Irrigation Project Washington Wines Ice Age Floods Institute

Lake Lewis

Ice Age Floods Wallula Gap by Stev Ominski

Ice Age Flood as it began to recede through Wallula Gap. Painting by Stev H. Ominski
(View looking northwest to massive Lake Lewis)

"Like roads to Rome, all scabland rivers led to Pasco Basin."

J Harlen Bretz


When Lake Missoula's ice dam failed, floodwaters gushing to the southwest quickly overwhelmed or bypassed everything that stood in their way--until these torrents funneled into the Pasco Basin. The result was Lake Lewis. Named for explorer Meriwether Lewis, this lake might be described as an Ice Age version of a gigantic stormwater runoff retention pond. This huge pond filled and emptied dozens of times.

The choke point that created Lake Lewis was the Horse Heaven Hills region just south of present-day Pasco, Wash. This range of basalt hills, which includes ridges whose elevations reach 2,000 feet or higher, is breached by the Columbia River at Wallula Gap. The gap is the only natural outlet for stream flows from the entire Columbia River Basin of eastern Washington. These terrain features were in place long before the Lake Missoula floods of 15,000-20,000 years ago. [The National Park Service has named Wallula Gap a National Natural Landmark, owing to its geographic significance.]

Constriction at Wallula Gap created Temporary Lake Lewis.
Wallula Gap - Floodwater overtopped high point on right by 80 feet!

During each of the Lake Missoula flood episodes, as much as 200 cubic miles of water per day might accumulate in the low country north of Wallula Gap. This created a temporary lake level of 1,200 feet elevation. Since Wallula Gap could only disgorge about 40 cubic miles of water daily, one or two weeks might elapse before Lake Lewis' flood water was drained from the basin. Outflows from the gap also were retarded as the Columbia River was slowed at similar choke points downstream--such as the narrows at Crown Point, east of Troutdale, Ore.


Lake Lewis covered the entire Pasco Basin, most of the Quincy Basin, and Yakima Valley, much of the Ahtanum Valley, and the valley of the Walla Walla River and its main tributaries. Floodwaters also backed up the Snake River well beyond present-day Lewiston, Idaho. An estimated 3,000 square miles of land were under water.

The locations later occupied by the region's principal cities all were submerged. Pasco and Richland would have been under 800 feet of water, and the only dry ground in the nearby area was atop a handful of promontories--Badger, Candy, and Red mountains, as well as Goose Hill. [Geologists refer to these high points as the "Lake Lewis Isles".]

Lake Lewis Isles by Bruce Bjornstad
- Image by Bruce Bjornstad -

Today's Walla Walla would have been covered by 50 feet of water, and Yakima would have been inundated by overflows backing up through Union Gap into the Ahtanum Valley. The vicinities of Quincy and Ephrata would have marked the temporary lake's northern limit, while much of the Saddle Mountains and Rattlesnake Hills, along with the upper elevations of Frenchman Hills, remained above the water line.

Geologic evidence suggests that Lake Missoula floods occurred at least 89 times, and Lake Lewis would have functioned as a temporary impoundment during most if not all of these events. But many geologists believe that there were many more floods--some of them from sources other than Lake Missoula, and others taking place at much earlier dates than the Lake Missoula floods. If so, Lake Lewis probably existed at these times as well, because the Columbia Basin, its rivers, the Horse Heaven Hills, and Wallula Gap all predated the Pleistocene Ice Age.

Lake Missoula Ice Age Flood bar erratic.
Teresa Foster examines erratic boulder stranded below Sentinel Gap - [Pasco Basin].

Rattlesnake Slope Erratic
- Click to enlarge -
erratic: a misplaced, oversized boulder that is of a different rock type than the underlying bedrock of the region. In the Mid-Columbia Basin, erratics were rafted in on icebergs and came to rest in slackwater areas during Ice Age flooding. (Bjornstad)

- Lake Lewis erratics shown in three images at top of this page -
Ice Age Floods rhythmites, Pasco Basin.

Hiker stands atop Lake Lewis rhythmites - Hanford Reach National Monument.

Dark layers shown consist of sand that settled to floor of Lake Lewis during Ice Age Flood events. Windblown material that accumulated between floods make up light-colored bands.

Columbia River top left - Saddle Mountains in the distance.

Red Fox along Columbia River near Pasco, WA. title=
Red Fox - Columbia River bank.

Click arrow above to play short video

Burlingame Canyon (Ravine) clue to Lake Missoula Ice Age Floods in Walla Walla Basin.

Rhythmites and volcanic ash layers exposed in Burlingame Canyon (private property near Touchet, WA.) provided evidence of at least 40 large Ice Age Floods to geologists in the early 80s. -Bruce Bjornstad photo

Ice Age Flood rhythmites (pictured top left of this page) visible from Interstate 82 near Zillah, WA.

Rhythmites (Touchet beds) and clastic dikes.
Clastic dikes within rhythmites (Walla Walla River).
Priest Rapids Bar

J Harlen Bretz noted "Gravel bars are the Basin's specialty" in 1959 Pasco Basin description.

. Ice-rafted erratics pushed to perimeter of Richland, WA housing development.

clastic dike: a discordant feature that cuts across sedimentary beds and contains the sedimentary material it transects. Believed to be the result of cracking and sediment filling or injection due to earthquake shaking during or soon after Ice Age flooding. (Bjornstad)

rhythmite: a graded sedimentary bed, several inches to several feet thick, deposited under slackwater conditions, especially in backflooded valleys during Ice Age flooding. Some believe that each rhythmite represents a separate Ice Age flood from Glacial Lake Missoula. (Bjornstad)

Lake Lewis map.

Lake Lewis at approximate high water level - 1,250 ft.

Major Ice Age Flood tracts - brown arrows

Tundra swans at McNary National Wildlife Refuge.

Tundra swans at McNary National Wildlife Refuge

Huge flocks of Canada geese and mallard ducks (Canada geese and mallard shown with swans above) use McNary National Wildlife Refuge in the lower Pasco Basin. The 15,000 acre refuge and surrounding irrigated farmlands are important to migrating waterfowl. The U.S. Fish & Wildlife Service estimates more than half the mallards in the Pacific Flyway overwinter at some time in this portion of the Columbia River Basin.

Hanford Reach Interpretive Center Tri Cities, WA.

Hanford Reach Interpretive Center on the Columbia River

The Hanford Reach Interpretive Center opened during the summer of 2014 - View from Pasco, WA. Some of the topics featured in displays are: Ice Age Floods, Ringold Formation, Columbia River Basalt Group, Hanford site history, wildlife of the Hanford Reach and area agriculture including the Columbia Basin Irrigation Project.

Respected Columbia Basin fieldman Dave Entel inspects seed placement in Pasco Basin field.
Respected Columbia Basin fieldman Dave Entel inspects seed placement in Pasco Basin field.
Pasco Basin pea harvest.
Pasco Basin pea harvest.

Hedges Estates.
Many of Washington State's premium wines are created with grapes sourced from vineyards within the Lake Lewis basin.

Lake Lewis Marker, Badger Mountain trail.
Marker placed at 1,250 Elev. along Badger Mountain trail.

Between fillings - Farmers produce crops that help feed the world on the floor of Lake Lewis.

Without irrigation water, farmers in the Quincy and Pasco basins would be limited to "dry land" crops due to the fact that much of the areas 6 to 10-inch annual precipitation arrives outside of the growing season.

Following development of the Columbia Basin Irrigation Project and other systems, the area once inundated by the waters of Lake Lewis encompasses some of the most productive farmland in the world. Slackwater flood sediments mixed with windblown deposits combine to create fertile well-drained soils with the capacity to produce high value crops.

Area food processors package and market the quality fruits, vegetables and grains produced during the region's long growing season . Well established transportation systems move products produced to domestic markets as well as world markets through the ports of Vancouver, Portland, Tacoma and Seattle.

Pasco Basin sweet corn harvest.
Sweet Corn harvest - Pasco Basin.

Lake Lewis Wikipedia page.

Ice Age Flood Features in the Vacinity of the Pasco Basin and the Hanford Reach National Monument.

Tri Cities Area Ice Age Floods Tour Map.

All photos by Tom Foster unless otherwise noted.*
*Photo at top right (Bruce Bjornstad on Badger Coulee erratic) by Gary Kleinkenecht.

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