Lake Lewis
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| Lake Lewis | |
|---|---|
 | |
| Name | Lake Lewis |
| Type | Pluvial lake / temporary flood lake |
| Inflow | Columbia River, Yakima River, Snake River (via floodwaters) |
| Outflow | overflow to Columbia River downstream channels |
| Basin countries | United States |
Lake Lewis Lake Lewis was a transient proglacial flood lake that existed in the Columbia Basin during late Pleistocene catastrophic flooding. Formed by ice-dammed and topographically constrained waters, it occupied parts of present-day Washington (state), inundating river valleys and creating depositional features influential to Pacific Northwest geomorphology. The lake is central to understanding connections among the Missoula Floods, Spokane Floods, and regional drainage reorganization during deglaciation.
Geography and Formation
The temporary basin formed where floodwaters impounded against glacial and volcanic highlands in the Columbia Plateau, lying between the Cascade Range and the Blue Mountains. Extents encompassed present-day Yakima County, Benton County, Franklin County, and parts of Grant County. Topographic control came from ridges including the Rattlesnake Hills and the Saddle Mountains, while lava flows of the Columbia River Basalt Group influenced basin morphology. Ice lobes from the Cordilleran Ice Sheet and ice-dammed channels near Okanogan altered drainage, producing backwater inundation upstream of constrictions like the Wallula Gap. Paleogeographic reconstructions use elevation markers such as the Ringold Formation surfaces and terraces along the Yakima River to delineate water levels.
Ice Age Floods and Missoula Floods Connection
Lake Lewis developed in response to episodic discharge events linked to the Missoula Floods generated by failures of the ice dam on Glacial Lake Missoula and rerouted flows from the Spokane Floods. Gigantic outburst floods traversed the Channeled Scablands, passed through the Grand Coulee and Dry Falls region, and were funneled into the Columbia River corridor. The convergence of flood paths at constrictions such as Wallula Gap and backwater effects produced the standing water body documented as Lake Lewis. Correlations between far-field gravel suites, varve stratigraphy, and scabland erosional features link the lake to multiple flood pulses associated with late Pleistocene deglaciation episodes dated by radiocarbon dating and optically stimulated luminescence techniques.
Paleoenvironment and Hydrology
Paleoenvironmental evidence indicates episodic lacustrine conditions with sharp transitions to fluvial scabland erosion, affecting regional Pleistocene ecosystems including refugia for taxa displaced by ice advance. Hydrologic modeling of peak discharges—estimated in the range of tens of millions of cubic meters per second—uses constraints from cross-sectional geometry at locations like Wallula Gap and deposit volumes in the Pasco Basin. Water temperatures, salinity, and residence time were controlled by meltwater provenance from the Cordilleran Ice Sheet and routing from basins such as Glacial Lake Columbia and Glacial Lake Missoula. Paleobotanical indicators from nearby sediments, including pollen assemblages correlated to Younger Dryas-age fluctuations, help reconstruct vegetation shifts in the Columbia Basin and adjacent Palouse landscapes.
Sedimentology and Deposits
Lake Lewis left a suite of lacustrine and slackwater deposits: rhythmites, coarse slackwater gravels, and fine silts preserved on terraces and in the Ringold Formation sequences. Sediment facies vary from imbricated cobble bars near paleochannels to laminated silts in protected embayments such as the area now occupied by Pasco and Richland. Correlative deposits include the Touchet Beds and high-level slackwater gravels correlated with ice-retreat chronologies. Petrographic analysis links clasts to source terranes including the Bitterroot Range and Rocky Mountains, reflecting long-distance transport. Stratigraphic relationships with Loess mantles and palagonite-bearing horizons assist in correlating flood events across the Snake River Plain and Columbia Basin.
Human History and Archaeology
Although the lake predates extensive human modification, landscapes shaped by Lake Lewis influenced postglacial migration routes of indigenous populations ancestral to groups now associated with the Yakama Nation, Nez Perce Tribe, Confederated Tribes of the Umatilla Indian Reservation, and Colville Confederated Tribes. Archaeological sites on high terraces and at archaeological localities near Kahlotus and Moses Lake preserve lithic scatter and resource procurement loci that reflect occupation of reworked habitats after inundation. Euro-American exploration and settlement by figures associated with the Oregon Trail, Lewis and Clark Expedition, and later Homestead Acts encountered terrain modified by the lake’s legacy, influencing route choices and agricultural development in the Yakima Valley and Columbia Basin Project irrigation districts.
Ecology and Modern Remnants
Modern ecological assemblages occupy relict habitats including riparian corridors along the Yakima River and saline flats in former lake depressions such as the Southeastern Washington playa systems. Remnants of Lake Lewis appear in contemporary geomorphic features: terrace benches, slackwater deposits accessible in Hanford Reach National Monument viewpoints, and ponded wetlands within the McNary Reservoir influence zone. Conservation efforts by agencies such as the U.S. Fish and Wildlife Service and the Washington State Department of Ecology engage with habitats shaped by Pleistocene flooding to manage migratory fish corridors for species like Chinook salmon and to preserve archaeological and geologic exposures for scientific study. Interpretive trails and educational resources at sites near Ice Age Floods National Geologic Trail units inform public understanding of the lake’s role in Pacific Northwest geohistory.
Category:Pleistocene lakes Category:Geology of Washington (state) Category:Glacial lakes of the United States