LATE PLEISTOCENE OUTBURST FLOOD FORMATION OF LAKES LOCATED ALONG SMALL DRAINAGE DIVIDES WITHIN THE NORTHERN COLUMBIA PLATEAU

Late Pleistocene outburst floods scoured out the basalt bedrock, leaving behind numerous lakes within the Channeled Scablands of the Columbia Plateau region. The location and formation of these scoured depressions have been directly attributed to the characteristics of the basalt bedrock and the hydrodynamics of the outburst flood channel flow. The larger, elongate lakes are almost always located in the mid-channel, high velocity zone. Some of these elongate lakes are serpentine, due to the highly jointed bedrock in linear fracture zones, while the more linear lakes are formed by headward erosion at flow contacts.

This is not the case in the northeastern Columbia Plateau, in the hilly region south and west of the Spokane West Plains. Moderately large, elongate lakes in this area formed not in mid-channel, but along the margins of the scabland channels near the headwaters of the local drainage divides. Water well logs and local borrow pits reveal that a deeply weathered saprolitic zone a hundred or more meters in thickness developed within the contact zone of the basalt and underlying granite. These zones of deep weathering within the basalt and underlying bedrock in the hilly divides adjacent to the Spokane West Plains were preferentially scoured by outburst floods, forming elongate lakes along the flanks of the scabland channels.

This study includes Audubon, West Medical, Medical, Silver, Granite, Willow, Meadow, and Fish lakes, located in a region between Cheney and Reardan. Each of these lakes was formed by outburst floods, which scoured out the highly weathered contact of basalt and adjacent older bedrock. Many of these lakes formed at or near local drainage divides. Audubon Lake has the distinction of occupying a depression at the head of three drainage divides: Crab Creek to the west, Spring Creek Canyon to the north, and Deep Creek to the east. West Medical, Medical, and Silver lakes formed along the flanks of the scabland channel, while Granite, Willow, and Meadow lakes formed at the head of a north-flowing paleochannel, but now drain in the opposite direction.