QUATERNARY HISTORY OF CANYON CARVING IN THE CHANNELED SCABLANDS

The Channeled Scablands landscape in eastern Washington, USA contains vast tracts of scoured bedrock and deep canyons carved by catastrophic outburst floods triggered by failure of the ice-dam that impounded Glacial Lake Missoula. In recent decades, much progress has been made in understanding the number and timing of floods that occurred during the Last Glacial Maximum (LGM). However, the response of the landscape to repeated flooding is poorly constrained, especially with respect to the possible role of canyon carving by outburst floods prior to the LGM. To constrain the processes, rates, and timing of landscape evolution in the Channeled Scablands, we analyzed topography, mapped terraces, re-constructed discharges, and dated bedrock surfaces and boulder deposits using cosmogenic nuclides. Many Scablands tracts contain numerous bedrock strath terraces that were abandoned by subsequent flows. Preliminary exposure dating results indicate there are likely a wide range of exposure ages, suggesting that canyon carving in the Channeled Scablands initiated prior to LGM. The results are consistent with loess and other sedimentary records in the Columbia Basin that suggest there were earlier, pre-LGM phases of flooding in the Channeled Scablands and provide insight into the evolution of landscapes subjected to multiple catastrophic outburst floods.