Reconstruction of Mega-Flood Hydraulics from Strath Terraces in the Channeled Scablands, Eastern Washington, USA

Abstract:

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. The Channeled Scablands are the closest terrestrial analog for the Martian Outflow Channels. Hence, understanding the interaction of flood hydraulics and erosion in the Channeled Scablands has key implications for reconstruction of flood magnitudes on Mars. In recent decades, much progress has been made in understanding the number and timing of floods that occurred in the Channeled Scablands during the last-glacial maximum. 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 last glacial period. Whether the Scablands were carved entirely during the last-glacial or progressively over multiple glacial cycles has important implications for the assumptions used to reconstruct flow hydraulics. To constrain the processes, rates, and timing of landscape evolution in the Channeled Scablands, we analyzed topography, mapped terraces, re-constructed discharges, modeled canyon incision, 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 the last-glacial maximum. The results suggest the canyons were progressively carved by a series of temporally distinct floods. The terrace topography and exposure ages hence suggest that last-glacial floods did not completely inundate the canyons, which has key implications for reconstructing the magnitude of outburst flood discharges on Earth and Mars.