GLACIAL ISOSTATIC ADJUSTMENT DIRECTED INCISION OF THE CHANNELED SCABLAND BY ICE-AGE MEGAFLOODS

During the last ice age, 22,000 to 14,000 years ago, dozens of catastrophic megafloods from an ice-dammed Glacial Lake Missoula scoured the landscapes of Idaho and eastern Washington, forming what are known as the Channeled Scabland. Although there is evidence for multiple floods and different flood routes, outstanding questions remain regarding the volume and discharge of meltwater associated with each flood event, and the routing of floodwater over time. Crustal deformation resulting from the solid Earth’s response to variations in ice loading, known as glacial isostatic adjustment, caused ~100 meters of uplift or subsidence relative to today in the Channeled Scabland, changing topographic slopes in this region. Here we hypothesize that glacial isostatic adjustment affected flood routing and controlled the spatio-temporal erosion of the Channeled Scabland.

We simulated floods using the 2-D shallow water equations by routing flow over the modern-day Channeled Scabland topography and topography corrected for glacial isostatic adjustment in response to ice unloading during the deglaciation of the Cordilleran Ice Sheet. From 18 to 15.5 ka, megafloods spilled into the Channeled Scabland from Glacial Lake Columbia, which formed when the Cordilleran Ice Sheet advanced southward to dam the Columbia River. We performed simulations of Glacial Lake Columbia spillover and modeled the evolution of flood routing over this time. Simulations on the present-day topography predict similar flood discharges through both the Telford-Crab Creek and Cheney-Palouse scabland tracts, conforming to previous interpretations that they eroded simultaneously. However, in simulations on topography corrected for glacial isostatic adjustment, floodwaters are disproportionately routed to the west into the Telford-Crab Creek scabland tract. For the same magnitude flood, the proportion of discharge diverted to the west increases from 18 to 15.5 ka. Thus, we found that glacial isostatic adjustment influences spatio-temporal patterns of megaflood routing.