CONSTRAINTS ON MEGAFLOOD DISCHARGE THROUGH UPPER GRAND COULEE IN THE CHANNELED SCABLAND OF EASTERN WASHINGTON

The topography of the Channeled Scablands in eastern Washington was formed by episodic megaflooding resulting from ice dam failure at the Purcell Trench Lobe during the last glacial maximum. Floods released by glacial Lake Missoula incised multiple channels across eastern Washington, including the 260 m deep Upper Grand Coulee. Field evidence suggests a complex relationship between the margins of glacial lobes, the evolution of the topography as incision progressed, and the paths that floodwater took throughout the Pleistocene. Upper Grand Coulee, the largest of these channels, formed via retreat of a large knickpoint that was later completely removed by erosion except for the island of Steamboat Rock in the center of the channel. Upper Grand Coulee was also blocked by ice of the Okanogan Lobe of the Cordilleran Ice Sheet. Because the bedrock and ice topography no longer exist, it is challenging to reconstruct the magnitude of the paleoflood discharges through Grand Coulee. Here we reconstruct the Upper Grand Coulee knickpoint and ice-filled channel and use these topographic boundary conditions, along with modern topography, to conduct paleoflood hydraulic analysis. We will present results from two-dimensional hydraulic models that constrain the range of possible flood discharges based on each topographic reconstruction, and discuss implications for the topographic evolution of Upper Grand Coulee.