MAPPING THE MISSOULA FLOODS, NORTH-CENTRAL WASHINGTON

The Columbia River corridor from Lake Pateros to Frenchman Coulee in north-central Washington was both glaciated and flooded during the last glacial maximum. Using a combination of lidar, 10-m DEMs, aerial photographs, and field observations, we are compiling a 1:250,000 scale geomorphic map and geodatabase to refine the Quaternary glacial and flood history of this section of the Columbia River as part of an overall effort to map Missoula flood features for the entire flood route. Mapped landforms include flood bars (of various types), eroded bedrock (scabland), glacial outwash terraces, moraines, dunes, and landslide deposits. Elevations and slopes from the DEMs help delineate boundaries between geomorphic features, and provide for elevation profiles of correlated features; together helping define the history and magnitude of flooding and aggradation along the Columbia Valley. Analysis of the orientation and elevation of moraines at the eastern end of Lake Chelan indicates direction of glacial advance at maximum ice extent. These correlations among features, further supported by morphology and sediment composition, indicate several distinct events relating to the Missoula floods, including large down-Columbia Missoula floods prior to the maximum extent of the Okanogan ice lobe, advance of the Cordilleran ice sheet and the Okanogan lobe into the Columbia River Valley, formation of outwash terraces, glacial retreat, and later floods from glacial Lake Columbia and other sources. Overall, geomorphic mapping of this section of the Columbia River allows for further interpretation of features related to the complicated Pleistocene history of glaciation and episodic flooding.