ICE AGE FLOODS RESEARCH AND MAPPING IN IDAHO: RESULTS FROM STATEMAP

The pioneer work of J Harlen Bretz’s “Spokane Floods” beginning in 1923 and of Joseph T. Pardee’s first recognition of Glacial Lake Missoula in 1910 is widely known to geologists. Although key to the ice age floods story, the Clark Fork ice dam and the Lake Missoula outburst area in northern Idaho remained essentially unexamined and largely unmapped until the Idaho Geological Survey began mapping the surficial geology as STATEMAP projects of the National Cooperative Geologic Mapping Program. In the ice dam and outburst flood area of the Idaho Panhandle the Idaho Geological Survey has completed digital geologic maps of twenty-eight 1:24,000 scale quadrangles and compilations of two 1:100,000 scale quadrangles. Our mapping includes substantial areas of surficial deposits and landform features formed by the Cordilleran ice sheet, alpine glaciers, and multiple catastrophic outburst floods from Glacial Lake Missoula. Field evidence, including paleomagnetic studies of lake and flood rhythmites, substantiates repeated failures of the ice dam and multiple outburst floods from Lake Missoula. These new maps redefine the limits and extent of ice, glacial lakes, and the routes of outburst floods as well as revealing numerous previously unrecognized glacial and flood features. These data are critical information for testing new hydrologic models of glacial outburst floods and water movement in glaciers. The Quaternary deposits, previously undivided, on maps have been subdivided and a stratigraphic framework was established. Glacial boulders at the late glacial ice limit of the Purcell Trench ice lobe and a post-flood recessional moraine have been sampled; the first cosmogenic surface exposure dating in the ice dam area. Maps of the surficial deposits have had wide geotechnical application in construction and land use planning. Glacial and flood deposits are the main aquifers in the Sandpoint, Coeur d'Alene, and Spokane region as well as the prime source of sand and gravel. These Idaho maps serve as a resource inventory for viewpoints and features for the newly authorized Ice Age Floods National Geologic Trail.