INUNDATION MAP, ICE-RAFTED ERRATICS, AND DEPOSITS OF LATE PLEISTOCENE MISSOULA FLOODS IN THE PORTLAND BASIN AND WILLAMETTE VALLEY, OREGON AND WASHINGTON

Glacial Lake Missoula, impounded by the Purcell Trench lobe of the late Pleistocene Cordilleran Icesheet, repeatedly breached its ice dam, sending floods as large as 2,500 cubic kilometers racing across the Channeled Scabland and down the Columbia River valley to the Pacific Ocean. Peak discharges for some floods exceeded 20 million cubic meters per second. At valley constrictions along the flood route, floodwaters temporarily ponded behind each narrow zone. One such constriction at Kalama Gap—northwest of Portland—backed water 120-150 meters high in the Portland basin, and backflooded 200 km south into Willamette Valley. Dozens of floods backed up into the Willamette Valley, eroding "scabland" channels, and depositing giant boulder gravel bars in areas of vigorous currents as well as bedded flood sand and silt in backwater areas. Also, large chunks of ice entrained from the breached glacier dam rafted hundreds of "erratic" rocks, leaving them scattered among the flanking foothills and valley bottom. From several sources and our own mapping, we have compiled information on many of these features and depict them on physiographic maps derived from digital elevation models of the Portland Basin and Willamette Valley. These maps show maximum flood inundation levels, inundation levels associated with stratigraphic evidence of repeated floodings, distribution of flood deposits, and sites of ice-rafted erratics. Accompanying these maps, a database lists locations, elevations, and descriptions of approximately 400 ice-rafted erratics—most compiled from early 20th-century maps and notes of A.M. Piper and I.S. Allison.