OUTBURST FLOODS FROM THE CORDILLERAN ICE SHEET AFFECTING AREAS NORTH OF THE CHANNELED SCABLAND

Paleo-outburst flood indicators such as large bedform trains and iceberg scour marks occur along Okanogan Valley and in other North-South trending valleys draining the southern margin of the Cordilleran Ice Sheet (CIS). Linkages between outburst floods and large bedform trains has been established in the Channeled Scabland and along other outburst flood tracts worldwide. Iceberg scour marks have modern analogues in Icelandic jokulhlaups. The indicators documented here occur north of Columbia River and outside the Channeled Scabland. However, routing of these outburst floods ultimately connected to some of the Channeled Scabland flood routes, most notably glacial Lake Columbia basin and Columbia River. Discharge estimates based on bedform morphometrics, and geometric parameters of iceberg scour marks suggest magnitudes reaching 10⁴-10⁵ m³s^-1, and possibly as high as 10⁶ m³s^-1. Absolute ages of these outburst floods are unknown but relative timing based on landform relationships establish these outburst floods close in time with the last glacial Lake Missoula drainages or postdating its last drainage. Taken together, these paleo-outburst flood indicators record a protracted history of outburst flooding and extends the geographic areas affected by outburst floods from the CIS. The routing and magnitude of these inferred outburst floods suggests that they may have been sufficiently energetic to leave a geomorphic record along the Columbia River, potentially modifying interpretations of flood landforms such as giant bars, currently ascribed to glacial Lake Missoula drainages, or other late-glacial lakes. Recognition of these landforms and their paleohydraulic reconstructions in northern Washington highlights the need for greater integration of CIS hydrology in the interpretive framework of the Channeled Scabland and glacial Lake Missoula flood routes.