EVALUATING THE GLACIAL UNDERBURST HYPOTHESIS IN THE OKANOGAN LOBE REGION OF THE CORDILLERAN ICE SHEET, NORTH-CENTRAL WASHINGTON (USA) AND SOUTH-CENTRAL BRITISH COLUMBIA (CANADA)

Drumlins and tunnel valleys or channels are the main evidence used to support the hypothesis that “catastrophic” subglacial floods occurred in the south-central Cordilleran Ice Sheet (CIS) landscape, including the Okanogan Lobe (OL) in north central Washington state. Such underbursts continue to be cited as a possible source, in addition to glacial Lake Missoula (GLM), for floods on a megaflood scale (>10⁶ m³/s of water) in the adjacent Channeled Scabland. However, examination of the glaciated and unglaciated landscapes, based on maps and studies by others, finds underbursts on a megaflood scale implausible.

Subaerial megaflood landforms are prominent across the Channeled Scabland. Similar features in the subglacial CIS-OL landscape are largely lacking, suggesting an absence of megaflooding there. Where present locally in the CIS landscape, giant gavel bars and current ripples have been interpreted to record floods from ice-dammed proglacial lakes during glacial recession.

Studies of megaflood sediment in the Channeled Scabland and the Pacific Ocean find a GLM provenance. Studies of flood rhythmites deposited in a lake ponded against the OL margin find lithologic signs of GLM provenance and flow coming from the GLM direction, not from the adjacent OL.

Behind the OL terminal moraine are drumlins which, by underburst theory, require a lobe-wide sheet flood as deep as drumlin heights. Beyond most of the terminal moraine is a loess-mantled, uneroded plateau, incongruent with a giant sheet flood.

Lines connecting erratic boulders from source to site align with CIS ice-flow direction indicators. Flow directions and possible maximum-elevation trimlines indicate the OL was thick. Recessional terraces in the Okanogan Valley, with kettles and ice-contact structures, suggest OL ice was thickest over the valley, rather than a thin ice shelf on a large subglacial lake. The suggested ice thickness, >1,000 m over the northern Okanogan Valley, is consistent with numeric models of the CIS and precludes a large supraglacial lake on the OL, leaving an underburst volume of water unaccommodated.

At low elevations, the base of the CIS was wet, and water played a role in the subglacial land-forming processes. However, combined evidence from the OL and the adjacent Channeled Scabland does not appear to support megaflooding from beneath the OL.