JöKULHLAUPS FROM THE SOUTHERN MARGIN OF THE CORDILLERAN ICE SHEET

During the Late Wisconsinan (Fraser) glaciation decay of the Cordilleran Ice Sheet (CIS) along its southern margin involved extensive meltwater ponding in glaciated valleys. In Okanagan Valley of southern British Columbia, meltwater ponding occurred in a subglacial lake and in a subaerial proglacial lake. Drainage of these lakes occurred as jökulhlaups originating mainly from Okanagan Valley and routed through valleys in northern Washington State and the Channeled Scabland.

Subglacial lake development is rationalized from regional landform relationships, valley fill characteristics, and perched sedimentary deposits interpreted as grounding line sediments marking the highest water plane of this subglacial reservoir. High geothermal activity in Okanagan Valley and subglacial volcanic eruptions within valley networks connecting with Okanagan Valley further support arguments for subglacial lake development. Subglacial lake drainage occurred as an underburst that evacuated valley ice, leading to development of a subaerial proglacial lake (glacial Lake Penticton – gLP). During the transition from subglacial to subaerial lake, ongoing drainage through bedrock spillways lowered lake level and increasingly confined gLP within Okanagan Valley. A stable water plane, marked by tributary deltas, was attained around 500 m asl as spillways became isolated and an ice and sediment dam developed in a valley constriction. Decay and eventual breaching of this dam led to a final drainage of gLP.

At least two catastrophic outbursts of decreasing magnitude were released from Okanagan Valley. The subglacial outburst consisted of at least 150 km³ and the final gLP drainage was one order of magnitude smaller. Although, timing of these drainages remains poorly constrained, such reconstructions may help in the interpretation of sedimentary sequences and process reconstructions associated with deglaciation of the southern margin of the CIS and its potential meltwater contributions to the Channeled Scabland flood(s).