RECONSTRUCTIONS OF GLACIAL LAKE PALEOHYDROLOGY IN SOUTHERN BRITISH COLUMBIA

Growth and especially decay of the Cordilleran Ice Sheet (CIS) along its southern margin involved extensive meltwater ponding in glaciated valleys. In the Okanagan Valley of southern British Columbia, glacial Lake Penticton (GLP) developed during the Late Wisconsin (Fraser) glaciation and may have extended into northern Washington State. Currently, basin-wide understanding of the GLP paleogeography is incomplete. Localized reconstructions of water plane elevations for GLP reach a maximum of ~520 m asl. Little is known about its drainage.

Dating of glacial lakes in southern British Columbia is complicated by a general lack of organic material in lake sediments. Lakes were short-lived (years – 100s years). Alternative dating methods (e.g. OSL) have difficulty resolving these short time spans. GLP occupied a bedrock-walled valley where wave-cut benches did not develop. Deltas are rare and sedimentary bodies consist of subaqueous fans that are poor water-plane indicators. Consequently, paleohydrologic reconstructions relying exclusively on these techniques and landforms are unfeasible in the GLP basin. Despite these limitations, new techniques and interpretations of lacustrine and glaciofluvial landforms have allowed reconstruction of the extent and drainage of GLP.

Mapping of lake-bottom sediments, deltas and subaqueous fans record lacustrine conditions at elevations reaching ~900 m asl. Steeply-dipping flights of terraces incised through these sediments record episodic lake drainages. These terraces are best explained by the process of ‘autocyclic incision' where base-level changes do not lead to immediate incision. Instead, aggradation of sedimentary bodies first occurs Autoincision thresholds control changes from aggradation to incision producing flights of terraces. Although individual terraces cannot be correlated throughout the basin, groups of terraces occurring within similar elevation ranges can be correlated. Bedrock sills along the valley axis also help constrain the amount of lake lowering. Drainage occurred as a series of jokulhlaups, routed through Okanagan Valley and connecting valleys in northern Washington State.

This presentation explores the techniques used for this reconstruction and the mechanisms of episodic drainage of GLP along the southern margin of the CIS.