FISHING FOR ANSWERS IN THE DEGLACIAL RIBBON LAKES OF THE THOMPSON BASIN, BRITISH COLUMBIA: ADVANCES IN CORDILLERAN PALEOHYDROLOGIC, PALEOCLIMATIC AND ISOSTATIC RECONSTRUCTIONS

During the decay of the Cordilleran Ice Sheet (CIS), ~9 to 12 ka BP, numerous deglacial ribbon lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia, yet they have rarely been studied beyond the reconnaissance level. This study integrates geomorphology, sedimentology, aerial photographs, differential GPS, GPR, DEMs and GIS to (i) investigate, survey and correlate paleolake levels, (ii) reconstruct paleolake geography, evolution and environment, and (iii) reconstruct glacio-isostatic rebound.

Two definable glacial paleolake levels were identified, associated with Glacial Lake Thompson (GLT) and Glacial Lake Deadman (GLD). DEMs of paleolake levels, inferred lake bottom and modern topography were integrated in a GIS to quantify lake parameters. Lakes were ribbon-shaped (width to length ratio of ~3:100), deep ( >140 and ~50 m, respectively), and of significant volumes (84 and 24 km³, respectively). Glacio-isostatic tilts of these lake shorelines (1.8 - 1.7 m km^-1) are among the highest measured in the world and are related to the unusual crustal properties of the Cordillera. Glacio-isostatic depression in the interior was likely hundreds of metres.

The sedimentary record of these lakes reflects the severe adjustments of a landscape undergoing deglaciation and hints at a new model of CIS decay, at least locally. Seventeen glaciolacustrine lithofacies were identified recording deltas, subaqueous fans, high rates of sedimentation, numerous river-generated underflows and a diversity of sediment dispersal and deposition processes. High sedimentation rates and numerous river-generated underflows suggest that ribbon lakes likely received their meltwater and sediment supply from ice remnant on the plateau. This inference is counter to the accepted model of CIS decay that describes plateaus becoming ice-free prior to valleys.

GLD drained catastrophically with the breach of an ice dam, producing drainage bedforms and erosional surfaces within the basin, and discharging ~20 km³ of water. It is possible that this event may have triggered the failure of glacial lakes downstream. Eventually the floodwaters reached the marine environment of Georgia Strait, a total distance of ~250 km, where exotic deposits dated at ~10,500 ¹⁴C yr BP may have been produced by this jokulhlaup.