During the decay of the Cordilleran Ice Sheet 10 000 – 13 000 BP, glacial lakes developed within valleys that dissect the Interior Plateau of British Columbia. In this paper, we (1) illustrate a procedure for assessing paleo water planes that has general application, (2) document lake paleogeography and evolution in the Thompson Valley, (3) provide new data on the glacio-isostatic response of the central Cordillera, and (4) present new evidence of its late-glacial environment. We employ geomorphology and sedimentology, digital elevation models, and new technologies (differential global positioning systems, ground p enetrating radar, and geographic information systems) to refine paleogeographic reconstructions of glacial lakes. Glacial Lake Thompson and Glacial Lake Deadman were ribb on-shaped (width to length ratio ≈ 3:100), deep (>>140 to ∼50 m) lakes that contained significant wate r volumes (84–24 km3). They lengthened to the west and their water level lowered as ice decayed. Final ice dam failure resulted in an ∼20 km3 jökulhlaup that eroded bedforms and deposited flood eddy bars within the lake basin, traveled ∼250 km along the Fraser River system, and may have deposited exotic mud offshore between 10 190 and 11 940 BP. Glacio-isostatic tilts of water planes are among the highest in the world (1.7–1.8 m km–1). Their orientations suggest that ice sheet loads were greater or longer-lived to the north-northwest of the study area, lending support to the notion of an ice divide centred on the Fraser Plateau.