The central interior plateau of British Columbia, Canada, is dissected by numerous elongate, glacially overdeepened lake basins akin to coastal fiords. An air-gun seismic-reflection investigation (242 km of trackline) of Okanagan Lake (120 km long, ∼3.5 km wide) shows that the Pleistocene sediment fill is up to 792 m thick and that bedrock has been excavated by repeated Pleistocene glacial erosion to nearly 650 m below sea level. The depth of bedrock incision below the surrounding plateau is more than 2000 m, exceeding that of the Grand Canyon in Arizona. The total volume of sediment infill within the narrow trenchlike basin is more than 90 km3, and several lines of evidence suggest that this fill accumulated rapidly during a single phase of glaciolacustrine sedimentation during late Wisconsin deglaciation of the region after 15 ka. Similar fiord-lake basins, up to 100 km long, are common in the interior of British Columbia. If the data from Okanagan are representative, these basins may represent a major previously unrecognized component of the total glacial sediment flux to the ocean. Overdeepening below sea level and the focusing of large volumes of glacial sediment and meltwater into such basins are of considerable glaciologic significance and may record the fast, unstable flow of "ice streams" during deglaciation of the Cordilleran ice sheet.