Uplift-driven expansion delayed by middle Holocene desiccation in Lake Winnipeg, Manitoba, Canada

New findings of paleoenvironmental change in Lake Winnipeg, southern Manito ba, reveal evidence of unexpectedly dry conditions from 7.5 to 4 ka (7500 t o 4000 radiocarbon years before present), with reduced lake area in the nor th and a desiccated lake basin in the south. Changes in extent of this larg e lake, now similar to 400 km long, can be explained by a combination of (1 ) expansion due to postglacial differential uplift (tilting), and (2) lake- area reduction due to drier climates associated with the former presence of dry-grassland vegetation. Comparing lake areas sustainable by grassland cl imate with computed potential lake areas based on the assumption of open (o verflowing) conditions, we quantify the atmospheric moisture reduction repr esented by the middle Holocene dry conditions. This approach holds promise for calibrating regional models of climate change and exploring the effects of dry paleoclimates in other large lake basins such as the Laurentian Gre at Lakes. The ongoing postglacial tilting is of societal concern because it contributes to long-term lakeshore erosion and to the decrease in discharg e capacity of the inflowing flood-prone Red River in a populated region.