Proglacial lakes develop along the front of glaciers from meltwater released along the ice margin. The weight of the glacier depresses the underlying ground; this depressed area is exposed as the ice retreats, and meltwater becomes trapped in this lower area in front of the glacial margin to form a proglacial lake. With the loss of the overlying weight of the ice, the land gradually rebounds upwards, a phenomenon called isostatic uplift, permitting the lake to drain. Numerous proglacial lakes formed with the retreat of the Laurentide ice sheet during the Wisconsinan glaciation. Some proglacial lakes were long-lived, covering large areas, while others existed only for a short time or covered small areas. Glacial meltwater typically is full of fine-grained sediments consisting of sand, silt and clay-sized particles often produced by glacial erosion processes. When this meltwater reaches a lake basin, it slows and deposits its load of sediment onto the floor of the lake. Former proglacial lake basins are identified by flat, relatively featureless plains composed of fine sediments identified as glaciolacustrine deposits. On occasion, large blocks of ice can break off the glacial margin to form icebergs that drift around the lake, scraping plough marks in the lakebed or depositing drop stones as they melt.
Proglacial lakes covered thousands of square kilometres of Saskatchewan; many, such as the former Glacial Lake Regina, lasted less than 1,000 years. Today the silts and clays from this former lake support rich agricultural soil but pose considerable engineering difficulties: the clays, dominated by montmorillonite, have a tremendous ability to expand with the addition of water and to contract on its absence. This results in heaving and cracking of sidewalks, roads, and foundations. It is believed that when Glacial Lake Regina drained, it did so quickly and catastrophically, eroding a large channel now occupied by the much smaller contemporary Souris River. Other sizable proglacial lakes include Glacial Lake Saskatchewan, whose basin area expanded and contracted following the retreat of the ice margin for almost 2,000 years. The lake was initiated southwest of Saskatoon, and gradually moved northeastwards with the retreating ice margin beyond what is now Prince Albert; eventually this lake joined Lake Agassiz to the east. Other proglacial lakes include Glacial Lake Melfort, Glacial Meadow Lake, Glacial Cree Lake, and Glacial Lake Athabasca. Along with the fine glaciolacustrine deposits, some former shorelines and deltas have been identified as associated with these former lakes.
Janis Dale