Lake Souris was a prehistoric glacial lake that occupied the Souris River basin in present-day North Dakota and Manitoba, extending into Saskatchewan and shaped by features such as Turtle Mountain and the Missouri Coteau. Formed as continental ice retreated, it changed repeatedly as outlets shifted, eroded, and lowered the water level until a later drainage route emptied it completely. Its legacy remained in the northern plains through the landforms and sediments it left behind. Those deposits later inspired brief, unsuccessful gold rushes near Denbigh and Towner after traces of glacier-borne gold were found in sands and gravels associated with the former lake.