Lake Winnipeg (together with Lake Manitoba) are remnants of the prehistoric Lake Agassiz from the last ice age. That lake was so massive that it changed the world's climate and was larger than the five Great Lakes combined. It disappeared around 4,000 years ago.
Lake Winnipeg is large, but it is also relatively shallow. It is Canada's sixth-largest freshwater lake and the third largest to be completely located within Canada. It is home to many islands of which most remain undeveloped.