The Laurentian River System is an ancient river in southern Ontario, Canada. The river predates the recent ice ages. The river valley was filled with glacial debris. Water still flows down this old valley—underground. The source of the aquifer is the Georgian Bay, approximately 200 kilometres (120 mi) away.
In 2003 it was discovered that the southern section of the Laurentian aquifer reaches under High Park in Toronto, Canada. This is believed to be where the Laurentian River System drains into Lake Ontario.
The bedrock under Toronto has several dips believed to be carved by the Laurentian River.
The Laurentian channel, more than 25 km wide, 100 km long and greater than 100 m deep, still lacks data regarding its total size and sediments, relying on well sites for information. Sediments the channel range from sands and gravels near the bottom and clayey silts near the top.