In most Ontario lakes in summer are said to be ‘stratified’ as there is a thin layer of warm water with a temperature of about 20 degrees celsius that lies above the cold clear deeper waters. This surface layer (called the ‘epilimnion’) is usually only 15 m thick and is much lighter in density that the cold < 4 degrees C water at depth. As surface waters cool in the fall, they become more dense and become completely mixed with underlying waters (the lake waters are said to ‘overturn’) such that the lake is no longer stratified. Winter cooling of surface water re-establishes a lid of less dense water.

The surface area of Georgian Bay is 1508 km2 whereas that of its surrounding watershed is three times as large. Most of the water circulation in Georgian Bay is wind-driven with cool water moving into the Bay from Lake Superior via the gap between the tip of the Bruce Peninsula and Manitoulin Island referred to as the Main Channel. During summer (and less so in winter) prevailing westerly winds push surface waters from Lake Huron into the southern part of the Bay and set up an anti-clockwise rotation (forming what is called a ‘gyre’) where water then flows north through the Thirty Thousand Islands area and thence leaves the Bay for Lake Huron. Water remains in Georgian Bay on average for about 8 years (‘residence time’) before being flushed out by return flows to Lake Huron through the Main Channel.