A huge bay of Lake Huron, one of the five of the Great Lakes. Georgian Bay is about 190 kilometres (120 mi) long by 80 kilometres (50 mi) wide. It covers approximately 15,000 square kilometres (5,800 sq mi), making it nearly 80% the size of Lake Ontario. If Georgian Bay were considered a lake in its own right, it would be the fourth largest lake located entirely within Canada after Great Bear Lake, Great Slave Lake and Lake Winnipeg.
Eastern Georgian Bay is part of the southern edge of the Canadian Shield, granite bedrock exposed by the glaciers at the end of the last ice age, about 11,000 years ago. The granite rock formations and windswept eastern white pine are characteristic of the islands and much of the shoreline of the bay. The rugged beauty of the area inspired landscapes by artists of the Group of Seven. The western part of the bay, from Collingwood north, and including Manitoulin, Drummond, Cockburn and Niagara St. Joseph islands, borders the Escarpment. Because of its size and narrowness of the straits joining it with the rest of Lake Huron, which is analogous to if not as pronounced as the separation of Lake Huron and Lake Michigan, Georgian Bay is sometimes called the “sixth Great Lake”.
There are tens of thousands of islands in Georgian Bay. Most of these islands are along the east side of the bay and are collectively known as the “Thirty Thousand Islands”, including the larger Parry Island. Manitoulin Island, lying along the northern side of the bay, is the world’s largest island in a freshwater lake. The Trent–Severn Waterway connects Georgian Bay to Lake Ontario, running from Port Severn in the southeastern corner of Georgian Bay through Lake Simcoe into Lake Ontario near Trenton. Further north, Lake Nipissing drains into Georgian Bay through the French River. In October 2004, the Georgian Bay Littoral was declared a Biosphere Reserve by UNESCO.