The Grand River, formerly known as the River Ouse, is a large river in Ontario, Canada. It lies along the western fringe of the Greater Golden Horseshoe region of Ontario which overlaps the eastern portion of southwestern Ontario, sometimes referred to as Midwestern Ontario, along the length of this river. From its source near Wareham, Ontario, it flows south through Grand Valley, Fergus, Elora, Waterloo, Kitchener, Cambridge, Paris, Brantford, Ohsweken, Six Nations of the Grand River, Caledonia, and Cayuga before emptying into the north shore of Lake Erie south of Dunnville at Port Maitland. One of the scenic and spectacular features of the river is the falls and Gorge at Elora.
The Grand River is the largest river that is entirely within southern Ontario's boundaries. The river owes its size to the unusual fact that its source is relatively close to the Georgian Bay of Lake Huron, yet it flows southwards to Lake Erie, rather than westward to the closer Lake Huron or northward to Georgian Bay (most southern Ontario rivers flow into the nearest Great Lake, which is why most of them are small), thus giving it more distance to take in more water from tributaries.