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.