Thousand Islands, a group of islands in the St. Lawrence River, just downstream from Lake Ontario. Some of the islands are in Ontario, and the rest are in New York. Structurally, the Thousand Islands are part of the Canadian Shield, an enormous block of ancient granitic rock extending through eastern Canada into the Adirondacks in New York. The group consists of about 1,800 islands; some, however, are no more than small exposures of bare rock. The largest is Wolfe Island, about 20 miles (32 km) long, in Ontario; other sizable ones include Howe, also in Ontario, and Wellesley and Grindstone, in New York.

The islands are a popular resort area with summer homes, hotels, and numerous recreational facilities. St. Lawrence Islands National Park includes part of the Ontario mainland and 17 of the islands. The Thousand Islands International Bridge, opened in 1938, utilizes several of the islands in connecting the New York and Ontario mainlands. (For color picture, .)