The Thousand Islands are located within the St. Lawrence River, specifically the portion of the river that acts as a border between the northern part of New York State and southeastern Ontario, which is a Canadian province. The region contains an estimated 1,864 islands.
To be considered a part of the Thousand Islands, an island in this region must meet three criteria. An island must remain above water level all year round, it must be larger than one square foot, and it must contain at least one living tree. The size of the islands range from those not much larger than one square foot to over 40 square miles.