Public parks. To be considered an island, a piece of land must stay above water throughout the year and support a living tree. Many of the islands are publicly owned. A group of 21 islands form the Thousand Islands National Park, the oldest of Canada's national parks east of the Rocky Mountains.

The Thousand Islands is an archipelago of 1,864 islands that straddles the border between the United States (New York) and Canada (Ontario). Ownership is split between the two nations, with roughly one-third of the islands belonging to the U.S. and two-thirds to Canada. On a more granular level, the islands are a patchwork of private, state/provincial, and federal ownership. Many are privately owned by individuals and feature stunning summer homes, ranging from modest cottages to the famous Boldt Castle on Heart Island (owned by the Thousand Islands Bridge Authority). Large portions are also protected as public land, such as Thousand Islands National Park in Canada and several New York State Parks like Wellesley Island State Park. The international border is clearly defined but often weaves between islands that are only yards apart, requiring boaters to be mindful of customs and border protection regulations when moving between the two jurisdictions.