The Thousand Islands constitute an archipelago of 1,864 islands that straddles the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about 50 miles (80 km) downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario, the U.S. islands in the state of New York.
The 1,864 islands range in size from over 40 square miles (100 km2) to smaller islands occupied by a single residence, or uninhabited outcroppings of rocks that are only home to migratory waterfowl. To count as one of the Thousand Islands these minimum criteria had to be met: 1) Above water level year round; 2) Have an area greater than 1 square foot (0.093 m2); and 3) Support at least one living tree.
Large freighters frequently ply the Saint Lawrence Seaway, but the area has so many shoals and rocks that vessels sometimes hire maritime pilots to help them travel through the hazardous waterway. Under the Canadian span, a vessel just less than 25 feet (7.6 m) offshore can find itself in over 200 feet (61 m) of water. Similarly, rocks and shoals less than two feet (61 cm) underwater can be found in the center of channels 90 feet (27 m) deep.
Because of the great number of rocks and shoals just above or below the water's surface, it is unwise to travel the waters at night, unless one stays in the main channels and has charts, a chart plotter, or knows the area well. The water is so clear in some areas, that a rocky bottom can be observed in 80 feet (24 m) of water. Before the advent of the zebra mussel, visibility of only ten to fifteen feet was usual, slightly decreasing as the years passed. Water clarity improved markedly in the mid-1990s with the arrival of zebra mussels, which feed on algae. The area has several shipwrecks, and although most of them are over 100 feet (30 m) underwater, some are a mere 15 feet (4.6 m) down and can be seen from the surface.
Geologically, the islands are located where a branch of the Canadian Shield runs south across the river to join with the Adirondacks. Around twenty of these islands form the Thousand Islands National Park, the oldest of Canada's national parks east of the Rockies. The park hosts campgrounds, inland walking trails, annual family events, as well as a national heritage building.
The Thousand Islands-Frontenac Arch region was designated a World Biosphere Reserve by UNESCO in 2002. The U.S. islands include numerous New York state parks, including Wellesley Island State Park, and Robert Moses State Park - Thousand Islands located on an island in the St. Lawrence.
The Thousand Islands Bridge connects New York State and Ontario by traversing Wellesley Island at the northernmost point of Interstate 81 in Jefferson County and meets Highway 137, which leads to Highway 401. The Thousand Islands Parkway provides a scenic view of many of the islands.
The largest island in the group, Wolfe Island, is located entirely in Ontario. Adjacent to Wolfe but part of New York is Carleton Island, the site of a ruined fort, Fort Haldiman, built in 1779 by the British during the American Revolutionary War. The island was captured by three American soldiers during the War of 1812 and remains part of the United States today.