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 are in the state of New York. The islands, which number 1,793 in all, range in size from over 40 square miles (100 km2) to smaller islands occupied by a single residence, to even smaller uninhabited outcroppings of rocks that are home to migratory waterfowl. The Thousand Islands is the name of an archipelago of islands that straddle the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. The number of islands was determined using the criteria that any island must be above water level all year round, bigger than one square foot (roughly 930 cm²), and support at least one living tree. The area is very popular among vacationers, campers, and boaters, and is often referred to as the "fresh water boating capital of the world".
The area is frequently traveled by large freighters traveling the St. Lawrence Seaway, but is so riddled with shoals and rocks that local navigators are hired to help the vessels 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. (It was very murky with a visibility of only a few feet until the arrival of the invading Zebra Mussels, roughly around the mid-1990s.) The area features several shipwrecks and is a great place for diving. Although most of the wrecks are over 100 feet (30 m) underwater, some are a mere 15 feet (4.6 m) below the water's surface and can be seen by looking overboard.
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 St. Lawrence Islands National Park, the smallest of Canada's national parks. 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 Robert Moses State Park - Thousand Islands, located on an island in the St. Lawrence.
