Great Lakes form a single, naturally interconnected body of fresh water, within the Great Lakes Basin. They form a chain connecting the east-central interior of North America to the Atlantic Ocean. From the interior to the outlet at the Saint Lawrence River, water flows from Superior to Huron and Michigan, southward to Erie, and finally northward to Lake Ontario. The lakes drain a large watershed via many rivers, and are studded with approximately 35,000 islands. There are also several thousand smaller lakes, often called "inland lakes," within the basin. The surface area of the five primary lakes combined is roughly equal to
Great Lakes to regulate water supply and use. Tourism and recreation are major industries on the Great Lakes. A few small cruise ships operate on the Great Lakes including a couple of sailing ships. Sport fishing, commercial fishing, and Native American fishing represent a U.S.$4 billion a year industry with salmon, whitefish, smelt, lake trout, bass and walleye being major catches. Many other water sports are practiced on the lakes such as yachting, sea kayaking, diving, kitesurfing, powerboating, and lake surfing. The Great Lakes Circle Tour is a designated scenic road system connecting all of the Great Lakes and the Saint Lawrence