Lake Michigan–Huron (also Huron–Michigan) is the combined waters of Lake Michigan and Lake Huron, which are joined through the 5-mile (8.0 km) wide, 20-fathom (120 ft; 37 m) deep, open-water Straits of Mackinac.
Huron and Michigan are hydrologically a single lake because the flow of water through the straits keeps their water levels in near-equilibrium. (Although the flow is generally eastward, the water moves in either direction depending on local conditions.)
Combined, Lake Michigan–Huron is the largest fresh water lake in US. If Lake Huron and Lake Michigan are considered two separate lakes, Lake Superior is larger than either. It is 117,600 square kilometers in area.