Waterton Lake is a mountain lake in southern Alberta, Canada and northern Montana, United States. The lake is composed of two bodies of water, connected by a shallow channel known locally as the Bosporus. The two parts are referred to as Lower Waterton Lake, and Upper Waterton Lake, the latter of which is crossed at the tip by the Canada–United States border. The United States Geological Survey gives the geocoordinates of for Upper Waterton Lake.
The northern, lower end of the main lake lies in Waterton Lakes National Park while the upper, southern part of the lake is located in Glacier National Park. In 1979, UNESCO established the Waterton Biosphere Reserve to protect the diverse habitats including prairie grasslands, aspen parkland, subalpine forests, alpine tundra and freshwater fens that surround the lake.
Waterton-Glacier International Peace Park was created by the US and Canada in 1932, and in 1976 it was designated an International Biosphere Reserve. Later, in 1995, it was inscribed as a World Heritage Site by UNESCO.
The upper and middle Waterton Lake system has a surface of 10.1km2, while the lower lake, which is in Canada only, has 1.5km2 and lies at an altitude of 1274m. Two soundings of the lake were 210ft and 317ft, done in 1910. More recent soundings have revealed a depth of 487 ft .
Highway 5 has its westernmost point on the shores of the lake.