St. Mary Lake is the second largest lake in Glacier National Park, Montana, United States. Located on the east side of the park, the Going-to-the-Sun Road parallels the lake along its north shore. Located at an altitude of 4,484 feet (1,367 m), St. Mary Lake's waters are colder and lie almost 1,500 feet (455 m) higher in altitude than Lake McDonald, the largest lake in the park, which is located on the west side of the Continental Divide. Here, the great plains end and the Rocky Mountains begin in an abrupt five thousand foot (1,500 m) altitude change, with Little Chief Mountain posing a formidable north flank above the west end of the lake. St. Mary lake has a surface area of 3,923 acres (15.8 km²).
The lake is 7 miles (11.3 km) long and several hundred feet (100 m) deep. The waters of the lake rarely rise above 50°F (10°C) and is home to various species of trout. During the winter, the lake oftentimes is frozen completely over with ice up to four feet (1 m) thick.
