Lake Superior - Hydrography

Hydrography

Lake Superior empties into Lake Huron via the St. Mary's River and the Soo Locks. Lake Superior is the largest freshwater lake in the world in area (if Lakes Michigan and Huron are taken separately; see Lake Michigan–Huron), and the third largest in volume, behind Lake Baikal in Siberia and Lake Tanganyika in East Africa. The Caspian Sea, while larger than Lake Superior in both surface area and volume, is brackish; though presently isolated, historically the Caspian has been repeatedly connected to and isolated from the Mediterranean via the Black Sea.

Lake Superior has a surface area of 31,700 square miles (82,103 km2), which is approximately the size of South Carolina. It has a maximum length of 350 statute miles (560 km; 300 nmi) and maximum breadth of 160 statute miles (257 km; 139 nmi). Its average depth is 80.5 fathoms (483 ft; 147 m) with a maximum depth of 222 fathoms (1,332 ft; 406 m). Lake Superior contains 2,900 cubic miles (12,100 km³) of water. There is enough water in Lake Superior to cover the entire land mass of North and South America with roughly 1 foot (30 cm) of water (roughly a depth of .93 feet) The shoreline of the lake stretches 2,726 miles (4,387 km) (including islands).

American limnologist J. Val Klump was the first person to reach the lowest depth of Lake Superior on July 30, 1985, as part of a scientific expedition, which, at 733 feet (223 m) below sea level, is the lowest spot on the continental interior of the United States and the second-lowest spot on the interior of the North American continent after the deeper Great Slave Lake in Canada (1,503 feet below sea level). (Though Crater Lake, not Lake Superior, is the deepest lake in the United States, Crater Lake's surface elevation is much higher and its deepest point is 4,229 feet (1,289 m) above sea level.)

While the temperature of the surface of the lake varies seasonally, the temperature below 660 feet (200 m) is an almost constant 39°F (4°C). This variation in temperature makes the lake seasonally stratigraphic. Twice per year, however, the water column reaches a uniform temperature of 39°F (4°C) from top to bottom, and the lake waters are able to completely mix. This feature makes the lake dimictic. Because of its size Superior has a retention time of 191 years.

Annual storms on Lake Superior regularly record wave heights of over 20 feet (6 m). Waves well over 30 feet (9 m) have been recorded.