Lake Superior receives the least amount of surface runoff, in relation to its volume, of all five Great Lakes. This is because most of its watershed is forested, and its watershed is small, when compared to its water surface area. No major tributaries enter the lake from the south. On the Canada side, a diversion of water from the Hudson Bay watershed provides an additional input of water to the basin.

FLUCTUATIONS OF LAKE SUPERIOR WATER LEVEL
Lake Superior is the largest of the Great Lakes, with a total area of over 82,000 km² . Waves on exposed shores, where fetches may exceed 500 km, can reach 6 m. Thunder Bay is much more sheltered: maximum fetch is no more than 50 km and waves rarely exceed 2 m in height. The water level of Lake Superior fluctuates on both long-term and seasonal bases. Since monitoring began in 1860, the lowest mean annual level of Lake Superior was recorded in 1925-26 at 182.8 m. The other extreme occurred in 1985-86, when new monthly record highs were set for 12 consecutive months, at levels exceeding 183.4 m. Long-term fluctuations appear to have an irregular cycle: high water levels were experienced in 1915-16, 1950-52, 1972-74 and 1985-86. Seasonal regimes are more predictable: the lake normally fluctuates about 30 cm, being lowest just before the snow melts in spring, and highest in September. More spectacular changes in level result from short-term (hourly) oscillations of the lake surface induced primarily by winds and differential barometric pressures. Wind set-up and seiche or barometric surface tilt may raise or lower the lake level along a given shore by as much as 1 m within a period of a few hours.
     The principal causes of long-term fluctuations in the level of Lake Superior (and the other Great Lakes) are natural, and can be explained in terms of the lake’s water budget. When inputs of precipitation, ground water, surface runoff and streamflow exceed the outputs of evaporation and outflow from the lake for a significant period of time, the lake level rises. The most important variables are precipitation and evaporation. An extended period of below-normal precipitation in the 1960’s was characterized by low lake levels. In contrast, precipitation was above normal for 13 of the 16 years between 1960 and 1985, resulting in high water levels in all five Great Lakes. On a long-term basis, Lake Superior basin precipitation and water levels have gradually increased since 1900. In addition, the Great Lakes have been in a cool air temperature regime since 1960, resulting in lower evaporation and higher runoff rates than would occur with average temperatures.
     Lake levels are also affected by human regulation systems. Outflow is regulated through the locks at Sault Ste Marie by the International Joint commission (IJC), while two diversion structures (Ogaki River and Long Lac) divert about 500 m³/s of streamflow from the Hudson Bay basin into Lake Superior. Working under an agreement known as Plan 1977, the IJC strives to keep Lake Superior’s mean level between 182.8 and 183.4 m, while balancing water levels between Lake Superior and Lakes Michigan, Huron and Erie. However, the lake’s natural water balance overrides the effects of human regulation systems. If the natural balance remains unchanged, then the greatest effect human control of outflow could have on the level of Lake Superior is less than 8 cm in six months.

FACTS
Length - 350 miles / 563 km.
Breadth - 160 miles / 257 km.
Ave. Depth - 483 ft. / 147 m.
Max. Depth - 1,332 ft. / 406 m.
Volume - 2,900 cubic miles / 12,100 cubic km.
Water Surface Area - 31,700 sq. miles / 82,100 sq. km.
Drainage Basin Area - 49,300 sq. miles / 127,700 sq. km.
Shoreline Length (including islands) - 2,726 miles / 4,385 km.
Elevation - 600 ft. / 183 m.
Outlet - St. Marys River to Lake Huron
Retention/Replacement Time - 191 years

Name - The first French explorers approaching the great inland sea by way of the Ottawa River and Lake Huron referred to their discovery as le lac superieur. Properly translated, the expression means "Upper Lake," that is, the lake above Lake Huron. Kitchi-gummi, a Chippewa Indian translation, signifies Great-water or Great-lake. A Jesuit name, Lac Tracy, was never officially adopted.

More facts about Lake Superior
Lake Superior is the largest of the Great Lakes in surface area and volume.
Lake Superior could contain all the other Great Lakes plus three more lakes the size of Lake Erie.
Water flows into the lake from many small rivers and streams.
The Long Lac and Ogoki diversions in Canada channel water into Lake Superior that would otherwise flow into Hudson Bay.
By order of the International Joint Commission, the lake's level, controlled by gates on the St. Marys River at Sault Ste. Marie, may not exceed 602 ft (183 m) above sea level.
Each year a small percentage of the lake's water flows out through the St. Marys River, and it takes almost two centuries for the water to be completely replaced (retention time).
The Lake Superior drainage basin is rich in natural resources and scenic beauty.
It is sparsely populated and economically dependent on its natural resources, which include metals, minerals, forests and recreation/tourism opportunities such as national lakeshores and national/state/provincial parks.
It is particularly known for its clear, cold water and agate beaches.
Many shipwrecks in Lake Superior are now protected in bottomland preserves and accessible to recreational divers.
In 1985, scientists using a submersible vessel descended for the first time to the deepest part (-1,333 ft./-405 m) of Lake Superior near the Pictured Rocks National Lakeshore in Michigan waters.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.