Lake Superior’s Natural Processes

Weather

With an average annual temperature of 40 degrees F (4 degrees C), Lake Superior moderates the climate, making winters warmer and summers cooler. The effect is strongest when the wind blows off the water, and is most pronounced on the lake shore and on slopes that face the lake.

Between late spring and late fall, the shore can be shrouded in fog when inland areas bask in sunshine. These warm-season fogs occur when moisture in the warm air condenses as it flows over the cold lake. Duluth gets an average of 52 days of heavy fog each year (the twin cities average 11 foggy days per year). These foggy days, punctuate with the sounds of gulls and fog horns, have a powerful appeal.

Increased snowfall along the shore is not as pronounced in Minnesota as it is in Wisconsin and Michigan. Portions of Michigan’s Upper Peninsula normally receive 350 inches of snow, while downtown Duluth receives an average of only 55 inches. Because air cools and releases moisture as it rises, more snow - 79 inches - falls over the hill in Duluth, a mile or more from the lake. Sometimes when snow is falling over the hill, it is raining near the shore.

During most winters, Lake Superior becomes 40-95 percent covered by ice. It occasionally freezes over completely. Open water often persists in the center of the lake because the ice that forms there is blown or broken by strong winds.

Seiches

Generally the prevailing winds blowing across Lake Superior come from the Northwest and sometimes from the East. These winds and passing weather fronts push the water in Lake Superior to the far shores setting-up the conditions for seiche activity once the wind dies down.

A seiche (pronounced “saysh”) is the rocking motion of water in a lake or similarly closed or partially closed water body. Scientists call the pendulum-like movements within seiches “free standing-wave oscillations.” Seiches, or sloshes as they are sometimes called on the Great Lakes, are almost always present on Lake Superior.

Alternating bands of blue and brown indicate a seiche powerful enough to reverse the flow of the St. Louis river at regular intervals. (Data and visualization from Water on the Web.)

Lake Superior Seiches:

• Create water-level changes ranging from imperceptible to at least three feet
• Have a period of 7.9 hours
• Stir nutrients (good stuff) and pollutants (bad stuff) into the water column
• Reverse the flow of rivers (for example, the St. Louis River can flow upstream for 11 miles when a seiche floods the harbor)
• Sustain a “mini-seiche” oscillation in the Duluth Superior Harbor (for about 2.1 hours water flows though the canals to Lake Superior and then the direction reverses).

See Also

• Bonanza for Lake Superior: Seiches Do More Than Move Water (Feb. 2000 Seiche newsletter)

Stratification & Turnover

Lake Superior is a monomictic lake, meaning that during the summer it separates into layers based on water density, then from fall to spring, the layers get mixed back together. Here's how the cycle goes:

Winter: Unlike more typical twice-mixing (dimictic) lakes of this region, monomictic Lake Superior is rarely completely ice-covered and winter is a single continuous wind-stirred event.

Spring: As the sun returns to the northern hemisphere, the lake continues to mix until the surface waters begin to warm past 4°C (which is not only the annual average temperature for Lake Superior but also when water is densest and therefore heaviest).

Summer: The water begins to stratify into layers of uniform, but different densities. Sun-warmed lighter water floats at the surface and is separated from the denser water by a transition zone where the temperature of the water column changes sharply. Limnologists call this transition zone a thermocline. In Lake Superior the thermocline hovers around 30 meters deep.

Autumn: The buoyant surface waters cool reducing the density difference between layers. Winds stir the lake to greater depths until the surface and bottom waters approach the same and density and a windstorm can mix the entire lake; the sinking of heavy water and mixing by wind results in the exchange of surface and bottom waters, an event referred to as lake turnover. In recent years, turnover in Lake Superior happens in late fall (early to mid-Dec). Global climate patterns have and will continue to influence the timing of the lake's turnover. Turnover is enormously important for redistributing nutrients and oxygen within the water column.

See Water on the Web for further information.

Creation of the Lake

Fire and ice, in the form of volcanoes and glaciers, created Lake Superior. One billion years ago, molten basalt erupted from the Mid-Continent Rift. This rift extended from near Detroit, Michigan, north through what is now Lake Superior, to Minnesota, and then south to Kansas. Lava flowed from the rift for 22 million years, resulting in a layer of basalt as much as ten miles (16 kilometers) thick. Billions of tons of molten rock, moving from underground to the surface, caused the surface to gradually sink as the earth’s crust pulled apart, forming the broad, shallow Superior basin.

Had the rift continued to evolve, the North American continent would have split in two, and Duluth might now be on the shore of an ocean instead of a lake. But the deep forces that fueled the rifting stopped, leaving behind the basalt-covered North Shore. The basin continued to sink, accumulating eroded sand and mud. Ancient rivers deposited the sand that became the Apostle Islands and Bayfield Peninsula in Wisconsin. Shallow seas flooded the southeastern part of the basin and deposited the sandstone found today in Pictured Rocks National Lakeshore in Michigan. The area became stable about 500 million years ago. About two million years ago glacial ice began to sculpt the basin into its present shape.

Ten thousand years ago, the last ice mass began a slow retreat from western Lake Superior while the eastern drainage outlet was still blocked by ice. Impounded water caught between the ice front and the southwestern highlands formed Glacial Lake Duluth. The wave-cut cliffs and terraces that line Duluth’s Skyline Drive about 600 feet (183 meters) above the present lake level are the remains of one of the highest former shorelines of Glacial Lake Duluth. Water flowed out of this large glacial lake through the Brule River Valley, Wisconsin, into the St. Croix River, and then to the Mississippi River. Another route took it past Carlton, Minnesota, and into the Kettle River. As the ice retreated, Lake Superior's present outlet was established through the St. Marys River, and the water level fell.