Lake Superior’s Natural Processes
With an average annual water temperature of 45°F (7°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, punctuated by 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. Ice cover helps the lake water retain heat and prevents evaporation. Over the past 30 years ice cover and duration have been diminishing.
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.
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).
- Bonanza for Lake Superior: Seiches Do More Than Move Water (Feb. 2000 Seiche newsletter)
- High Tide on Lake Superior (6:08), Aired 1/6/10
1.2 MB mp3 | Stream
- Seiches in Lake Superior (5:02), Aired 3/11/08 and 3/13/08
4.6 MB mp3 | Stream | Transcript
Stratification & Turnover
June 2008-June 2009 data from a mooring in central Lake Superior with an array of thermistors (temperature recording tools used in oceanography). Measuring temperatures at varying depths over a year captures the twice-per-year formation and destruction of stratification in Lake Superior. Lake Superior exhibits a positive stratification in the summer, with a warm layer of water developing over a colder layer. In the winter, colder-over-warmer negative stratification develops. Notice how the thermocline in winter is deeper than the thermocline that develops in the summer. Lake Superior is definitely cold!
Lake Superior is dimictic, meaning that twice each year the water column is able to freely circulate from the surface to the floor (turnover). During the rest of the year, the water column separates into layers based on water density (stratification).
Water's tricky and unusual property — the one that enables lakes to stratify — is density. Water is most dense, and therefore heaviest, at 3.94°C (39.2°F). Thermal layers begin to form in July as Superior's surface waters warm beyond 3.94°C. The sun-warmed lighter water in the epilimnion layer becomes separated from the denser water in the hypolimnion layer by a transition zone where the temperature of the water column changes sharply (the thermocline, also know as the metalimnion). The thermocline creates a thermal barrier between the surface and bottom waters. In Lake Superior the summer thermocline hovers around 30 meters deep, in part due to water clarity and wind.
Stratification becomes more pronounced and deeper as Lake Superior's surface warms to a September high. As the winter solstice approaches and the buoyant surface waters cool, the density between layers becomes increasingly similar. When the density is similar enough, a windstorm can mix the entire lake. This sinking of heavy water and mixing by wind results in the exchange of surface and bottom waters, an event referred to as lake turnover. Global climate patterns influence the timing of the lake's turnover. Turnover is enormously important for redistributing nutrients and oxygen within the water column. The lake is thought to mix rather completely until the surface waters cool enough to show negative stratification, a condition in which positively frigid water (0-3.93°C) lies above very cold water. This period of stratification breaks down in June, after the returning sun has warmed the surface water back to 3.94°C and its maximum density 1.000 gm/ml. This is when spring turnover occurs.
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. The land sunk under the weight of the basalt as the Earth's crust pulled apart. The rifting and sinking formed 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.