Superior Pursuit: Facts About the Greatest Great Lake

From the sunny calm of July to the gray crashing waves of November, the lake helps shape the climate, landscape, economy, and quality of life along the shore. Here are some facts about Lake Superior that highlight the greatness of this inland sea.

Size

Lake Superior covers 31,700 square miles (82,100 square kilometers), about the same size as Maine or the combined provinces of New Brunswick and Prince Edward Island. It is the greatest Great Lake--the largest freshwater lake in the world by surface area, and the third largest by volume. Only Lake Baikal in Siberia and Lake Tanganyika in East Africa contain a greater volume of fresh water.

Lake Superior could hold the water from all of the other Great Lakes, along with three more Lake Eries. Because of its great size, Superior has a retention time of 191 years. Retention time measures how long water stays in the lake, based on volume and the mean rate of outflow.

Lake Superior is the deepest of the Great Lakes. Along much of Minnesota's North Shore, the lake is 700 feet deep (213 meters) within 3 miles (4.8 kilometers) of land. If placed at this depth, Montreal's landmark, Mount Royal, would be a small island. The deepest spot, 1,332 feet (406 meters), is 40 miles (64 kilometers) off Munising, Michigan. One of the world's tallest buildings, Chicago's Sears Tower, could fit in that hole with only a few floors sticking above the water.

If you drive the 350 miles (563 kilometers) from Ottawa to Niagara Falls, you have traveled the length of Lake Superior. To appreciate the lake's width, drive the 160 miles (257 kilometers) from Duluth to Minneapolis. If Lake Superior's shoreline were unraveled into a highway, it would extend 1,826 miles (2,938 kilometers), the distance from Duluth to Miami. Lake Superior's watershed covers 49,300 square miles (127,700 square kilometers). This basin is small for the size of the lake.

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.

H₂0

Lake Superior contains three quadrillion (3,000,000,000,000,000) gallons of water (or 11,400,000,000,000,000 liters). That's ten percent of the world's fresh surface water and over half of the water in the Great Lakes. Lake Superior contains enough water to submerge all of North and South America under one foot of water.

Each year, about 2.3 feet (70 centimeters) of water fall on the lake as rain or snow, and nearly two more feet enter through streams or ground water. The Nipigon River in Ontario and the St. Louis River in Duluth/Superior are two of the largest tributaries; their combined input makes up roughly one-fourth of the total river inflow into Lake Superior. Of the roughly 4 feet of water coming into the lake, 1.6 feet (49 centimeters) evaporate and the remainder flows to the lower Great Lakes. The average annual outflow from Lake Superior into Lake Huron is 70,600 cubic feet (2,000 cubic meters) of water per second. That's about 28 times the flow of the St. Louis River as it enters Lake Superior.

Tidal motion, caused by the gravitational pull of the sun and moon is negligible in all lakes, including Lake Superior. Instead, weather conditions produce a tidal-like rise and fall called a seiche ("saysh"). Seiches occur after water is piled up on one side of the lake by wind or high barometric pressure. When this force shifts or diminishes, the water rocks back and forth from one shore to the other with decreasing amplitude, like water in a pan that's been tipped then laid flat. In Lake Superior, the period of a seiche (the time it takes for water to slosh between the two farthest coasts) is about 8 hours. Such seiches can affect water levels by as little as 3 inches (7 centimeters), or as much as 3 feet (90 centimeters), or more.

Weather

Lake Superior, with an average annual temperature of 45° F (7° C), 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 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 with the sounds of gulls and fog horns, have a powerful appeal.

Fall is the season for "northeasters." These gales, with strong winds and rain, occur when low pressure systems pass over the lake. Most of Lake Superior's 350 shipwrecks occurred during such storms.

In winter, temperatures near the lake rarely dip below -30° F (-35° C). Inland temperatures, on the other hand, can reach -45° F (-43° C).

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 (889 centimeters) of snow, while downtown Duluth receives an average of only 55 inches (140 centimeters). Because the air cools and releases moisture as it rises, more snow — 79 inches (200 centimeters) — 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 to 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.

Tourism

The Lake Superior basin's natural beauty attracted visitors long before the phrase ecotourism became vogue. The basin is about 90 percent forested and is home to about 425,000 U.S. citizens and 182,000 Canadians. There are only two large metropolitan areas: Duluth/Superior and Thunder Bay, Ontario.

An estimated 3.5 million visitors came to Duluth and the North Shore in 1997. Sightseeing, fishing, and pleasure boating draw people there. Lake Superior's natural air conditioning offers a break from summer heat. The Superior Hiking Trail is one of the premier trails in the country. Snow season brings people for downhill and cross-country skiing, snowmobiling, ice fishing, and dog sledding. Outdoor events such as a waterfront blues music festival, a fisherman's picnic, a nationally acclaimed in-line skating and a marathon running race, to a premier sled dog marathon, are activities that attract thousands year-round. Tourism had an economic impact of $583 million for the three coastal Minnesota counties in 1995.

Local governments are developing new boat launches, a kayak water trail, protected harbors, and marinas to provide better access for boaters to enjoy the North Shore.

Although the water is cold, it is also clear, making SCUBA diving in Lake Superior exciting. Divers are attracted to the North Shore's craggy coast, the final resting place for many shipwrecks, five of which are favorite dive sites. Interesting underwater rock formations and shallow rock reefs also attract divers.

Shipping

Lake Superior is connected to the Atlantic Ocean by the St. Lawrence Seaway. The 2,343 mile (3,770 kilometer) trip by boat from Duluth/Superior to the ocean takes about seven days.

Duluth/Superior is the busiest inland port in the country, with more than 1,000 vessels visiting annually. In 2007, 1,231 vessels visited the port -- 781 U.S., 292 Canadian, and 158 overseas vessels. Taconite (pelletized iron ore) and coal are the major domestic cargoes. Taconite is shipped to steel mills on the lower Great Lakes. Low-sulfur western coal is shipped to Michigan, where it is burned to generate electricity.

Water Quality

Lake Superior is the cleanest, clearest, and coldest of the Great Lakes. Low concentrations of nutrients, suspended sediments, and organic material create an underwater visibility averaging 27 feet (8 meters). The water is usually more turbid at stream mouths, especially near the St. Louis River. After storms and on windy days, the water in the western tip of Lake Superior often becomes reddish-brown as red clay erodes from the Wisconsin shore.

Acid precipitation falls on Lake Superior as rain and snow but the lake is so large that it has not been noticeably affected. Spring runoff from the melting of acid snow could cause "pulses" of acidity in streams, which could effect spawning in fish. Acid precipitation should not be a serious problem in the near future, however.

Toxic pollution in Lake Superior is low compared to the other Great Lakes. Nevertheless, toxins such as mercury, dioxin, and PCBs pose a threat because they do not break down. They remain in the water and sediment, and accumulate in the tissues of plants and animals, particularly those high on the food chain such as lake trout, loons, and even human beings. Most of these toxic substances come from the atmosphere, entering the lake through precipitation or dust. Scientists believe that air currents carry contaminants from hundreds or even thousands of miles away.

As a result of the Binational Agreement in 1991, the United States and Canada, along with the governments of Michigan, Minnesota, Wisconsin, and the province of Ontario, agreed to establish a demonstration program on Lake Superior. The goal of the program is to achieve zero discharge of 9 persistent toxic substances. Despite progress towards this goal, state advisories still recommend that anglers limit their consumption of some kinds of fish.

The Fishery

For all its size, Lake Superior has less capacity to support aquatic life than do the other Great Lakes. Lake Superior is ultra-oligotrophic, which means it has less dissolved nutrients. As a result, the Lake Superior fishery produces only about 10 percent of what Lake Michigan, a comparatively nutrient-rich lake, produces annually.

Another major factor impacting the Lake Superior fishery is the intentional and unintentional introduction of new fish and other aquatic species. Introductions began before 1900, with stocking of rainbow trout (steelhead) and brown trout. After 1965, coho, chinook, and Atlantic salmon, and splake (a cross between lake trout and brook trout) were added. Only two kinds of trout (also known as char) are native to Lake Superior: lake trout and brook trout.

Lake Superior's fishery has improved tremendously during the last 30 years due to stocking, natural reproduction, and lamprey control. The health of the lake trout population has eliminated the need to stock them in Lake Superior. Lake trout provide an excellent sport fishery and, in a few parts of Lake Superior, a viable commercial fishery.

In Minnesota, the commercial catch of lake trout is limited to an assessment fishery: commercial gill-netters harvest a limited number of lake trout in exchange for providing biological information to the Minnesota Department of Natural Resources. Species currently harvested from Lake Superior commercially include lake trout, whitefish, herring, chubs (deepwater ciscoes), siskowet (deepwater lake trout), and smelt.

Both trout and salmon support charter and recreational fishing fleets that contribute to the region's economy.

Aquatic Invasive Species

Sea Lamprey and Smelt

Sea lamprey and rainbow smelt spread to the Great Lakes accidentally. The lamprey, a parasitic fish, entered the Upper Great Lakes from the Atlantic Ocean after the Welland Canal, which lets ships bypass Niagara Falls, was reconstructed in 1919. Adult sea lamprey feed by attaching themselves to their prey, rasping a hole in the skin, and consuming blood and body fluids. Each sea lamprey can destroy up to 40 pounds (18 kilograms) of fish in the 12 to 18 months it spends in the lake.

Sea lamprey reached Lake Superior in 1938. Together with overfishing, they helped decimate populations of lake trout and other large fish. The lake's annual lake trout harvest, which had averaged 4.5 million pounds (1,814,000 kilograms) from the 1920s through the late 1940s, fell to 500,000 pounds (227,000 kilograms) by 1960.

Lamprey still cause severe losses of Great Lakes fish, but their numbers are controlled chemically, mechanically, and biologically.

By the 1960s, rainbow smelt, which were introduced in the 1930s, became the most abundant fish in Lake Superior. Smelt increased rapidly following the decline of the lake trout. Native lake herring, the major food of lake trout and other predators, once provided an 18-million-pound (8-million-kilogram) annual commercial harvest, but declined to near extinction by the mid-1970s. Smelt, along with intense commercial fishing, may have played a key role in the decline of herring. Since 1979, however, smelt have declined significantly and herring have rebounded.

Eurasian Ruffe

New harmful exotics continue to cause changes that could have severe economic repercussions in the Great Lakes. The Eurasian ruffe (pronounced "rough"), a perch-like fish from Europe, was discovered in the St. Louis River in 1986. By 1992, ruffe had become the most numerous fish in the river, as measured by trawl samples.

Spiny Waterflea

Bythotrephes longimanus, a European zooplankton (a tiny crustacean) known as spiny waterflea, first appeared in Lake Ontario in 1982 and in Lake Superior in 1987. It is now commonly seen on fishing lines in Lake Superior during late summer. B.C., which can grow to about one-half inch long, might compete with small fish for food by eating smaller zooplankton. Unfortunately, larval fish that feed on zooplankton cannot eat spiny waterflea because it is fairly large and has a long, spiny tail.

Zebra Mussels and Quagga Mussels

Zebra mussels and quagga mussels are causing severe problems in the other Great Lakes but are still scarce in Lake Superior beyond the Duluth-Superior Harbor. Large numbers can attach to and clog the interior surfaces of boat motors and water intake pipes. They can also attach to native mussels and smother them.

Round Goby

Round Gobies were found in the Duluth-Superior harbor in 1995 after first appearing in the Great Lakes in the St. Clair River in 1990. Once introduced into new waters, the population of this bottom-dwelling fish increases rapidly. The round goby displaces native fish, eats their young, takes over optimal habitat, spawns up to six times per season, and survives in poor quality water -- giving them a competitive advantage.

Many non-native species are thought to have entered the Great Lakes in ballast water discharged from oceangoing vessels. Requirements that ships exchange ballast water in mid-ocean may reduce the likelihood of other exotics entering and disrupting the Great Lakes ecosystem.