Lake Superior empties into Lake Huron via the St. Marys River and the Soo Locks. Lake Superior is the largest freshwater lake in the world based on surface area (if Lakes Michigan and Huron are taken separately), and the third largest in volume, behind Lake Baikal in Russia and Lake Tanganyika in 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 31,820 square miles (82,413 km²), which is approximately the size of South Carolina. It has a maximum length of 350 miles (563 km) and maximum breadth of 160 miles (257 km). Its average depth is 482 feet (147 m) with a maximum depth of 1,332 feet (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 1 foot (30 cm) of water. 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 (458 metres (1,503 ft) 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.)

The average temperature of the lake during the summer is about 40 °F (4.4 °C). Lake Superior is the largest, deepest and coldest of the Great Lakes. Superior could contain the volume of all the other Great Lakes and three more Lake Eries. 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.

The lake is fed by over 200 rivers. The largest include the Nipigon River, the St. Louis River, the Pigeon River, the Pic River, the White River, the Michipicoten River, the Bois Brule River and the Kaministiquia River. Lake Superior drains into Lake Huron by the St. Marys River. The rapids on the river necessitate the Sault Locks (pronounced "soo"), a part of the Great Lakes Waterway, to move boats over the 25 feet (8 m) height difference from Lake Huron.

The lake's average surface elevation is 600 feet (183 m) above sea level. Until approximately 1887 the natural hydraulic conveyance through the St. Marys River rapids determined outflow from Lake Superior. By 1921 development in support of transportation and hydropower resulted in gates, locks, power canals and other control structures completely spanning St. Marys rapids. The regulating structure is known as the Compensating Works and is operated according to a regulation plan known as Plan 1977-A. The current water levels, including diversions of water from the Hudson Bay watershed, are governed by the International Lake Superior Board of Control which was established in 1914 by the International Joint Commission.

Superior's water levels temporarily reached a new low in September 2007, slightly less than the previous record low in 1926. However, the water levels returned within a few days.

Historic High Water The lake fluctuates from month to month with the highest lake levels in October and November. The normal highwater mark is 1.17 feet (0.36 m) above datum (601.1 ft or 183.2 metres). In the summer of 1985, Lake Superior reached its highest level at 2.33 feet (0.71 m) above datum. The winter of 1986 set new highwater records through the winter and spring months (January - June), ranging from 1.33 feet (0.41 m) to 1.833 feet (0.559 m) above Chart Datum.

Historic Low Water The lake fluctuates from month to month with the lowest lake levels in March and April. The normal lowwater mark is 0.33 feet (0.10 m) below datum (601.1 ft or 183.2 metres). In the winter of 1926 Lake Superior reached its lowest level at 1.58 feet (0.48 m) below datum. Additionally, the entire first half of the year (January - June) saw record low months. The low water was a continuation of the dropping lake levels from the previous year, 1925; which set lowwater records for October through December. During this nine month period (Oct 1925 - June 1926) water levels ranged from 1.58 feet (0.48 m) to 0.33 feet (0.10 m) below Chart Datum. In the summer of 2007 monthly historic lows were set; August at 0.66 feet (0.20 m), September at 0.58 feet (0.18 m).

According to a study by professors at the University of Minnesota Duluth, Lake Superior may have warmed faster than its surrounding climate. Summer surface temperatures in the lake appeared to have increased by about 4.5 °F (2.5 °C) since 1979, compared with an approximately 2.7 °F (1.5 °C) increase in the surrounding average air temperature. The increase in the lake’s surface temperature may be related to the decreasing ice cover. Less winter ice cover allows more solar radiation to penetrate the lake and warm the water. If trends continue Lake Superior, which freezes over completely once every 20 years, could routinely be ice free by 2040. These warmer temperatures can actually lead to more snow in the lake effect snow belts along the shores of the lake, especially in the Upper Peninsula of Michigan.

The largest island in Lake Superior is Isle Royale in the state of Michigan. Isle Royale contains several lakes, some of which also contain islands. Other large famous islands include Madeline Island in the state of Wisconsin, Michipicoten Island in the province of Ontario, and Grand Island (location of the Grand Island National Recreation Area) in the state of Michigan.

The larger cities on Lake Superior include: the twin ports of Duluth, Minnesota, and Superior, Wisconsin; Thunder Bay, Ontario; Marquette, Michigan; and the twin cities of Sault Ste. Marie, Michigan, and Sault Ste. Marie, Ontario. Duluth, at the western tip of Lake Superior, is the most inland point on the St. Lawrence Seaway and the most inland port in the world.

Among the scenic places on the lake are: the Apostle Islands National Lakeshore, Isle Royale National Park, Porcupine Mountains Wilderness State Park, Pukaskwa National Park, Lake Superior Provincial Park, Grand Island National Recreation Area, Sleeping Giant (Ontario) and Pictured Rocks National Lakeshore.

The Great Lakes Circle Tour is a designated scenic road system connecting all of the Great Lakes and the St. Lawrence River.

Lake Superior's size creates a localized oceanic or maritime climate (more typically seen in locations like Nova Scotia). The water surface's slow reaction to temperature changes, seasonally ranging between 32 and 55 °F (0 - 13 °C) around 1970, helps to moderate surrounding air temperatures in the summer and winter, and creates lake effect snow in colder months. The hills and mountains that border the lake hold moisture and fog, particularly in the fall.

The lake's surface temperature has risen by 4.5 °F (2.5 °C) since 1979.

The rocks of Lake Superior's North Shore date back to the early history of the earth. During the Precambrian (between 4.5 billion and 540 million years ago) magma forcing its way to the surface created the intrusive granites of the Canadian Shield. These ancient granites can be seen on the North Shore today. It was during the Penokean orogeny, part of the process that created the Great Lakes Tectonic Zone, that many valuable metals were deposited. The region surrounding the lake has proved to be rich in minerals. Copper, iron, silver, gold and nickel are or were the most frequently mined. Examples include the Hemlo gold mine near Marathon, copper at Point Mamainse, silver at Silver Islet and uranium at Theano Point.

The mountains steadily eroded, depositing layers of sediments which compacted and became limestone, dolostone, taconite and the shale at Kakabeka Falls.

The continent was later riven, creating one of the deepest rifts in the world. The lake lies in this long extinct Mesoproterozoic rift valley, the Midcontinent Rift. Magma was injected between layers of sedimentary rock, forming diabase sills. This hard diabase protects the layers of sedimentary rock below, forming the flat topped mesas in the Thunder Bay area.

Amethyst formed in some of the cavities created by the Midcontinent Rift and there are several amethyst mines in the Thunder Bay area.

Lava erupted from the rift and formed the black basalt rock of Michipicoten Island, Black Bay Peninsula, and St. Ignace Island.

During the Wisconsin glaciation 10,000 years ago, ice covered the region at a thickness of 1.25 miles (2.01 km). The land contours familiar today were carved by the advance and retreat of the ice sheet. The retreat left gravel, sand, clay and boulder deposits. Glacial meltwaters gathered in the Superior basin creating Lake Minong, a precursor to Lake Superior. Without the immense weight of the ice, the land rebounded, and a drainage outlet formed at Sault Ste. Marie, which would become known as St. Mary's River.

The first people came to the Lake Superior region 10,000 years ago after the retreat of the glaciers in the last Ice Age. They are known as the Plano, and they used stone - tipped spears to hunt caribou on the northwestern side of Lake Minong.

The next documented people were known as the Shield Archaic (c. 5000 - 500 BC). Evidence of this culture can be found at the eastern and western ends of the Canadian shore. They used bows and arrows, dugout canoes, fished, hunted, mined copper for tools and weapons, and established trading networks. They are believed to be the direct ancestors of the Ojibwe and Cree.

The Laurel people (c. 500 BC to AD 500) developed seine net fishing, evidence being found at rivers around Superior such as the Pic and Michipicoten.

Another culture known as the Terminal Woodland Indians (c. AD 900 - 1650) has been found. They were Algonkian people who hunted, fished and gathered berries. They used snow shoes, birch bark canoes and conical or domed lodges. At the mouth of the Michipicoten River, nine layers of encampments have been discovered. Most of the Pukaskwa Pits were likely made during this time.

The Anishinaabe, also known as the Ojibwe or Chippewa, have inhabited the Lake Superior region for over five hundred years and were preceded by the Dakota, Fox, Menominee, Nipigon, Noquet and Gros Ventres. They called Lake Superior Anishnaabe Gichgamiing, or "the Ojibwe's Ocean". After the arrival of Europeans, the Anishinaabe made themselves the middle - men between the French fur traders and other Native peoples. They soon became the dominant Indian nation in the region: they forced out the Sioux and Fox and won a victory against the Iroquois west of Sault Ste. Marie in 1662. By the mid 18th century, the Ojibwe occupied all of Lake Superior's shores.

In the 18th century, the fur trade in the region was booming, with the Hudson's Bay Company having a virtual monopoly. In 1783, however, the North West Company was formed to rival Hudson's Bay Company. The North West Company built forts on Lake Superior at Grand Portage, Nipigon, the Pic River, the Michipicoten River, and Sault Ste. Marie. But by 1821, with competition taking too great a toll on both, the companies merged under the Hudson's Bay Company name.

Many towns around the lake are either current or former mining areas, or engaged in processing or shipping. Today, tourism is another significant industry; the sparsely populated Lake Superior country, with its rugged shorelines and wilderness, attracts tourists and adventurers.

Lake Superior has been an important link in the Great Lakes Waterway, providing a route for the transportation of iron ore and other mined and manufactured materials. Large cargo vessels called lake freighters, as well as smaller ocean going freighters, transport these commodities across Lake Superior.

According to shipwreck historian Frederick Stonehouse, the southern shore of Lake Superior between Grand Marais, Michigan, and Whitefish Point is known as the "Graveyard of the Great Lakes" and more ships have been lost around the Whitefish Point area than any other part of Lake Superior. These shipwrecks are now protected by the Whitefish Point Underwater Preserve. The SS Edmund Fitzgerald was the last major shipwreck on Lake Superior, sinking 17 miles (27 km) from Whitefish Point on November 10, 1975.

According to legend, "Lake Superior seldom gives up her dead". This is because of the unusually low temperature of the water, estimated at under 36 °F (2 °C) on average around 1970. Normally bacteria feeding on a sunken decaying body will generate gas inside the body, causing it to float to the surface after a few days. The water in Lake Superior is cold enough year round to inhibit bacterial growth, and bodies tend to sink and never surface. This is alluded to in Gordon Lightfoot's ballad, "The Wreck of the Edmund Fitzgerald". The Edmund Fitzgerald's 29 crew members all perished. Edmund Fitzgerald adventurer Joe MacInnis reported that in July 1994, explorer Frederick Shannon's Expedition 94 to the Fitzgerald discovered and filmed a man's body near the port side of her pilothouse, not far from the open door, "fully clothed, wearing an orange life jacket, and lying face down in the sediment." No crew members were ever recovered. The Fitzgerald was swallowed up so intensely by Lake Superior that the 729 foot (222 m) ship split in half. Her two pieces are sitting approximately 170 feet (52 m) apart in a depth of 550 feet (170 m).

Storms that claimed multiple ships include the Mataafa Storm on November 28, 1905, and the Great Lakes Storm of 1913.

In August 2007 wreckage was found of the Cyprus, a 420 foot (130 m) ore carrier which sank during a Lake Superior storm in 460 feet (140 m) of water. All but Charles G. Pitz of the Cyprus’ 23 crew perished on October 11, 1907. The ore carrier sank in Lake Superior on her second voyage, while hauling iron ore from Superior, Wisconsin, to Buffalo, New York. Built in Lorain, Ohio, the Cyprus was launched August 17, 1907.

In 1918 the last warships ever to sink in the great lakes, Inkerman and Cerisoles Minesweepers vanished in a Lake Superior storm marking the largest loss of life on Lake Superior to date.

Over 80 species of fish have been found in Lake Superior. Species native to the lake include: bloater, brook trout, burbot, cisco, lake sturgeon, lake trout, lake whitefish, longnose sucker, muskellunge, northern pike, pumpkinseed, rock bass, round whitefish, smallmouth bass, walleye, white sucker and yellow perch. In addition, many fish species have been either intentionally or accidentally introduced to Lake Superior: atlantic salmon, brown trout, carp, chinook salmon, coho salmon, freshwater drum, pink salmon, rainbow smelt, rainbow trout, round goby, ruffe, sea lamprey and white perch.

Lake Superior has fewer dissolved nutrients relative to its water volume compared to the other Great Lakes and so is less productive in terms of fish populations and is an oligotrophic lake. This is a result of the underdeveloped soils found in its relatively small watershed. However, nitrate concentrations in the lake have been continuously rising for more than a century. They are still much lower than levels considered dangerous to human health; but this steady, long term rise is an unusual record of environmental nitrogen buildup. It may relate to anthropogenic alternations to the regional Nitrogen Cycle, but researchers are still unsure of the causes of this change to the lake's ecology.

As for other Great Lakes fish populations have also been impacted by the accidental or intentional introduction of foreign species such as the sea lamprey and Eurasian ruffe. Accidental introductions have occurred in part by the removal of natural barriers to navigation between the Great Lakes. Overfishing has also been a factor in the decline of fish populations.