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Great Lakes

Related subjects: Geography

Background Information

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A satellite image of the Great Lakes.
A map showing the Great Lakes.

The Laurentian Great Lakes are a chain of freshwater lakes located in eastern North America, on the Canada-United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth. They are sometimes referred to as inland seas or Canada and the United States' Third Coast.


The Great Lakes region contains not only the five main lakes themselves, but also numerous minor lakes and rivers, as well as approximately 35,000 islands.

Great Lakes

Lake Lake Erie Lake Huron Lake Michigan Lake Ontario Lake Superior
Surface area 9,940 sq mi (25,700 km2) 23,010 sq mi (59,600 km2) 22,400 sq mi (58,000 km2) 7,540 sq mi (19,500 km2) 31,820 sq mi (82,400 km2)
Water volume 116 cu mi (480 km3) 849 cu mi (3,540 km3) 1,180 cu mi (4,900 km3) 393 cu mi (1,640 km3) 2,900 cu mi (12,000 km3)
Elevation 571 ft (174 m) 577 ft (176 m) 577 ft (176 m) 246 ft (75 m) 609 ft (186 m)
Average depth 62 ft (19 m) 195 ft (59 m) 279 ft (85 m) 283 ft (86 m) 483 ft (147 m)
Maximum depth 210 ft (64 m) 770 ft (230 m) 923 ft (281 m) 808 ft (246 m) 1,332 ft (406 m)
Major settlements Buffalo, NY
Cleveland, OH
Erie, PA
Toledo, OH
Sarnia, ON
Port Huron, MI
Bay City, MI
Chicago, IL
Gary, IN
Green Bay, WI
Milwaukee, WI
Hamilton, ON
Kingston, ON
Oshawa, ON
Rochester, NY
Toronto, ON
Mississauga, ON
Duluth, MN
Sault Ste. Marie, ON
Thunder Bay, ON
Marquette, MI
Relative elevations, average depths, maximum depths, and volumes of the Great Lakes.

Notes: The area of each rectangle is proportionate to the volume of each lake. All measurements at Low Water Datum.
Source: EPA

Lake Michigan-Huron

Lakes Michigan and Huron are hydrologically a single lake, sometimes called Lake Michigan-Huron; they have the same surface elevation of 577 feet (176 m), and are not connected by a river but by the 295-foot (90 m) deep Straits of Mackinac.


Sarnia, ON, the largest city on Lake Huron, and the St. Clair River shoreline. The smokestacks of Chemical Valley along the river are visible in the background.
  • The St. Marys River connects Lake Superior to Lake Huron.
  • The St. Clair River connects Lake Huron to Lake St. Clair
  • The Detroit River connects Lake St. Clair to Lake Erie.
  • The Niagara River, including Niagara Falls, connects Lake Erie to Lake Ontario.
  • The St. Lawrence River connects Lake Ontario to the Atlantic Ocean

Other bodies of water

  • Georgian Bay is a large bay located within Lake Huron, separated by the Bruce Peninsula and Manitoulin Island. It contains the majority of the islands of the Great Lakes, with a count of approximately 30,000.
  • The Straits of Mackinac connects Lake Michigan to Lake Huron.
  • The Welland Canal connects Lake Erie to Lake Ontario, bypassing the Niagara River which cannot be fully navigated due to the presence of Niagara Falls.
  • Lake St. Clair is the smallest lake in the Great Lake system but due to its relatively small size (compared to the five "Great Lakes"), it's rarely, if ever, considered a Great Lake.
The shoreline of a beach in the Apostle Islands, Lake Superior


Dispersed throughout the Great Lakes are approximately 35,000 islands. The largest among them is Manitoulin Island in Lake Huron, the largest island in any inland body of water and home to the world's largest lake within a lake, Lake Manitou. The second-largest island is Isle Royale in Lake Superior. Both of these islands are large enough to contain multiple lakes themselves.

Connection to ocean and open water

The Saint Lawrence Seaway and Great Lakes Waterway opened the Great Lakes to ocean-going vessels. The move to wider ocean-going container ships — which do not fit through the locks on these routes — has limited shipping on the lakes. Despite their vast size, large sections of the Great Lakes freeze over in winter, interrupting most shipping. Some icebreakers ply the lakes.

The Great Lakes are also connected to the Gulf of Mexico by way of the Illinois River (from Chicago), to the Mississippi, to the Gulf. An alternate track is via the Illinois River (from Chicago), to the Mississippi, to the Ohio, up the Ohio, and then through the Tennessee-Tombigbee Waterway (combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf. Commercial tug-and-barge traffic on these waterways is heavy.

Pleasure boats can also enter or exit the Great Lakes by way of the Erie Canal and Hudson River in New York. The Erie Canal connects to the Great Lakes at the east end of Lake Erie (at Buffalo, NY) and at the south side of Lake Ontario (at Oswego, NY).


The lakes are bounded by the Canadian province of Ontario and the U.S. states of Minnesota, Wisconsin, Michigan, Illinois, Indiana, Ohio, Pennsylvania, and New York; however, not all of the lakes border on all of these regions. Four of the five lakes form part of the Canada-United States border; the fifth, Lake Michigan, is contained entirely within the United States. The Saint Lawrence River, which marks the same international border for a portion of its course, is the primary outlet of these interconnected lakes, and flows through Quebec and past the Gaspé Peninsula to the northern Atlantic Ocean.


The Great Lakes contain roughly 22% of the world’s fresh surface water: 5,472 cubic miles (22,810 km3), or 6.0×1015 U.S. gallons (2.3×1016 liters). This is enough water to cover the 48 contiguous U.S. states to a uniform depth of 9.5 feet (2.9 m).

The combined surface area of the lakes is approximately 94,250 square miles (244,100 km2)—nearly the same size as the United Kingdom, and larger than the U.S. states of New York, New Jersey, Connecticut, Rhode Island, Massachusetts, Vermont and New Hampshire combined.

The Great Lakes coast measures approximately 10,500 miles (16,900 km); however, the length of a coastline is impossible to measure exactly and is not a well-defined measure (see Coastline paradox).

Geological history

A diagram of the formation of the Great Lakes.

The foundation of the Great Lakes was laid approximately two billion years ago, when two tectonic plates fused and created the Midcontinent Rift, forming a valley that was the basis of Lake Superior. When a second fault line, the Saint Lawrence rift, formed approximately 570 million years ago, the basis for Lakes Ontario and Erie was created, along with what would become the St. Lawrence River.

The Great Lakes were formed at the end of the last ice age about 10,000 years ago, when the Laurentide ice sheet receded. When this happened, the glaciers left behind a large amount of meltwater (see Lake Agassiz) which filled up the basins that the glaciers had carved, thus creating the Great Lakes as we know them today. Because of the uneven nature of glacier erosion, some higher hills became Great Lakes islands. The Niagara Escarpment follows the contour of the Great Lakes between New York and Wisconsin.


Lake effect

The effect of Great Lakes on weather in the region is called the lake effect. In winter, the moisture picked up by the prevailing winds from the west can produce very heavy snowfall, especially along lakeshores to the east such as Michigan, Ohio, Pennsylvania, Ontario, and New York. The lakes also moderate seasonal temperatures somewhat, by absorbing heat and cooling the air in summer, then slowly radiating that heat in autumn. This temperature buffering produces areas known as "fruit belts", where fruit typically grown farther south can be produced. Western Michigan has apple and cherry orchards, and vineyards adjacent to the lakeshore as far north as the Grand Traverse Bay. The eastern shore of Lake Michigan and the southern shore of Lake Erie have many wineries as a result of this, as does the Niagara Peninsula between Lake Erie and Lake Ontario. A similar phenomenon occurs in the Finger Lakes region of New York as well as Prince Edward county on the northeast shore of Lake Ontario. Related to lake effect, is the occurrence of fog over medium-sized areas, particularly along the shorelines of the lakes. This is most noticeable along Lake Superior's shores, due to its maritime climate.

The Great Lakes have been observed to help strengthen storms, such as Hurricane Hazel in 1954, and a frontal system in 2007 that spawned a few tornadoes in Michigan and Ontario, picking up warmth from the lakes to fuel them. Also observed in 1996, was a rare subtropical cyclone forming in Lake Huron, dubbed the 1996 Lake Huron cyclone.


The lakes are extensively used for transport, though cargo traffic has decreased considerably in recent years. The Great Lakes Waterway makes each of the lakes accessible.

Historical economy

During settlement, the Great Lakes and its rivers were the only practical means of moving people and freight. Barges from middle North America were able to reach the Atlantic Ocean from the Great Lakes when the Erie Canal opened in 1825. By 1848, with the opening of the Illinois and Michigan Canal at Chicago, direct access to the Mississippi River was possible from the lakes. With these two canals an all-inland water route was provided between New York City and New Orleans.

The main business of many of the passenger lines in the 1800s was transporting immigrants. Many of the larger cities owe their existence to their position on the lakes as a freight destination as well as for being a magnet for immigrants. After railroads and surface roads developed, the freight and passenger businesses dwindled and except for ferries and a few foreign cruise ships, now has vanished.

The immigration routes still have an effect today. Immigrants often formed their own communities and some areas have a pronounced ethnicity, such as Dutch, German, Polish, Finnish, and many others. Since many immigrants settled for a time in New England before moving westward, many areas on the U.S. side of the Great Lakes also have a New England feel, especially in home styles and accent.

Since general freight these days is transported by railroads and trucks, domestic ships mostly move bulk cargoes, such as iron ore, coal and limestone for the steel industry. The domestic bulk freight developed because of the nearby mines. It was more economical to transport the ingredients for steel to centralized plants rather than try to make steel on the spot. Ingredients for steel, however, are not the only bulk shipments made. Grain exports are also a major cargo on the lakes.

In the 19th and early 20th centuries, iron and other ores such as copper were shipped south on (downbound ships), and supplies, food, and coal were shipped north (upbound). Because of the location of the coal fields in Pennsylvania and West Virginia, and the general northeast track of the Appalachian Mountains, railroads naturally developed shipping routes that went due north to ports such as Erie, Pennsylvania and Ashtabula, Ohio.

Because the lake maritime community largely developed independently, it has its own language. Ships, no matter the size, are called boats. When the sailing ships gave way to steamships, they were called steamboats—the same term used on the Mississippi. The ships also have a distinctive design. Ships that primarily trade on the lakes are known as lakers. Foreign boats are known as salties.

One of the more common sights on the lakes is the 1,000‑by‑105 foot (305-by-32 m), 78,850-long-ton (80,120-metric-ton) self-unloader. This is a laker with a conveyor belt system that can unload itself by swinging a crane over the side. Today, the Great Lakes fleet is much smaller in numbers than it once was because of the increased use of overland freight, and a few larger ships replacing many small ones.

A woodcut of Le Griffon

Modern economy

The Great Lakes are used as a major mode of transport for bulk goods. The brigantine Le Griffon (The Griffon), which was commissioned by René Robert Cavelier, Sieur de La Salle, was built at Cayuga Creek, near the southern end of the Niagara River, to become the first sailing ship to travel the upper Great Lakes on August 7, 1679.

In 2002, 162 million net tons of dry bulk cargo were moved on the Lakes. This was, in order of volume: iron ore, grain, and potash. The iron ore and much of the stone and coal are used in the steel industry. There is also some shipping of liquid and containerized cargo but most container ships cannot pass the locks on the Saint Lawrence Seaway because they are too wide. The total amount of shipping on the lakes has been on a downward trend for several years.

Recreational boating and tourism are major industries on the Great Lakes. A few small cruise ships operate on the Great Lakes including a couple of sailing ships. Sport fishing, commercial fishing, and Native American fishing represent a US$4 billion a year industry with salmon, whitefish, smelt, lake trout, and walleye being major catches.

The Great Lakes are used to supply drinking water to tens of millions of people in bordering areas. This valuable resource is collectively administered by the state and provincial governments adjacent to the lakes.

Great Lakes Passenger Steamers

From 1844 through 1857, palace steamers carried passengers and cargo around the Great Lakes. Throughout the 20th century, large luxurious passenger steamers sailed from Chicago all the way to Detroit and Cleveland. These were primarily operated by the Detroit & Cleveland Navigation Company. Several ferries currently operate on the Great Lakes to carry passengers to various islands, including Isle Royale, Pelee Island, Mackinac Island, Beaver Island, both Bois Blanc Islands, Kelleys Island, South Bass Island, North Manitou Island, South Manitou Island, Harsens Island, Manitoulin Island, and the Toronto Islands. As of 2007, two car ferry services cross the Great Lakes, both on Lake Michigan: a steamer from Ludington, Michigan to Manitowoc, Wisconsin and a high speed catamaran from Milwaukee to Muskegon, Michigan. An international ferry across Lake Ontario from Rochester, New York to Toronto ran during 2004 and 2005, but is no longer in operation.


The large size of the Great Lakes increases the risk of water travel; storms and reefs are a common threat. The lakes are prone to sudden and severe storms, particularly in the autumn, from late October until early December. The greatest concentration of shipwrecks lies near Thunder Bay, beneath Lake Huron, near the point where eastbound and westbound shipping lanes converge.

Two notable ships that have sunk on the Great Lakes are the SS Edmund Fitzgerald and Le Griffon. The SS Edmund Fitzgerald, which sank November 10, 1975, was the last major freighter lost on the lakes, sinking just over 30 miles (50 km) offshore from Whitefish Point in Lake Superior.

In August 2007, the Great Lakes Shipwreck Historical Society announced that it had found the wreckage of Cyprus, a 420-foot (130 m) long, century-old ore carrier. Cyprus sank during a Lake Superior storm on October 11, 1907, during its second voyage while hauling iron ore from Superior, Wisconsin, to Buffalo, New York. The entire crew of 23 drowned, except one, a man named Charles Pitz, who floated on a life raft for almost seven hours.

In June 2008 deep sea divers found the wreck of the 1780 Royal Navy warship, HMS Ontario in Lake Ontario and has been described as an archaeological miracle. There are no plans to raise her as it is being treated as a war grave.

Political issues

Great Lakes water use and diversions

The International Joint Commission was established in 1909 to help prevent and resolve disputes relating to the use and quality of boundary waters, and to advise Canada and the United States on questions related to water resources. Concerns over diversion of Lake water are of concern to both Americans and Canadians. Some water is diverted through the Chicago River to operate the Illinois Waterway but the flow is limited by treaty. Possible schemes for bottled water plants and diversion to dry regions of the continent raise concerns. Under the U.S. "Water Resources Development Act" , diversion of water from the Great Lakes Basin requires the approval of all eight Great Lakes governors, which rarely occurs. International treaties regulate large diversions. In 1998, the Canadian company Nova Group won approval from the Province of Ontario to withdraw 158,000,000 US gallons (600,000 m3) of Lake Superior water annually to ship by tanker to Asian countries. Public outcry forced the company to abandon the plan before it began. Since that time, the eight Great Lakes Governors and the Premiers of Ontario and Quebec have negotiated the Great Lakes-St. Lawrence River Basin Sustainable Water Resources Agreement and the Great Lakes-St. Lawrence River Basin Water Resources Compact that would prevent most future diversion proposals and all long-distance ones. The agreements also strengthen protection against abusive water withdrawal practices within the Great Lakes basin. On December 13, 2005, the Governors and Premiers signed these two agreements, the first of which is between all ten jurisdictions. It is somewhat more detailed and protective, but cannot be enforced in court because enforcement arrangements can be made only between the federal governments. The second, The Great Lakes Compact, has been approved by the state legislatures of all eight states that border the Great Lakes and was submitted for approval to the U.S. Congress on July 23, 2008; this legislation could be enforced in U.S. federal court.

Coast Guard live fire exercises

In 2006, the United States Coast Guard (USCG) proposed a plan to designate 34 areas in the Great Lakes, at least five miles (8 km) offshore, as permanent safety zones for live fire machine gun practice. In August, 2006 the plan was published in the Federal Register. The USCG reserved the right to hold target practice whenever the weather allowed with a two hour notice. These firing ranges would be open to the public when not in use. In response to requests from the public, the Coast Guard held a series of public meetings in nine U.S. cities to solicit comment. During these meetings many people voiced concerns about the plan and its impact on the environment.

A preliminary health risk assessment stated that the “proposed training will result in no elevated risks for a freshwater system such as the Great Lakes using ”realistic worst case” assumptions, and further investigation is not recommended … if typical rather than worst case assumptions were used, the predicted risk would be even less.” However, the assessment was based on lead levels after five years, and so one could infer that lead levels could meet or exceed EPA safe levels for lead after fifteen years. The Coast Guard established an information page about their proposal at

On December 18, 2006, the Coast Guard announced its decision to withdraw the entire proposal. Officials said they would look into alternative ammunition, modifying the proposed zones and have more public dialogue before proposing a new plan.

Great Lakes Collaboration Implementation Act

During the 109th United States Congress in 2006, the Great Lakes Collaboration Implementation Act (Bill HR5100) was introduced to enact the recommendations of the Great Lakes Regional Collaboration, an effort established in 2004 to produce a strategy for restoring and maintaining the Great Lakes. The bill was introduced by U.S. senators Mike DeWine and Carl Levin, along with representatives Vern Ehlers and Rahm Emanuel.

The bill states that "the Great Lakes are on the brink of an ecologic catastrophe" and that "if the pattern of deterioration is not reversed immediately, the damage could be irreparable". It cites the closing of over 1,800 beaches in 2003, the 6,300-square-mile (16,300 km2) dead zone in Lake Erie, and the US$500 million damage each year due to the zebra mussel as evidences that "a comprehensive restoration of the system is needed to prevent the Great Lakes from collapsing".

A press release states that the bill aims to stop the introduction and spreading of invasive species, prevent the Asian carp from invading the Great Lakes, phase out mercury, restore animal habitats, and prevent sewage contamination.

A coalition called Healthy Lakes, Healthy Lives was formed by several environmental groups and foundations in 2005 to educate and assist citizens in advocating for the cleanup of the Great Lakes.

Additions to the five Great Lakes

Lake Champlain, a lake on the border between upstate New York and northwestern Vermont that is part of the Saint Lawrence-Great Lakes Watershed, briefly became labeled by the U.S. government as the sixth "Great Lake of the United States" on March 6, 1998, when President Clinton signed Senate Bill 927. This bill, which reauthorized the National Sea Grant Program, contained a line penned by Senator Patrick Leahy (D-VT) declaring Lake Champlain to be a Great Lake. Not coincidentally, this status allows neighboring states to apply for additional federal research and education funds allocated to these national resources. The claim was viewed with some amusement by other countries, particularly in the Canadian media, and the lake is small compared to other Canadian lakes (such as Great Bear Lake which has over 27 times more surface area). Following a small uproar (and several New York Times and Time Magazine articles), the Great Lake status was rescinded on March 24, 1998 (although Vermont universities continue to receive funds to monitor and study the lake).

Similarly, there has been interest in making Lake St. Clair a Great Lake. In October 2002, backers planned to present such a proposal at the Great Lakes Commission annual meeting, but ultimately withheld it as it appeared to them to have too little support.


Ecological challenges

Before the arrival of Europeans, the Great Lakes provided fish to the indigenous groups who lived near them. Early European settlers were astounded by both the variety and quantity of fishes; there were 150 different species in the Great Lakes. Historically, fish populations were the early indicator of the condition of the Lakes, and have remained one of the key indicators even in the current era of sophisticated analyses and measuring instruments. According to the bi-national (U.S. and Canadian) resource book, The Great Lakes: An Environmental Atlas and Resource Book, "the largest Great Lakes fish harvests were recorded in 1889 and 1899 at some 67,000 tonnes [147 million pounds]," though the beginning of environmental impacts on the fish can be traced back nearly a century prior to those years.

By 1801, the New York Legislature found it necessary to pass regulations curtailing obstructions to the natural migrations of Atlantic salmon from Lake Erie into their spawning channels. In the early nineteenth century, Upper Canada’s government found it necessary to introduce similar legislation prohibiting the use of weirs and nets at the mouths of Lake Ontario’s tributaries. Other protective legislation was passed as well, but enforcement remained difficult and often quite spotty.

On both sides of the Canada–United States border, the proliferation of dams and impoundments multiplied, necessitating more regulatory efforts. The decline in fish populations was unmistakable by the middle of the nineteenth century, as the obstructions in the rivers prevented salmon and sturgeon from reaching their spawning grounds. The decline in salmon was recognized by Canadian officials and reported as virtually a complete absence by the end of the 1860s. The Wisconsin Fisheries Commission noted a reduction of roughly 25 percent in general fish harvests by 1875. Many Michigan rivers sport multiple dams that range from mere relics to those with serious loss of life potential. The state's dam removal budget has been frozen in recent years; in the 1990s, the state was removing 1 dam per year.

Overfishing was cited as responsible for the decline of the population of various whitefish, important because of their culinary desirability and, hence, economic consequence. Moreover, between 1879 and 1899, reported whitefish harvests declined from some 24.3 million pounds (11 million kg) to just over 9 million pounds (4 million kg). Recorded sturgeon catches fell from 7.8 million pounds (1.5 million kg) in 1879 to 1.7 million pounds (770,000 kg) in 1899. The population of giant freshwater mussels was eliminated as the mussels were harvested for use as buttons by early Great Lakes entrepreneurs.

There were, however, other factors in the population declines besides overfishing and the problems posed by water obstructions. Logging in the Great Lakes region removed tree cover near stream channels which provide spawning grounds, and this affected necessary shade and temperature-moderating conditions. Removal of tree cover also destabilized soil, allowing soil to be carried in greater quantity into the streambeds, and even brought about more frequent flooding. Running cut logs down the Lakes’ tributary rivers also stirred bottom sediments. In 1884, the New York Fish Commission determined that the dumping of sawmill waste (chips and sawdust) was impacting fish populations.

The Great Lakes are international, and in situations that require regulation, a lack of cooperation between the U.S. and Canada might be predicted to have disastrous consequences. In the development of ecological problems in the Great Lakes, it was the influx of parasitic lamprey populations after the development of the Erie Canal and the much later Welland Canal that led to the two federal governments attempting to work together – which proved a very complicated and troubled road.

Nevertheless, despite the ever more sophisticated efforts to eliminate or minimize the lamprey, by the mid 1950s the lake trout populations of Lakes Michigan and Huron were reduced by about 99%, with the lamprey deemed largely to blame. This led to the launch of the bi-national Great Lakes Fishery Commission.

Other ecological problems in the Lakes and their surroundings have stemmed from urban sprawl, sewage disposal, and toxic industrial effluent. These, of course, also affect aquatic food chains and fish populations. Some of these glaring problem areas are what attracted the high-level publicity of Great Lakes ecological troubles in the 1960s and 1970s. Evidence of chemical pollution in the Lakes and their tributaries now stretches back for decades. In the late 1960s, the recurrent phenomenon of the surface of river stretches (see Ohio’s Cuyahoga River) catching fire from a combination of oil, chemicals, and combustible materials floating on the water’s surface, came to the attention of a public growing more environmentally aware. Another aspect that caught popular attention was the “toxic blobs” (expanses of lake bed covered by various combinations of such substances as solvents, wood preservatives, coal tar, and metals) found in Lake Superior, the St. Clair River, and other portions of the Great Lakes region.

According to the authoritative bi-national source The Great Lakes: An Environmental Atlas and Resource Book, "only pockets remain of the once large commercial fishery."

The annual Great Lakes Bioneers Conference held in Traverse City, Michigan addresses many of these problems with local speakers, workshops and tools. The conference is a satellite conference of the Bioneers Conference in San Rafael, California. The Traverse City site focuses on durable ecological and socially just solutions to a diverse set of issues in the Great Lakes bioregion.

Invasive species

The Great Lakes have suffered from the introduction of many non-native and invasive species. Since the 1800s, more than 160 species have invaded the Great Lakes ecosystem from around the world, causing severe economic and ecological impacts. According to the Inland Seas Education Association, on average a new invasive species enters the Great Lakes every eight months.

A zebra mussel-encrusted Vector Averaging Current Meter from Lake Michigan.

One such infestation in the Great Lakes was the introduction of the zebra mussel, which was first discovered in 1988. The mollusk is an efficient feeder, competing with native mussels. It also reduces available food and spawning grounds for fishes. The zebra mussel also hurts utility and manufacturing industries by clogging or blocking pipes. The U.S. Fish and Wildlife Service estimates that the economic impact of the zebra mussel will be about $5 billion over the next decade.

Approximately 10 percent of nonindigenous aquatic species introduced into the Great Lakes have had significant impacts, both economic and ecological. The remaining 90 percent have potentially harmful impacts but are insufficiently researched and understood. Besides the zebra mussel, several other species have been particularly harmful. The invasion of the sea lamprey, a parasite that attaches to large fishes with a sucker mouth armed with teeth that consume flesh and fluid from its prey, has resulted in substantial economic losses to recreational and commercial fisheries. Protection of the Great Lakes fishery (both native and nonindigenous species) from sea lamprey predation has required annual expenditures of millions of dollars to finance chemical control programs.

Alewife, introduced through the canal systems built in the Great Lakes, littered beaches each spring and altered food webs, causing increased water turbidity. These impacts subsided with the intentional introduction of salmonids that were stocked as predators to keep alewife populations under control. The ruffe, a small percid fish, became the most abundant fish species in Lake Superior's St. Louis River within five years of its detection in 1986. Its range, which has expanded to Lake Huron, poses a significant threat to the lower lake fishery. Five years after first being observed in the St. Clair River, the round goby can now be found in all of the Great Lakes. The goby is considered undesirable for several reasons: It preys upon bottom-feeding fishes, overruns optimal habitat, spawns multiple times a season, and can survive poor water quality conditions.

Several species of water fleas have accidentally been introduced into the Great lakes such as Bythotrephes cederstroemi and the Fishhook waterflea potentially having an effect on the zooplankton population. Several species of crayfish have also been introduced that may contend with native crayfish populations

An electric fence has been set up across the mouth of the Great Lakes across the Chicago Sanitary and Ship Canal in order to keep several species of invasive Asian carps out of the area. These fast-growing planktivorous fishes are thought to have the potential to cause substantial ecological damage to the Great Lakes, through changes in the food chain and water quality.

Even more rare, there have been reports of bull sharks that have apparently made their way up the Illinois River and into Lake Michigan such as the encounter off the coast of Chicago, Illinois.

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