Though the five lakes have separate basins, they form a single, connected body of freshwater. The lakes connect the east-central interior of North America to the Atlantic Ocean. Lakes Michigan, Huron and Erie are approximately equally high and ships can easily pass from one to the next. Water flows from Lake Superior and Lake Michigan into Lake Huron; then through the Detroit River into Lake Erie; then through Niagara Falls into Lake Ontario; and then through the Saint Lawrence River to the Atlantic Ocean. Water also drains from the Chicago River on the south.
Many rivers flow through a large watershed into the lakes. The lakes have about 35,000 islands. The Great Lakes region includes the five lakes and many thousands of smaller lakes, often called "inland lakes".
Lake Michigan and Lake Huron hit all-time record low levels in 2013.
The unusual shape of the Great Lakes has created the possibility of large waves called seiche. If a storm causes a fast, strong increase in air pressure on one side of a lake, the water level on that side of the lake will drop and suddenly push up the water level on the opposite side of the lake. A 10 foot tall wave in Chicago caused several deaths in 1954.
The Great Lakes are home to a variety of species of fish and other organisms. In recent years, overfishing caused a decline in lake trout. The drop in lake trout increased the alewife population. In response, the government introduced salmon as a predator to decrease the alewife population. This program was so successful that the salmon population rose rapidly, and the states surrounding Lake Michigan promoted 'salmon snagging'. This has been made illegal in all of the Great Lakes states except for a limited season in Illinois. Lake Michigan is now being stocked with several species of fish. However, several invader species such as lampreys, round goby, and zebra mussels threaten the native fish populations.
Accidentally introduced species are a big problem. Since the 19th century about 160 species have invaded the Great Lakes ecosystem, causing severe economic and ecological impacts. According to the Inland Seas Education Association, they deprive fish of food, cause blooms of toxic algae, and foul boats, spawning areas and drinking water intakes. On average a new invasive species enters the Great Lakes every eight months.
Two important infestations in the Great Lakes are the zebra mussel, first discovered in 1988, and the quagga mussel in 1989. These molluscs are efficient filter feeders. They compete with native mussels, and also reduce available food and spawning grounds for fish.
Also, the mussels hurt 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. Because the quagga mussel is good at filtering plankton from the lake water, sunlight reaches deeper into the lake. This increases the growth of algae.
Chemicals from industrial plants run off the land into rivers and arrive in the lakes. Some of these chemicals are highly toxic, such as mercury. Contaminated water from sewer overflows also reaches the lakes, and beaches get closed because of the threat of pathogenic bacteria.
GNU Free Documentation License
The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for open content such as software. It was made by the Free Software Foundation (FSF) for the GNU project. It was initially created for use with software documentation, but can be applied to other types of works as well, such as Wikipedia.
As a copyright license, the GFDL is a type of contract between the creator of a copyrightable work (such as a book, an encyclopedia article, a painting, or a piece of music) and anyone else who might want to use it. The GFDL is considered "copyleft" because the license is meant to make it easier to use and re-use the copyrighted work, not to restrict its use.
If a copyrightable work is released under the GFDL, the creator of the work is saying that anyone else may reproduce, distribute, or modify the work, as long as they follow a set of requirements specified in the GFDL. Among the requirements of the GFDL are that any new work created from the original work is also licensed under the GFDL—that is, once something is licensed as GFDL, it will always stay licensed as GFDL, and anything which uses it also is licensed as GFDL.
The GFDL also says that in order to distribute or modify a work licensed with the GFDL, the re-user must give credit to any previous authors of the work, and include a list of changes they made to the work.
Finally, any work licensed with the GFDL must contain, somewhere, the entire text of the license. This provision has been criticized, because it is not always easy to include an entire, long license with a copyrighted work. In a book, for example, it is easy to include one extra page with the license, but if the work is something like a song, or a photograph, it is not easy.
The GFDL has other requirements that are more complicated. For example, if part of the work is labeled as an "invariant section," it cannot ever be removed or changed by someone using the work ("invariant" means "does not change").
Works licensed under the GFDL may be included in with non-GFDL-licensed works only if it is clear which parts of the work are licensed as the GFDL. For example, in a book of poetry it would be easy to label some poems as licensed under the GFDL and some not licensed under it. But it would not be easy to label if part of a song was licensed as GFDL and the rest was not, so this would not be allowed.
Any use of GFDL material which violates the terms of the GFDL is potentially copyright infringement. Infringement issues are managed through a community based approach with the approval and assistance of the Free Software Foundation.
A number of online projects use the GFDL. An online project to license its content under the GFDL is Wikipedia.
The GFDL has been criticized by many people who wish that it made it even easier for content to be re-used. Among the criticisms are that it is very hard to combine GFDL material with other copyleft licenses, that it is not always clear and easy to understand, and that some of its requirements, such as the "invariant sections", are not free at all.
The GFDL was released in draft form for feedback in September 1999. After revisions, version 1.1 was issued in March 2000, version 1.2 in November 2002, and version 1.3 in November 2008. The current state of the license is version 1.3.
Material licensed under the current version of the license can be used for any purpose, as long as the use meets certain conditions.
Glass
Glass is a hard material that can be made in many shapes. It is usually transparent, but it can also be made in colours. Glass is mainly made of silica; glass made of silica only is called silica glass.
Glass used to make windows and bottles is a specific type called soda-lime glass, composed of about 75% silicon dioxide (SiO), sodium oxide (NaO) from sodium carbonate (NaCO), calcium oxide, also called lime (CaO), and several minor additives.
By changing the proportions, and adding different ingredients, many kinds of glass can be made. Coloured glass is made by adding small amounts of metal oxides. For example, a blue colour is given by tiny amounts of cobalt oxide.
Crystal glass is made by adding lead and zinc oxides. It is not actually a crystal because all glass is a non-crystalline solid. Crystal glass is called cut glass if it has been cut by hand:
Because glass is used to make lenses, the word "glasses" often means eyeglasses.