A lake can be defined broadly as a body of standing water located inland. Lakes originate in several ways. Many lakes are created each year by man, either by digging a lake basin or by damming a natural valley. Natural lakes can be formed in several ways:
  • By glaciers gouging basins and melting and depositing materials in such a way as to form natural dams
  • By landslides which close off open ends of valleys
  • Extinct craters which fill with water
  • Changes in the earth's crust, as can happen during earthquakes, forming basins which fill with water
  • By changes in a river or stream course which isolate parts of the old course forming lakes, called oxbow lakes
A lake, like its inhabitants, has a life span. This lifetime may be thousands of years for a large lake or just a few years for a pond. This process of a lake aging is known generally as eutrophication. It is a natural process which is usually accelerated by man's activities. Human sewage, industrial waste, and the drainage from agricultural lands increases the nutrients in a lake which in turn increases the growth of algae and other plants. As plants die, the chemical process of decomposition depletes the water's supply of oxygen necessary for fish and other animal life. These life forms then disappear from the lake, and the lake becomes a marsh or swamp.

Shallow Lakes
Shallow lakes are extremely susceptible to increases in the rate of eutrophication resulting from discharges of waste and nutrient laden-runoff waters. Temperature stratification does not normally occur in shallow lakes. Efficient bottom-to-surface circulation of water in these shallow lakes moves nutrients to the surface, photosynthetic zone encouraging increased biotic productivity. Large quantities of organic matter are produced under these conditions. Upon decomposition, heavy demands are made on the dissolved oxygen content of shallow lakes. Eventually, the oxygen level drops and some fish and other life forms die.

Lake Ecosystem
The entire ecosystem of a lake can be altered by man. By removing the surrounding forest for lumber or to provide a building site or farm land, erosion into the lake is accelerated. Fertilizers, whether agricultural or those used by homeowners, can enter the lake either from runoff or leaching along with other chemicals that interfere with the intricate balance of living organisms. The construction of bulkheads to control erosion and filling behind them to enlarge individual properties can rob small fish and amphibians of their habitats.

The indiscriminate construction of piers, docks, and boathouses, can deprive all of the waterfront owners and the general public of a serene natural view and reduce the lake's surface (see Use Activity Policies and Regulations).