Below is an overview of some elemental information related to limnology and lake management that are important to know in understanding lakes and the big picture of lake management. For more detailed information please visit the NALMS Bookstore, which has numerous publications on general limnology and lake-related topics.
Basin Types
Events that occurred on or beneath the Earth’s surface thousands of years ago formed many of our lakes. As a result, lakes are usually concentrated into areas that have a large number of waterbodies. Most lakes can be found in the Northern Hemisphere, where large areas were covered by huge ice formations. On a scale of human life spans, lakes seem to be permanent features of our landscape, but they are really only geologically temporary. They are created, mature (fill-in) and eventually disappear.
The origins of the lake basins and their characteristics ultimately reflect the physical, chemical and biological events taking place within the area surrounding them. These events play an important role in how the lake responds to surrounding activities.
Glacial Lakes: By far the most important agents in the formation of lakes are the catastrophic effects of glacial ice movements that occurred 10,000 to 12,000 years ago. Gigantic sheets of ice and snow are created in climates where snow falls but does not melt. The glaciers covered an area from the Atlantic Ocean to the Rocky Mountains in ice that was more than a mile high. Although these glaciers did eventually melt, ten percent of the earth is presently covered with glaciers. Some of these glaciers can still be seen in the mountainous areas of the United States and Canada.
As a glacier moves back and forth across the land, scraping off the tops of hills and bluffs and taking rocks with it, lakes are formed. The material picked up by the glacier is later dropped off at other sites. This back and forth and stop and go movement of the glaciers permanently alters the landscape. This movement creates several important landforms. When the glacier stops, it leaves behind piles of rocks and materials that it carried over time, called moraines. These dam up rivers and smaller streams to form lakes. Sometimes, huge blocks of ice are broken off and covered by sand and gravel. When the ice melts, the sand and gravel cave in, leaving a large hole behind. These kettles may form large marshes or lakes. As the large mass of ice melts, rivers form beneath the glaciers.
Solution Lakes: Lakes can form when underground deposits of soluble rocks are dissolved by water running through the area, making a depression in the ground. Rock formations made of sodium chloride (salt), or calcium carbonate (limestone), are most likely to be dissolved by acidic waters. Once the groundwater has dissolved the rocks below the surface, the top of the land caves in, usually forming a round-shaped lake, called a solution lake. Typically, the depressions are deep enough to extend below the groundwater table and are permanently filled with water. Solution lakes are common in Michigan, Indiana, Kentucky and particularly in Florida.
Oxbow Lakes: The flow of water from rivers has a great deal of energy and erosive strength that may create lake basins. As a river winds over the earth’s surface, a greater amount of erosion occurs on the outer river bend, where the flow of water is the fastest. Materials carried by the river are deposited on the inner portion of the bend, where currents are reduced. As time passes, erosion continues and more materials are left off until the U-shaped meander of the river closes in. The main course of the river cuts a new channel to the inner end of the meander. Oxbow lakes are usually shaped like the letter C.
Man-made or Animal-made Lakes: Many small lakes in North America have been formed by the activities of the American beaver. Sticks, aquatic plants and mud are used to build dams across small streams to form an impoundment of the water. These ponds are usually very shallow and are rich in nutrients and plant life. Humans have constructed artificial lakes (reservoirs) to supply drinking water to the public, to provide power, to aid in navigation, to provide flood control and for recreational purposes. These reservoirs are usually well engineered by humans to hold back a certain quantity of water with the use of dams.
Volcanic Lakes: Sometimes, disastrous events associated with volcanic activity form lake basins. The formation of volcanic lakes can occur in different ways. As volcanic material, including magma, is discharged out of the volcano, empty depressions or cavities are formed within the volcano. Some of these depressions cannot drain and become sealed holes on top of the volcano. Rainfall and runoff eventually fill the depression with water and a new
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