Hutchinson (1957) defined 76 different types of lakes based on formation, grouped under 11 processes that shaped the lake basin. Often, regions with lakes have only a smaller subset of these lake types present. Lake types formed by different processes include glacial lakes, tectonic basin lakes, volcanic lakes, landslide lakes, human-made lakes, and lakes formed by other natural processes.

Image: Kettle lakes in Siberia [Photo: Jesse Allen, NASA]	Glacial Glacial lakes are the most common type of lakes found in North America. Many of the lakes found today were formed during the most recent cycle of glacial activity, approximately 10,000 to 20,000 years ago. Cirque lakes were formed when glacial debris formed dams in mountainous valleys which then filled with water. Pluvial lakes were also formed in the last glacial period, in dry regions that did not form typical snowmelt-driven river and lake drainage basins. As the climate became wetter, increased precipitation formed pluvial lakes in natural basins, which once again became dry or reduced in size in drier times. Kettle lakes were formed by blocks of glacial ice left by retreating glaciers that gradually melted, causing the overlying land to collapse and create a hole.
Image: Lake Champlain, a tectonically formed lake [Photo: NASA]	Tectonic Basin Tectonic basin lakes were formed by tectonic activity, or the movement of the earth's crust. Tectonic uplift, tilting, or folding created depressions to form lake basins, sometimes along tectonic faults.
Image: Crater Lake, a volcanically formed lake [Photo: Mike Doukas, USGS]	Volcanic Volcanic processes that created calderas and lava dams in turn formed distinctive lake types. Calderas, areas of land that collapsed following volcanic eruptions, formed crater-shaped basins which then filled with water. Lava dams blocked off valleys to form basins.
Image: Landslide formed lake, India [Photo: NASA]	Landslide Landslide lakes were formed by mudslides or rockfall that were sizable enough to dam the path of a stream or river. These lakes are typically shorter-lived, lasting from a year to several centuries before the river is able to break through the dam and drain the lake.
Image: Sinkhole lake, Mt. Gambier, Florida [Photo: NASA]	Other Natural Processes Other lake types that have been formed by natural processes include solution lakes, plunge pools, oxbow lakes, beaver-formed lakes, and coastal lakes. Solution lakes, particularly common in Florida, were formed in areas with underlying limestone deposits where percolating water creates sinkholes. Plunge pools were formed in previous geologic eras when waterfalls scoured deep pools and glacial activity diverted river flow. Oxbow lakes formed as rivers meandered and former channels became isolated from the river. Beaver-formed lakes form where beavers dam rivers and streams, creating a backlog of water behind the dam.
Image: Lake Mead [Photo: NASA]	Human-made Human-made lakes and reservoirs have been created throughout the U.S. from small to large scales. Many free-flowing rivers in the U.S. have been dammed for hydropower, navigation, or flood control, creating reservoirs and impoundments behind the dam. Many artificial impoundments have also been created by filling artificial or natural depressions with water, or supplementing existing ponds and lakes.

Sources:

• Hutchinson, G.E. 1957. A treatise on limnology. Vol. 1. Geography, physics and chemistry. John Wiley and Sons, New York, NY.
• EPA Watershed Academy Web

Lakeshore and Lake Layers [Image: EPA National Lakes Assessment Final Report]

Another common way to classify lakes is by mixing regime or thermal stratification. Wind and wave action, gravity, atmospheric pressure, and density differences due to chemistry and temperature act together to influence the movement of water in lakes and the formation of layers or stratification.

Six types of lake mixing regimes have been identified for lakes deep enough to form a hypolimnion, or bottom layer (Hutchinson and Loffler 1956).

• Amictic lakes are always covered with ice and never mix.
• Cold monomictic lakes are covered with ice for most of the year and warm enough to thaw but not at temperatures above 4 degrees C.
• Dimictic lakes are covered with ice for part of the year, stratified into distinct and stable layers for part of the year, and mix in the spring and the fall. This is a very common type of mixing for temperate North American lakes.
• Warm monomictic lakes are never ice-covered, and mix once per year. They are stably stratified the rest of the year.
• Oligomictic lakes are never ice-covered, and are usually stratified but mix at irregular intervals.
• Polymictic lakes are never ice-covered and are sometimes stratified but mix completely several times per year.

Another type of lake outside of this schema is a meromictic lake, which is one that mixes incompletely because of a persistently dense bottom layer of water, often attributable to salinity differences.

Source:
Hutchinson, G.E. and H. Loffler. 1956. The thermal stratification of lakes. Proc. Na. Acad. Sci. 42: 84-86.

Comparison of oligotrophic (left) and eutrophic (right) lakes in Minnesota [Photo: NASA GLRSAC]

Classifying lakes by trophic status provides an indication of the nutrient dynamics in lakes, including nitrogen content, phosphorus content, organic matter, transparency, oxygen levels, chlorophyll levels, and primary production. These factors are in turn related to aquatic food webs and fish population dynamics. Oligotrophic lakes are typically low in primary productivity, nutrients, littoral zone area, and often contain deep, cold, clear water. Eutrophic lakes are typically shallow and have high primary productivity and nutrient levels. Water in eutrophic lakes is usually cloudy because of high concentrations of phytoplankton, and aquatic vegetation thrives in these lakes. Dissolved oxygen can become depleted and become a limiting factor in these lakes. Mesotrophic lakes lie in the middle of this spectrum with respect to depth, productivity, and nutrients. The trophic status of lakes can change through time, due to natural processes or human-caused processes such as excessive nutrient inputs to lakes and watersheds.

Lake Turnover [Image: US EPA]

Dimictic lakes are commonly found in temperate regions, and their twice-yearly mixing is also referred to as lake turnover. These lakes are covered with ice during the winter, but warming temperatures and rain act to melt the surface ice in the spring. As the surface ice melts, it warms to 4 degrees C, which is the temperature at which water is most dense, and sinks. This sinking water forces the less dense water below upward, resulting in an overall mixing or "turnover" of the lake. Mixing of the lake continues until the surface water or epilimnion is warm and less dense than the layer of colder water below in the hypolimnion. The transition zone between these layers is the metalimnion. At this point, the lake is thermally stratified, with little mixing occurring between layers.

In the fall, the lake turns over again as cooler air temperatures chill the surface water to 4 degrees C. The denser surface water once again sinks into the warmer water below and mixes until the lake is again covered by a sheet of ice. Lake turnover is an important process that distributes oxygen and nutrients necessary for aquatic life throughout the entire water column.