The Lakes Portal
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Introduction

Lac Gentau in the Ossau Valley of the Pyrenees, France

A lake is a naturally occurring, relatively large and fixed body of water on the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from the ocean, although they may be connected with the ocean by rivers, such as Lake Ontario. Most lakes are freshwater and account for almost all the world's surface freshwater, but some are salt lakes with salinities even higher than that of seawater. Lakes vary significantly in surface area and volume.

Lakes are typically larger and deeper than ponds, which are also water-filled basins on land, although there are no official definitions or scientific criteria distinguishing the two. Lakes are also distinct from lagoons, which are shallow tidal pools dammed by sandbars at coastal regions of oceans or large lakes. Most lakes are fed by springs, and both fed and drained by creeks and rivers, but some lakes are endorheic without any outflow, while volcanic lakes are filled directly by precipitation runoffs and do not have any inflow streams.

Natural lakes are generally found in mountainous areas (i.e. alpine lakes), dormant volcanic craters, rift zones and areas with ongoing glaciation. Other lakes are found in depressed landforms or along the courses of mature rivers, where a river channel has widened over a basin formed by eroded floodplains and wetlands. Some parts of the world have many lakes formed by the chaotic drainage patterns left over from the last ice age. All lakes are temporary over long periods of time, as they will slowly fill in with sediments or spill out of the basin containing them. (Full article...)

Landsat 7 image of Karikkoselkä.

Karikkoselkä is a lake formed in an impact crater in Petäjävesi, Finland. Karikkoselkä is located approximately 30 km east from the centre of Keurusselkä, a much older and larger impact crater. Most lakes in the region are elongated in northwest–southeast direction due to glaciation, but Karikkoselkä is strikingly round.

Many shatter cones, rock formations that form under the extreme pressures of impact, have been found around the lake. Further evidence comes from aeromagnetic maps, which show a clear magnetic anomaly in the impact crater area. In addition, samples collected from deep drillings into the lake bottom confirm the impact origin of the structure.

The crater is the smallest identified in Finland, 1.4 km diameter and 150 m deep. Due to sediments the lake has a maximum depth of 26m which is unusually deep for a lake in the region. Karikkoselkä is estimated to be between 230 Ma and 450 Ma (million years old), most likely near 240 Ma (Triassic or earlier). Some sources give an unreasonably young age of 1.88 Ma, which is likely a misquotation – the discovery paper mentions that the bedrock in the region (known as the Central Finland Granite Complex) formed about 1.88 Ga (thousand million years ago), in the late Paleoproterozoic era. (Full article...)
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Tulainyo Lake is a freshwater alpine lake in the eastern Sierra Nevada in the U.S. state of California
Tulainyo Lake is a freshwater alpine lake in the eastern Sierra Nevada in the U.S. state of California

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