Are lakes fed by rivers?

Due to the unusual relationship between the temperature of the water and its density, lakes form layers known as thermoclines, i.e. layers with drastic differences in temperature relative to depth. Lakes are discrete, largely isolated ecosystems where the interplay of physical, biogeochemical and organismal processes can be studied, understood and used for effective management. As warmer water floats, energy must be added to the water column to mix the heat deeper, and in most lakes, wind provides this energy. For a lake to retain its water over time, it needs to be replenished.

The bar on the right shows the distribution of fresh water at the surface.

How do lakes and ponds get water?

Although Lake Baikal is less than half the surface area of Lake Superior, one of the Great Lakes of North America, it is about four times as deep and holds almost as much water as all five Great Lakes combined. Dams that beavers build from branches can block rivers or streams and form large ponds or swamps. Freshwater is defined as water with a dissolved solids content of less than 1,000 milligrams per litre, usually salt. Surface waters include lakes, reservoirs (man-made lakes), ponds, streams (from large rivers to small creeks), canals (man-made lakes and streams) and freshwater wetlands.

How do lakes gain and lose water?

To understand the role of living organisms in the water cycle, it is helpful to know the basics of the cycle itself. When water evaporates from the world's oceans, lakes and rivers, it becomes water vapour and enters the atmosphere where it forms clouds. Terraces, natural dams and abandoned river meanders are common landscape features in large river valleys, and wetlands and lakes are usually associated with these features. Apart from this, lakes and ponds should (and do) theoretically dry out after a certain time, between a few weeks and a few months depending on the size of the water body.

Evaporation generally has a greater effect on the water level of lakes than on the water level of streams, because the surface of lakes is generally larger and less shaded than many river sections, and because the water in lakes is not replenished as quickly as in a river section.

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Amie Pross

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