Lakes

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Lakes

Introduction

Lakes are bodies of water that are completely surrounded by land. Lakes are not directly connected with the ocean, although a lake may drain to the ocean via a river. They range in size from small bodies that may be only a few hundred yards at their widest point to bodies of water such as the Great Lakes that are dozens of miles in width with depths that easily accommodate the passage of huge cargo ships.

The estimated number of lakes around the globe exceeds 300 million. The 122 largest lakes, each with an area of 386 square mi (1,000 square km) or more, contains almost 30% of Earth’s total inland water.

Almost all lakes contain freshwater. Exceptions include Utah’s Great Salt Lake, Caspian Sea (at over 134,750 square mi [349,000 square km] the largest lake in the world), Aral Sea, Dead Sea (which separates Israel and Jordan, and, at 1,378 ft (420 m) below sea level, represents the lowest point of land on the planet), and Bras d’Or Lake in Cape Breton Island, Canada (which empties directly to the ocean at its northern end). Most lakes are located in the Northern Hemisphere, particularly at higher latitudes. Canada claims over 60% of the planet’s lakes.

Lakes are particularly susceptible to pollution, as runoff from the surrounding land empties into the water. If the rate at which the volume of water in the lake is completely replaced is long, the slow turnover of the total volume of water may support the growth of potentially harmful microorganisms, and chemicals that can be hazardous to health even at low concentrations will be present in the water for a longer time. Many municipalities that have small lakes within their boundaries will regularly check the water quality during the summer months, when they are used for recreation. Tests that detect unacceptable levels of indicators of water quality can lead to the closure of the affected lake to swimming until the source of the problem has been identified and corrected.

Runoff of chemicals such as phosphate and nitrogen, which are used as a food source by some microorganisms in the water, can lead to an explosive increase in the number of the microbes with the subsequent depletion of oxygen in the water, which can be lethal to fish and other life in the lake. This phenomenon, which is called eutrophication, can occur even in large lakes; an example is Lake Erie, which in the 1960s was in danger of becoming incapable of supporting life.

Lakes can also be affected by human activities, both directly (such as by excessive withdraw of water) and indirectly (drying up due to hotter and drier weather that is thought to be due at least in part to global warming).

Historical Background and Scientific Foundations

Lakes are formed naturally in a number of ways. As a portion of Earth’s surface heaves upward, or as two adjacent plates of Earth’s pull apart, depressions that are created will fill with water over time. Lake Baikal located in southern Siberia and Lake Tanganyika located in central Africa were both formed by tectonic activity. These lakes are over 20 million years old and are the deepest lakes known. Lake Baikal is the world’s deepest lake at 5,371 ft (1,637 m).

In present-day Canada, Scandinavia, and Russia, where glaciers have advanced southward and retreated northward periodically, the moving walls of ice scoured out depressions, which subsequently filled with water. A notable example of glacier-created lakes are North America’s Great Lakes. The system of five lakes that make up the Great Lakes are among the 13 largest lakes on Earth’s surface.

Lakes with a high content of salt can form as a consequence of the underlying geology of the region, or when a lake has little if any outlet to drain off water and so becomes elevated in salt that is present in the runoff entering the lake.

The depression that is left behind in an extinct volcano can also fill with water over time to create a lake. One example is Crater Lake in Oregon.

A type of lake known as an oxbow lake can also form from the changing flow of rivers that can occur with time. Rivers tend to adopt a more winding path as they age. Sometimes this meandering can become so pronounced that a loop forms and erosion eventually joins the sections of the river near the bottom of the loop. When this occurs, water flows straight through the newly created path, progressively isolating the looped portion, which is eventually closed off to form the crescent-shaped oxbow lake.

Lakes can also be artificially created. Most commonly, this occurs following the construction of a dam, with the impeded water collecting behind the retaining wall. Excavation of land can also be done to create small lakes; this is typically done to provide for water recreation at resorts, in planned communities, or to provide a scenic challenge in golf courses. As an artificial lake has no natural outflow, an exit for water movement must be created or the water quality of the lake must be monitored and maintained to prevent eutrophication.

A eutrophic lake contains an abundance of nutrients, which encourages the growth of microorganisms

WORDS TO KNOW

COLIFORMS: Bacteria that live in the intestinal tract of warm-blooded animals. The presence of coliforms in water indicates fecal pollution.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

HYDROLOGY: The study of the distribution, movement, and physical-chemical properties of water in Earth’s atmosphere, surface, and near-surface crust.

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

such as some bacteria and algae. The opposite type of lake, in which nutrients are scarce, is known as an oligotrophic lake. The crystal clear appearance of an oligotrophic lake can be misinterpreted as a sign that the water body is healthy. In fact, the lake is plant-free and has few fish. Lakes that have been affected by acidic precipitation (acid rain) are oligotrophic. The mid-ground between oligotrophic and eutrophic is mesotrophic. A mesotrophic lake is acceptably clear and has good but not over-abundant plant and fish life. Finally, a lake that has received a great deal of nutrients and has become very murky is known as a hypertrophic lake. This type of lake is also depleted of oxygen and is considered to be dead.

The volume of water in a lake is a balance between the amount of water entering the lake (input or inflow) and the amount of water leaving the lake (output or inflow). If the inflow exceeds the outflow, the volume of the lake will increase and the level will rise. If the outflow is greater than the inflow, the volume of the lake will decrease and the lake level will drop. Precipitation, runoff, surface watercourses, and underground water emptying into the lake are sources of inflowing water. Outflow is due to evaporation, exit via underground routes or watercourses such as streams or rivers, and the deliberate removal of water (such as occurs to provide water for drinking or crop irrigation).

Lakes that are shallow enough for sunlight to penetrate close to or down to the bottom (lake bed) will tend to have a fairly uniform temperature throughout the volume. However, in deeper lakes the sunlit portion will be warmer than the deeper waters. Because warmer water tends to more biologically active, the upper zone of such lakes will be enriched in oxygen more so than the deeper regions.

Lakes can become smaller in size, become marshland, and finally disappear over time. Normally, this process is not noticeable as it can take millions of years to complete. However, the process can be accelerated in small and slow draining lakes that receive a high input of nutrients or sediment. A lake in the process of converting from an open body of water to a more marshy area often is ringed by floating plants and rushes along the shoreline. Rarely, the process can occur much more quickly. An example is the draining of Lake Beloye in Russia, which occurred in minutes on June 3, 2005, presumably due to activity on the lakebed that created an opening to an adjacent river. Lakes can also become depleted due to the excessive removal of water.

Impacts and Issues

Lakes can become polluted by natural activities (such as runoff from rocks containing increased levels of harmful compounds such as lead or arsenic) and, more commonly, from human activities. For example, lakes that undergo eutrophication have usually received runoff of fertilizer components including phosphorus and nitrogen, which are food sources for microorganisms.

Even large lakes can become contaminated. For example, the Great Lakes are known to contain over 360 chemical compounds such as DDT, mercury, and mirex. Some chemicals do not readily degrade and so can remain toxic for a long time. These can be ingested by

fish and, in turn, by creatures including humans who eat the fish. Contaminated lakes lose their biodiversity. In the Great Lakes, for example, populations of 7 of the 10 most commercially valuable fish have greatly lessened.

Efforts to restore the quality of affected lakes is ongoing at the municipal, state, national, and international level. An example of the latter is the Great Lakes Water Quality Agreement, which was first signed by the United States and Canada in 1972, and which has been modified several times since then. The agreement sets reduction targets for industrial pollutants and noxious chemicals including airborne pollutants.

Lakes are also bellwethers of climate change. In regions that are becoming drier and hotter, increased evaporation and decreased input of water is causing decreases in lake sizes. This is occurring even in northern regions such as the Kenai Peninsula of Alaska, where the average temperature from 1980 to 2005 rose by almost 1.8°F (1°C). In another example, the average temperature of Lake Baikal—the world’s largest and deepest lake, that also holds 20% of the world’s freshwater—has risen by 2°F (1.2°C) between 1946 and 2008. Scientists who have studied the lake are concerned that its extensive biodiversity is at risk. The findings at Lake Baikal imply that lakes with less volume could also be at risk in an increasingly warming world.