If people had to pick their favorite water body (vote for yours in our opinion survey), they'd probably choose a crystal-clear lake nestled in the mountains (my opinion, or course!). Not all lakes are clear or are near mountains, though. The
world is full of lakes of all types and sizes.
A lake really is just another component of Earth's surface water. A lake is where surface-water runoff (and maybe some ground-water seepage) have accumulated in a low spot, relative to the surrounding countryside. It's not that the water that forms lakes get trapped, but that the water entering a lake comes in faster than it can escape, either via outflow in a river, seepage into the ground, or by evaporation.
A reservoir is the same thing as a lake in many peoples' minds. But, in fact, a reservoir is a manmade lake that is created when a dam is built on a river. River water backs up behind the dam creating a reservoir.
Here's a question for you: when a beaver dams a creek, is the pond that it creates a lake or a reservoir?
The Earth has a tremendous variety of freshwater lakes, from fishing ponds to Lake Superior (the world's largest), to many reservoirs. Most lakes contain fresh water, but some, especially those where water cannot escape via a river, can be salty. In fact, some lakes, such as the Great Salt Lake, are saltier than the oceans. Most lakes support a lot of aquatic life, but the Dead Sea isn't called "Dead" for nothing -- it is too salty for aquatic life! Lakes formed by the erosive force of ancient glaciers, such as the Great Lakes, can be thousands of feet deep. Some very large lakes may be only a few dozen feet deep -- Lake Pontchartrain in Louisiana has a maximum depth of only about 15 feet.
Some of the salty lakes were formed in ancient times when they were connected to seas and when rainfall may have been heavier. These lakes have been shrinking since the last ice age. The ancient Lake Bonneville in the United States was once as big as Lake Michigan, and the Great Salt Lake was once about 14 times as large as it is now.
Lakes are highly valued for their recreational, aesthetic and scenic qualities, and the water they contain is one of the most treasured of our natural resources. Lakes constitute important habitats and food resources for a diverse array of fish, aquatic life, and wildlife, but lake ecosystems are fragile. Lake ecosystems can undergo rapid environmental changes, often leading to significant declines in their aesthetic, recreational, and aquatic ecosystem functions. Exposed to external effects from the atmosphere, their watersheds, and ground water, lakes are subject to change through time. Human activities can further accelerate the rates of change. If the causes of the changes are known, however, human intervention (lake-management practices) sometimes can control, or even reverse, detrimental changes.
Limnology - The study of lakes
Limnology (the study of lakes and other freshwater systems) is the science that can provide improved understanding of lake ecosystem dynamics and information that can lead to sound management policies. As more studies are conducted on a variety of lake systems, the accumulated information leads to the development of general concepts about how lakes function and respond to environmental changes.
The condition of a lake at a given time is the result of the interaction of many factors—its watershed, climate, geology, human influence, and characteristics of the lake itself. With constantly expanding databases and increased knowledge, limnologists and hydrologists are able to better understand problems that develop in particular lakes, and further develop comprehensive models that can be used to predict how lakes might change in the future.
While the development of a limnological database and knowledge is important, no amount of generalization can provide a full understanding or predict conditions of any particular lake. Each lake system is unique, and its dynamics can be understood only to a limited degree based on information from other lakes. Just as a physician would not diagnose an individual's medical condition or prescribe treatment without a personal medical examination, a limnologist or hydrologist cannot accurately assess a lake system or suggest a management strategy without data and analysis from that particular lake and its environment.
Characteristics of lakes
The following are some of the most important basic factors that give unique character to each lake ecosystem.
Climate: Temperature, wind, precipitation, and solar radiation all critically affect the lake's hydrologic and chemical characteristics, and indirectly affect the composition of the biological community. Precipitation is the main factor affecting runoff and the delivery of nutrients and sediments. Temperature, wind, and energy from the sun affect lake stratification and mixing, plant growth, and evaporation.
Atmospheric inputs: The surface of a lake is directly exposed to atmospheric inputs. Not only wet precipitation, but also dry particles, can be major sources of certain contaminants to a lake. Each lake also receives indirect atmospheric inputs by way of the runoff from its watershed.
Geologic substrate and soils in the basin: The soil type affects the potential for runoff and erosion. The physical characteristics of the substrate determine the extent, nature, and quality of ground-water inflows and outflows. These are primary factors affecting the lake's chemistry, because of transfers between water and sediments, and input of sediment, minerals, and nutrients from the watershed by runoff water flowing into the lake.
Physiography: The area, surface topography, existence of upstream lakes and wetlands, altitude, and land slope of the lake's watershed affect surface-water runoff and the amount and nature of chemicals and sediments entering the lake. The physiography of the region affects the size of a lake's watershed and ground-water contributing area. The boundaries of a lake watershed and ground-water contributing area may not necessarily coincide. Interactions with land use by people can appreciably change how these factors affect runoff and the export of nutrients and sediment.
Land use: The type, location, extent, and history of land cover/land use (such as agriculture, rural, and urban developed areas) can greatly affect the quantity of surfacewater and ground-water inflows and outflows, as well as the amounts and types of sediment, nutrients and chemicals (natural or synthetic) that are transported into the lake from the watershed.
Lake morphometry: Size, shape, and depth characteristics of a lake are critical in determining currents and mixing of the lake, as well as its thermal and chemical stratification characteristics.
Common environmental problems in lakes and probable causes
Eutrophication is the natural process of physical, chemical, and biological changes ("aging") associated with nutrient, organic matter, and silt enrichment of a lake. If the natural process is accelerated by human influences, it is termed "cultural" eutrophication. Lakes are subject to a variety of physical, chemical, and biological problems that can diminish their aesthetic beauty, recreational value, water quality, and habitat suitability. Among the most common lake problems, and the conditions that often occur with eutrophication are the following.
Algal blooms Extensive and rapid growth of planktonic (floating and suspended) algae, caused by an increased input of nutrients (primarily phosphorus, but occasionally can also be caused by nitrogen), is a common problem in lakes (fig. 3). Lakes normally undergo aging over timescales of centuries or thousands of years, but the process can be accelerated rapidly to only decades by human activities that cause increases in sedimentation and nutrient inflow to the lake. Accelerated eutrophication and excessive algal growth reduces water clarity, inhibits growth of other plants, and can lead to extensive oxygen depletion, accumulation of unsightly and decaying organic matter, unpleasant odors, and fish kills.
Sedimentation/turbidity Increases in accumulation and/or resuspension of sediments can be a detriment to water quality and habitat for many aquatic species. Such events usually are caused by heavy rains that produce erosion and intense runoff, carrying heavy sediment loads into lakes. High winds, boating activity, and bottom-feeding fish, such as carp, may also resuspend bottom sediments and increase turbidity.
Oxygen depletion Decreases in dissolved oxygen to less than 3 mg/L (milligrams per liter) in the water can be harmful or lethal to many desirable species of aquatic life. The primary mechanism of oxygen loss is consumption by high rates of respiration and organic decomposition. Ideally, such consumption is offset by oxygen inputs from the atmosphere and from photosynthesis by aquatic plants. However, in stratified lakes, the atmospheric source is cut off from the hypolimnion (deep lake layer), and oxygen concentrations in the hypolimnion may decline to zero (anoxia) until the lake mixes again. Under anoxic conditions, phosphorus may be released from the bottom sediments into the overlying water. This "internal loading" may be considerable with phosphorus-enriched sediments and prolonged anoxia. Prolonged low oxygen concentrations in the summer or under ice in the winter can lead to fish kills.
Growth of aquatic plants (macrophytes) Normal macrophytic growth generally is beneficial for the lake ecosystem; among other benefits, the plants provide refuge for fish and other organisms. However, in some lakes, the growth of aquatic plants ("weeds") can become excessive and create a serious nuisance for lake users, interfering with swimming, boating, and other recreational activities. Excessive macrophytes commonly are caused by increased nutrients, invasion of exotic species, or accumulation of organic sediment. The improvement of water clarity resulting from management actions designed to control algal production can provide better conditions for growth of rooted plants.
Water-level changes Wide fluctuations in stage (lake level) can create major hardships for lakeside residences, marinas, and businesses, and they also may impair the habitat suitability for nearshore biota. These changes most commonly are linked to weather anomalies (extended periods of abnormally high or low precipitation), but also may be associated with human activities such as withdrawals for water use.
Species shifts Populations of desirable animal and plant species might decline sharply or disappear, to be replaced by other species. Usually, the new dominant species will become a nuisance and degrade some or all desirable qualities of the lake. Species shifts can be caused by introduction of invasive species that may have little or no natural controls on their population growth, or are stimulated by changes in environmental conditions (for example, climate changes, acidification from "acid rain" or other changes in water chemistry, or physical changes).