The movement of water within lake basins is facilitated by several forces. Currents caused by winds and water inflows and outflows are the primary movers of water in lakes. However, surface waves and seiches can also affect water flow. The movement of water powers the processes of erosion and deposition that affect the bottom morphology of lake basins. In addition, the maintenance of the basin’s water level depends on balancing inputs from groundwater, precipitation, and other sources with outflows from evaporation, streams that allow water to exit the basin, and other sinks.
The principal forces acting to initiate water movements in lakes are those due to hydraulic gradients, wind stress, and factors that cause horizontal or vertical density gradients. Lake water movement is usually classified as being turbulent.
Hydraulic effects are frequently the result of inflows and outflows of water. These may be substantial and continuous or weak and sporadic. In terms of the ratio of the volume of the inflow or outflow to the lake volume, the latter is the most frequently observed situation.
The stress of wind moving over the lake surface causes a transport of water within the lake, as well as the movement of energy downwind through the mechanism of surface waves. The wind is therefore one of the most important external forces on a lake. It can be relatively consistent in speed and direction, or it can be highly variable in either or both.
Water movements can occur as a result of internal pressure gradients and from density gradients caused by variations in temperature, sediment concentration, or the concentration of dissolved substances. Surface water in lakes can become denser than underlying water either by cooling or heating, because the temperature of maximum density for pure lake water is about 4 °C (39 °F). Water entering a lake from rivers with a high concentration of dissolved substances will sink to a lake level of similar density. These movements are both horizontal and vertical, but the net effect is downward, if not vertical, motion.
Horizontal pressure gradients can result from many different processes that act to produce density gradients. One example is the situation of solar heating in a shallow nearshore region, where the heat is committed to the warming of a relatively small volume of water. This produces a water of lower density than the near-surface water of an adjacent deep region, where the heat is spread throughout a greater volume. Consequently, the pressure gradient force will act to move the warmer water offshore and to replace it from below with cooler water.
Lake currents are the result of complex interactions of forces, but in many cases a small number of particular forces dominate. In the case of horizontal flow in the absence of horizontal pressure gradients, assuming no friction, water set in motion will curve to the right in the Northern Hemisphere because the Earth rotates from west to east. This effect is called the Coriolis force, and it will continue to influence water motion until there is a balance with the centrifugal force. This movement causes free-floating markers to move in an elliptical manner with a period that depends upon the latitude. In Lake Ontario, for example, it is about 17 hours. Where a dominating pressure gradient exists, the balance of the pressure-gradient force with the Coriolis force results in the so-called geostrophic flow, at right angles to the pressure gradient, with low pressure on the left (Northern Hemisphere). These conditions are most nearly realized only in very large lakes and in the oceans.
In those small lakes where hydraulic effects dominate, steady flow conditions may be achieved through balance with friction. This situation is commonly encountered in rivers, and relationships exist between mean current speed and the slope and mean depth of the river or narrow lake. These are called gradient currents and occur following situations where the wind or atmosphere pressure gradient causes a tilting of the lake surface (denivellation). In cases where the Coriolis force is a significant factor, the flow down a lake will tend to move toward the right (in the Northern Hemisphere). The development of a deeper countercurrent to the left will occur to compensate for the piling up of water on the right side.
Horizontal pressure gradients will be important in lakes where there are significant inflows of water with markedly different density from ambient lake density or where significant differential surface heating occurs.
Currents resulting from wind stress are the most common in lakes. Considerable research is still underway into the mechanism of transfer of wind momentum to water momentum. The stress on the lake is proportional to some power of wind speed, usually taken to be 2, although it evidently varies with wind speed, wave conditions, and atmospheric stability. In large, deep lakes, away from the boundaries, where wind-stress effects may be balanced by Coriolis-force effects, theory suggests that the surface current will move in a direction 45° to the right of the wind and that deeper currents are progressively weaker and directed farther to the right. The depth at which flow is opposite to the wind direction is effectively the depth below which there is no influence from the wind. This depth, designated D, can theoretically occur at about 100 metres (300 feet) in large, deep, midlatitude lakes. Observations show varying degrees of fidelity to theory because of complications from coastal effects and thermal stratification.
In coastal regions, if water depth is a significant fraction of or greater than D, winds blowing parallel to the shore will transport water either onshore or offshore. In the latter case, where the coast is to the left of the wind flow (Northern Hemisphere), the water driven offshore is replaced by cooler, deeper water (upwelling).
Under stratified conditions a strong thermocline will essentially separate a lake into two layers. Shearing forces that develop between these layers cause a motion, termed internal waves, that may serve to directly dissipate a substantial portion of a lake’s kinetic energy and act as a coupling between motion in the epilimnion and hypolimnion. A great range of periodicities is observed in the oscillations of the thermoclines, particularly in large lakes. Internal seiches, which are responsible for relatively long-period internal waves, are discussed later.
A small-scale circulation phenomenon that has aroused considerable attention on lakes is Langmuir circulation. On windy days, parallel “streaks” can be observed to develop on the water surface and exhibit continuity for some distance. These streaks may be caused by convergence zones where surface froth and debris collect. Langmuir circulation thus appears to be a relatively organized mixing mechanism wherein sinking occurs at the streaks and upwelling occurs between the streaks. Under favourable circumstances, this appears to be a key process for mixing heat downward in lakes.
Wind blowing over a calm lake surface first produces an effect that may appear as a widely varying and fluctuating ruffling of the surface. The first wave motion to develop is relatively regular, consisting of small, uniformly developed waves called capillary waves. These are quite transient, dissipating rapidly if the wind dies away or developing to the more commonly observed and more persistent gravity waves.
Energy will be continually fed to the waves by the frictional drag of the air moving over the water and by the direct force of the wind on the upwind face of the waves. The latter effect occurs only while the waves move more slowly than the wind. Pressure differences at the air–water interface also contribute energy to surface waves. Energy losses occur due mainly to turbulence in the water and, to a smaller extent, to the effects of viscosity.
Waves will continue to grow as long as there is a net addition of energy to them. Their height will increase as a function of wind speed and duration and the distance over which it blows (fetch). Most lakes are so small that fetch considerations are unimportant. Studies in larger lakes, however, have shown that the height of the highest waves are related to the fetch. In these lakes, waves as high as several metres are common, although waves of about seven metres (23 feet) are the highest to be expected. Wave heights in a given portion of a lake may vary considerably, due to interactions that suppress some waves and amplify others. As waves develop, their lengths increase, even after their height has stopped increasing. The phenomenon of swell, commonly observed in the oceans, is not truly realized, even in the largest lakes.
Waves travel in the same direction as the wind that generated them and at right angles to their crests. If they meet a solid object rather than a sloping beach, much of their energy will be reflected. If they enter shallow water obliquely, they are refracted. Wave speed, for waves longer than four times the depth of the water, is approximately equal to the square root of the product of the depth and the gravitational acceleration. For waves in relatively deep water, the wave speed is proportional to the square root of the wavelength.
Waves crest and roll across Lake Superior, in the United States, on a stormy day. Wind creates waves and determines the direction in which they travel. Anne Rippy/The Image Bank/Getty Images
As wave height increases, the sharpening of the wave crest may result in instability and a breaking off of the crest, a process hastened by the wind. This results in the familiar whitecaps. Waves that run ashore break up in surf. The wave height first decreases slightly, then increases, and the speed decreases, and eventually the wave form disappears as it crumbles into breakers. These can be plunging forms, in which the top curls right over the forward face, or of the spilling type, in which the crest spills down the forward face. A particular wave may break several times before reaching shore.
If a denivellation, or tilting of a lake’s surface, occurs as a result of a persistent wind stress or atmospheric pressure gradient, the cessation of the external forcing mechanism will result in a flow of water to restore the lake level. The flow would be periodic and uniform with depth, except for the damping effects of the lake-bottom friction and internal turbulence. Because of this, each successive tilt of the lake surface in the opposite direction occurs at a level slightly less than the previous one. The oscillation proceeds, moving the water back and forth until damping levels the water or until wind and pressure effect another tilt. This process is seiching; the lake oscillation is a seiche. The basic seiche has a single node, but harmonics of the oscillation occur, with several nodes being possible.
The period of the uninodal seiche can be estimated from a formula that equates it to twice the length in the direction of the tilt, divided by the square root of the product of the mean lake depth and the gravitational acceleration.
Seiches have been noted, recorded, and studied for hundreds of years. Lake Geneva in Switzerland, which has an observed uninodal period of about 74 minutes and a binodal period of about 35 minutes, was one of the first lakes to be studied in connection with seiching. The observed uninodal periods of Loch Treig and Loch Earn, Scotland; Lago di Garda, Italy; Lake Vetter, Sweden; and Lake Erie, North America, are approximately 9, 14.5, 43, 179, and 880 minutes, respectively.
Long, relatively narrow lakes that are exposed to a predominance of wind flow along their major axes are most likely to exhibit so-called longitudinal seiches. Transverse seiching can occur across the narrower dimension of a lake. That observed in Lake Geneva, for example, has a period of about 10 minutes.
The height of the denivellation depends upon the strength and duration of the forcing mechanism, as well as on the lake size and dimensions. In small lakes, level changes of a few centimetres are common, whereas, in the Great Lakes, intense storms can produce changes as great as two metres (seven feet). If the disturbance causing the tilting moves across the lake at close to the speed of the shallow-water wave speed, a profound amplification can occur, with possible disastrous consequences.
In addition, seiches can be caused by the pull of other nearby celestial bodies, such as the Sun and Moon, or occur as the result of changes in heating with depth. True tides that result from the gravitational effects of the Moon and Sun are rarely measurable in lakes, but small values of tidal components occasionally have been discerned. In contrast, internal seiching results from thermal stratification. The layers separated by the thermoclines oscillate relative to one another. Observed uninodal periods for Loch Earn, Lake Geneva, Lake Baikal, and Cayuga Lake (New York) are approximately 16, 96, 900 (binodal), and 65 hours, respectively.
Because hypolimnion water is very different from epilimnion water with regard to both thermal and biological characteristics, the massive movements of water and the turbulent exchanges that can occur during internal seiching are very important. Substantial portions of the bottom of shallow lakes can experience periodic alternation of exposure to hypolimnetic and epilimnetic water, and hypolimnetic water can be periodically exposed to the surface.
In a lake’s early stages of existence, its shore is most susceptible to changes from wave and current action. As these changes occur, there is a tendency over time to an equilibrium condition—a balance between form and processes that depends upon the nature of the materials present (e.g., the size of sand and gravel present). The effectiveness of waves in the erosion process depends in part upon the depth and slope of the lake bottom. Where the shore consists of a sheer cliff adjacent to deep water, wave energy will be reflected away without much erosional effect. The refraction of waves in zones of irregular coastline tends to concentrate wave energy at some locations and dilute it in others. Thus, features extended out into the lake will receive more wave energy, and the tendency is to smooth out an irregular coastline. Other net effects of shore erosion are an increase in the surface area of a lake and a reduction in its mean depth.
As erosion takes place, the distribution of erosion products results in transport of finer material offshore. The resulting terrace is called the beach in its above-water manifestation and the littoral shelf where it is below water. Landward, beyond the beach, a wave-cut cliff is usually found. The steeper slope that often separates the littoral shelf from the benthos (bottom) zone in the central part of the lake is called the step-off by some limnologists.
Water movement directed at an angle to the coastline will result in the generation of currents along the shore. Erosion products will then be transported down the coast and may be deposited in locations where transport energy is dissipated due to movement around a bend or past an obstruction. A buildup of such material is called a spit. If a bay becomes completely enclosed in this way, the spit is called a bar.
Water in very shallow lakes that are subjected to strong winds may be piled against the lee shore to such an extent that countercurrents will develop from along the lee shore around each side of the lake. The cutting effects of these currents are known as end-current erosion and may characteristically alter the shape of a lake frequently subjected to winds from a particular direction.
The bottom morphology of a lake can be greatly influenced by deposition of sediment carried by inflowing rivers and streams. Although this process can be modified by wave and current action, most lakes are sufficiently quiet to permit the formation of substantial deltas. In very old lake basins the relief may become so extensively decreased due to the great buildup of deltaic deposits and the long-term effects of river widening, that deposition on the outer portions of a delta will fail to balance the effects of wave erosion. A delta, in these circumstances, will begin to shrink in size.
It is very important to understand lake processes that affect the basin morphology and to be able to predict their trends and their impact on human activities. Increasingly, man is imposing his ability to change natural events in lakes, and he has often encountered problems by not anticipating a lake’s reaction to his projects. The actual creation of a lake by damming a river is a major undertaking of this type. One fairly recent example is Lake Diefenbaker, in Saskatchewan. In this region of prairie farmland, the banks of the new lake are extremely vulnerable to erosion, and planners have had to contend with the consequences of bank cutting and infilling of the basin. There are many examples of lesser engineering undertakings that have had to face the consequences of a lake’s reaction. The building of jetties or breakwaters, for example, may interfere with natural circulation features. In some cases this has resulted in the reduction of flow past a harbour and increase in flow past a previously stable shoreline, with the result that the harbour has filled in or been blocked by sediment deposition, while the stable shoreline has become badly eroded.
The role of lakes within the global hydrologic cycle has been described earlier. Lakes depend for their very existence upon a balance between their many sources of water and the losses that they experience. This so-called water budget of lakes is important enough to have warranted considerable study throughout the world, with each lake or lake system possessing its own hydrologic idiosyncrasies. Aside from being of scientific interest, water-budget studies serve to reveal the dependence of each lake on particular hydrologic factors, thus enabling better management practices. These may include restrictions on water utilization during drought conditions, dike construction and evacuations prior to flooding, control of water levels to ensure efficient power production, and major decisions associated with diversions of watercourses in order to enhance water-quantity- and water-quality-management activities.
The net water balance for a particular lake will vary according to the periodic and nonperiodic variations of the inputs and outputs and is reflected in the fluctuations of the lake level. Because the prime influencing factors are meteorological, the periodicity of seasonal events are often seen in water-level records.
Ultimately, a lake will decline as a result of water loss or sediment infill. Over time, a number of biological, physical, and chemical factors work together to transform the lake and its basin into another physical entity.
While people may accommodate to predicted imbalances in the hydrologic budget, it is usually difficult to influence the basic natural factors that cause the imbalances. Precipitation and evaporation, for the most part, are uncontrollable, although some advances have been made in evaporation suppression from small lakes through the use of monomolecular surface films. Groundwater flow is not controllable, except where highly restricted flow can be tapped. Rivers and streams, however, can be subjected to regulation by well-established practices through the use of dams, storage reservoirs, and diversions. It is mainly through these controls that efforts are made to make the most efficient usage of water as a resource.
When engineers take steps to alter elements of a basin’s water budget, careful consideration must be given to the consequences of the hydrology and ecology of the entire watershed. Dredging operations for the purpose of harbour clearance or improvements to a navigable channel, for example, may increase the outflow from an upstream lake, increase shore erosion, or regenerate undesirable sedimentary constituents into the lake or river water. The damming of a river or a lake outlet to increase local water storage may also result in undesirable effects, such as an increased evaporation from the larger surface area, the restriction of fish movement, or changes in the thermal climate of the downstream flow. Diversions and dam-site construction may also result in flooding of important bird-breeding areas or a lowering of other lakes in the system, resulting in undesirable consequences.
Workers operating a dredger on a lake created for boating in Havasu City, Ariz. (U.S.). Photoshot/Hulton Archive/Getty Images
The usual major input of water to a lake derives from streams and rivers, precipitation, and groundwater. In some cases inflow may come directly from glacier melt. The relative importance of each of the major sources varies from lake to lake.
Stream and river flow are usually seasonally variable, depending upon precipitation cycles and snowmelt. At low altitudes some rivers exhibit a peak during a high precipitation period in winter and then a second peak associated with a subsequent spring snowmelt that feeds the nearby high-altitude tributaries. In regions where precipitation can occur in great quantities at high rates, streams swell quickly and water is delivered in relatively large volumes to downstream lakes.
A great deal of work has been done to improve the ability to measure and record streamflow. Consequently, it is usually the most accurately known of the inflow terms in the water budget. Most frequently, the height of the river level (stage) correlates well with the water discharge. In other cases, direct river-flow measurements are taken periodically with flow meters.
Precipitation reaching a lake’s surface directly may be the major input. This is true of Lake Victoria, in eastern Africa. In other cases, where the lake basin is large with well-developed drainage to a deep lake of small surface area, precipitation may be a small component. Precipitation that falls elsewhere in the lake basin may reach the lake through either surface or groundwater flow, or it may be lost due to evapotranspiration.
Measurements or estimates of precipitation for a basin are difficult to achieve. Even where elaborate networks of rain gauges exist or where these are supplemented by meteorological radar installations, total basin-precipitation data are still considered to be poor. Measurements of direct precipitation over lakes are exceedingly rare. This situation is especially serious in the case of a large lake for which nearby land data are not necessarily representative of conditions over the lake. Each climatic region throughout the world has its typical precipitation pattern, and the lakes within the regions are affected accordingly.
Groundwater reaches lakes either through general seepage or through fissures (springs). Groundwater is taken to be water in that zone of saturation that has as its surface the water table. The depth of the water table can be determined by digging a well into the saturated zone and noting the level of water—unless the water is under pressure, in which case it will rise in the well to a level above the water table. Clearly, it is possible for a lake level to coincide with the water table. In fact, unless impermeable material intervenes, the water table will drop to, rise to, or lie level with a lake surface. Groundwater that is lost from the saturated zone to a lake is termed groundwater discharge. Groundwater introduced to the saturated zone from a lake is termed recharge. The rate at which groundwater is exchanged between a lake and the saturated zone depends mainly upon the level of the water table and the pressure conditions within the saturated zone.
In permeable materials the zone above the water table is called the zone of aeration, and water within it is called soil moisture. Soil moisture is classified into three types: hygroscopic water adsorbed on the surface of soil particles, water held by surface tension in capillary spaces in the soil and moving in response to capillary forces, and water that drains through the soil under gravitational influence. The latter will most significantly contribute to groundwater recharge and to the water balance of a lake. The second category will generally be subject to loss due to transpiration by plants.
Lakes that have no outlets, either above or below surface, are termed closed lakes, whereas those from which water is lost through surface or groundwater flows are called open lakes. Closed lakes, therefore, lose water only through evaporation. In these cases, the loss of water that is less saline than the source water results in an increasing lake salinity.
Evaporation results from a vertical gradient of vapour pressure over the water surface. Next to the water surface, saturation conditions exist that are a function of the temperature at the interface. The vapour pressure in the air above the surface is calculated from the temperature of the air and the wet-bulb temperature. The rate at which evaporation occurs also depends upon the factors that affect the removal of the saturated air above the surface, such as wind speed and thermal convection.
Studies of evaporation must surely constitute a sizable proportion of all hydrological and oceanographic work. The principal categories of evaporation studies are water budget, energy budget, bulk aerodynamic techniques, and direct measurements of vapour flux.
The so-called aerodynamic technique is based upon Dalton’s formula, which correlates evaporation with the product of the vapour pressure gradient and the wind speed. Studies during the past 20 years have produced a host of variations of this equation, determined empirically using independent measurements of evaporation. One of the most often used of these was developed in a study of Lake Hefner, and even this work has been subsequently modified to suit other climates and conditions. Few workers are satisfied with the present state of the art in the use of the aerodynamic equations. Nevertheless, once an equation of this type is satisfactorily developed for a particular lake, having been checked with independent methods, it is attractive because it usually employs data that can be routinely observed.
The direct measurement of vapour fluxes is an extremely intricate proposition, as motions over a water surface are usually turbulent, and instruments capable of measuring rapidly changing vertical motions and humidities are required. Not the least of the difficulties is the likelihood that the kind of turbulence over large bodies far from land is significantly different from that over land. Recent advances in theoretical developments and instrumentation continue to encourage this type of study. In turn, successes in this field offer the opportunity for the refinement of empirical techniques more practically suited for general lake investigators.
Evaporation pans attempt to simulate, but cannot completely duplicate, the climactic conditions on a lake. They do, however, help scientists estimate the rate of lake-water evaporation. NOAA/National Weather Service/North Indiana Weather Forecast Office
In many lake studies, data from evaporation pans have been used to determine lake evaporation. Pans have even been developed for flotation on lakes. Pans cannot truly simulate lakes, however, as they constitute a different type of system (they are not exposed to the atmosphere in the same way, they exchange heat through their sides, and they do not store heat in the same way as lakes).
Some examples of evaporation estimates include annual totals of between 60 and 90 cm (two and three feet) for Lake Ontario (using different techniques and for different years); about 75 cm (2.5 feet) for Lake Mendota, Wisconsin; over 210 cm (seven feet) for Lake Mead, Arizona and Nevada; about 140 cm (4.5 feet) for Lake Hefner; about 660 mm (26 inches) for the IJsselmeer, in the Netherlands; and about 109 mm (4.25 inches) for Lake Baikal.
Water output from a lake in the form of surface-water outflow generally depends upon the lake level and the capacity of the effluent channel. Although lakes often have many surface inflows or at least several incoming streams or rivers, they generally have but one surface effluent.
Lake-level rises generally coincide with or closely follow seasons of high precipitation, and falls of level generally coincide with seasons of high evaporation. Complications are introduced by a variety of factors, however. The storage of heavy winter precipitation as snowpack is one example. The release of this water during the spring thaw may also be hampered by the presence of river ice, resulting in late-spring or summer peaks. In large drainage basins the full effects of heavy precipitation may not be immediately realized in the lake-water balance because of the time required for basin drainage. Where glacier melt is a major input to a lake, the changes in level respond to seasonal heating as well as seasonal precipitation.
Although artificial controls, in the form of diversions, river dredging, and dams, affect the levels of the Great Lakes, the latter provide good examples of seasonal variations because of the lengthy record of levels available. The rivers draining to these large lakes are relatively stable; that is, the ratio of maximum to minimum flow is about 2 or 3 to 1, compared to 30 to 1 for the Mississippi River and 35 to 1 for the Columbia River. A 67-year average of lake levels by month shows that high water occurs, on the average, in September for Lake Superior and in June for Lake Ontario. Lows occur in March and December–January, respectively. The mean range in seasonal levels, for this period, is about 30 cm (1 foot) for Lake Superior and about 45 cm (1.5 feet) for Lake Ontario. The pattern varies considerably from year to year, however, and periods of exceptional precipitation and drought are shown in the records. These events ultimately affect the downstream lakes, but, because of their relatively small discharge volumes, it takes 3.5 years for 60 percent of the full effect of a supply change to Lake Huron–Michigan to appear in the outflow from Lake Ontario.
An Aymara Indian poling a reed boat on Lake Titicaca, near the Bolivian shore. Titicaca’s water level fluctuates seasonally and over a cycle of years. © Tony Morrison/South American Pictures
The seasonal changes in a lake’s level may be superimposed on longer-term trends, which in some cases dominate. Several of the large lakes of the world have water-level records that illustrate long-term periods of relative abundance of water and drought. In Central Africa, Lakes Victoria, Albert, Tanganyika, and Nyasa exhibit substantial long-term features, some of which are consistent, suggesting that a common climatological factor is responsible. Nevertheless, others of these features are not consistent within the lakes and have not been adequately explained.
The principal climatological factors that would most affect long-term lake-level variations have not been recorded for long periods at many locations. Regular precipitation observations were not made before about 1850. Some useful evidence is found in such natural records as tree rings and peat-bog stratigraphy.
On a worldwide basis, there is evidence of a period of low levels in the middle 19th century and near the end of the first quarter of the 20th century. Lake George, in Australia, the Caspian Sea, several lakes in western North America, and Pangong Lake, in Tibet, are examples that have exhibited these features.
The life history of a lake may take place over just a few days, in the case of one formed by a beaver dam, or, for the largest lakes, it may cover geologic time periods. A lake may come to its end physically through loss of its water or through infilling by sediments and other materials. Reference has previously been made to the chemical-biological death of a lake, which is not necessarily the end of it as a physical entity but may in fact be its termination as a desirable body of water.
Geologic processes involving the uplift and subsequent erosion of mountains and the advance and retreat of glaciers establish lake basins and then proceed to destroy them through infilling. Lake basins may also lose their water through drought or through changes in the drainage pattern that result in depletion of water inflows or enhancement of outflows.
The chemical-biological changes within a lake’s history offer a fine example of ecological succession. In the early stages a lake contains little organic material and has a poorly developed littoral zone. Particularly in temperate zones, such conditions favour a plentiful oxygen content, and the lake is said to be oligotrophic. As erosion progresses and as lake enrichment and organic content increase, the lake may become sufficiently productive to place an excessive demand upon the oxygen content. When periods of oxygen depletion occur, a lake is said to be eutrophic. An intermediate stage in this course of events is called mesotrophy. In the case of oligotrophy the vertical oxygen distribution is essentially uniform, or orthograde. Under eutrophic conditions, oxygen values decrease with depth, and the vertical distribution is called clinograde.
The limits of oligotrophic and eutrophic conditions have been set in terms of the rate at which oxygen is depleted from the hypolimnion. These limits are arbitrary but are approximately 0.03 and 0.05 milligrams per square centimetre per day as the upper limit of oligotrophy and the lower limit of eutrophy, respectively.
As eutrophic conditions develop, bottom sediments become enriched in organic material, and bottom plants spread throughout the littoral zone. As infilling proceeds, the plant-choked littoral zone spreads lakeward. Eventually, the littoral zone becomes a marsh, and the central part of the lake diminishes to a pond. When the lake finally ceases to exist, terrestrial vegetation may flourish, even to the extent of forestation.
