lakes

lakes A lake is a standing body of fresh or saline water on the land surface, into which and from which rivers may flow. Although lakes originate in many ways, they may be as prolific today as at any time in the geological past. This is attributable to the legacy of the Pleistocene glaciation during which ice scraped across the land surface, accentuating hollows, picking out weak rocks, and depressing great areas of the Earth's crust to entrap vast volumes of water, as in the Great Lakes of North America. However, glacial activity is by no means the only process whereby lakes are formed. Small lakes may form as parts of river channels become separated from the main flow channel during flood. Other ephemeral lakes develop in volcanic craters or collapsed caldera systems. These are destined to be lost during subsequent volcanism. Most lakes will become filled with sediments in due course, but cores show that some of the largest, such as Lakes Baikal and Tanganyika, are at least 20 million years old. Anthropogenic lakes, reservoirs for water storage or for power generation, abound in both the developed and developing worlds, and many of these are filling so rapidly with sediment that their useful life is gauged as a few decades.

Large lakes may develop as rivers become blocked when mountain belts rise across their former routes, as with the Tibetan rivers encountering the rising Himalayas. In major rift valleys lowering of the graben floor creates linear depressions, as in East Africa, but the uplift of the marginal horsts may equally block pre-existing rivers draining across the area to create lakes such as Lake Kyoga. Some such rift valley floors may fall below sea level, as in the Dead Sea, so that their lakes are below sea level. Downfaulting away from rift systems may also create substantial basins isolated from the surrounding areas and within which streams flow to the low points where water accumulates. Such waters carry salts in solution and become increasingly saline as the water evaporates from the lake surface; the result is that salts are precipitated on to the bed of the lake. Short-lived saline lakes or playas are particularly common at the margins of many deserts, where waters from flash floods become ponded up, but evaporate away in time (see playas). In other arid areas, where the wind blows particles away from the floors of the hollows, large deflation hollows (e.g. Quattara) may form whose floor level is defined by the presence of the water table below surface. The capillary action of the water serves to retain the sand particles, and water seeps may form. Many oasis lakes are of this origin.

Whatever their mode of origin, lake basins contain bodies of water which are dynamic in that they respond to solar energy input to give warmth to the waters, and to the volumes of water being carried to and from the basin. The influent waters carry not only solute loads, but also transport sediments both in suspension and along the bed of the streams. At the points of entry of streams deltas may build into the water body, and where these extend to the opposite shore they subdivide the lake. Factors such as wind activity, which creates surface waves capable of eroding shorelines, and the generation of ‘seiches’, which cause the water mass to oscillate within the basin, contribute to the creation of a range of associated, often submerged, geomorphological features.

The solar energy input is related to the position of the lake on the Earth's surface. In tropical areas there is always substantial heat input, but in temperate or sub-arctic zones the lake waters may be very cold or frozen for part of the year when solar energy input is low, increasing to a thermal peak in the summer months. Under such conditions the density of the water mass varies as the water temperature changes and the lake waters become layered. The warmer surface waters are less dense than those at depth, and form a distinct layer which responds readily to wind-induced current generation and wave formation. The lower water mass remains cool and little of the energy input from sun or wind penetrates across the thermocline, a barrier across which water temperatures may fall by as much as 6 °C, marking a horizon at which settling suspended sediment may be held during its descent to the lake bed. Many fish species feed at the thermocline, although its level falls as the summer progresses. With the first frosts of approaching winter, the surface waters are chilled, increasing in density, and they begin to sink into and below the summer bottom layer. The turnover of water leads to drastic changes in the colour of the water if materials from the lake bed are carried into the upper waters. The position at which influent water penetrates the lake water depends on the relative densities of the water masses. In summer, influent waters are commonly warm and enter the uppermost waters (overflow), but cooler rainwater falling at night may sink to the level of the thermocline before moving out into the basin (interflow) (Fig. 1). Cold rain derived from thunderstorms may hug the bed as it flows along the basin floor (underflow). Although it is possible to calculate residence times for the waters entering and leaving a lake system using influent and effluent figures, the relative importance of the overflows, interflows, and underflows is the defining factor for the geomorphological behaviour of the lake.

The behaviour of chemicals entering the water with the influent rivers or directly from the atmosphere is complex, being dependent on the chemicals concerned, their relative solubilities, and the pH and Eh of the systems concerned. This fascinating topic is discussed in the entry on the geochemistry of lakes.

Casual observers may regard the beds of lakes as being inherently rather dull with little variety of sediment texture throughout. The belief that the floor is normally flat may be derived from memories of dry lake-bed salt flats used for high-speed vehicle testing. Few lakes have this characteristic, and many have substantial deltas ending in steep slopes where streams enter the water body. In others there are submerged, often broad, benches and platforms which French and German workers have shown to be related to a widely recognized period of low water levels at about 4500 years BP. At this stage the vegetation of the shallower parts of the lake floor was replaced by emergent species before a return to wet conditions again raised water levels. Many ancient lake-dwellings in Scotland (‘crannogs’) and elsewhere have their foundations on this now submerged surface or on rocky outcrops protruding above it. It appears that as lake levels rose slowly the people built upwards, strengthened their access causeways, and continued to occupy the site. Archaeologists have shown that some were occupied for over 3000 years before being abandoned in the fifteenth century.

Steep-sided glacially formed lakes occupying deep U-profile valleys, as in northern Switzerland, show slopes at angles close to those of the angle of stability of the sediments. Silts and muds are known to slide from these slopes into the deeper parts of the basin, sometimes creating small density flows in which the additional sediment load in the sliding waters provides the power enabling the flow to extend for considerable distances from the originating collapse. The atmospherically controlled seiching phenomenon which causes very long-period waves to become established, both at the water surface and also deeper at the thermocline, may exert large enough shear stresses to help in dislodging this material.

In ice-marginal lakes, notably those in Norway, where the active ice alternates between retreat and advance, the lake basin is subject to very rapid sedimentation. Some of the sediments settling to the bed are contributed directly by melt waters, but much is also derived from calving of the glacier. The upward-calving ice from the bed of the glacier is usually heavily charged with morainic matter. The bergs, driven towards the lake outlet by katabatic winds, become stranded as they move into shallower waters. Here melting continues, with accompanying release of sediment to the bed, and a broad shallow shelf is commonly created, with an erratic-strewn beach lining the shore. The finer sediments are frequently moved by the small tsunami-like waves created as the upward calving of the glacier ice continues. During advance phases the deposited morainic and layered bottom sediments become pushed by the base of the ice, and sequences of push-moraines may form beside the glacier margin during retreat dominance.

In the deeper ice-marginal lakes where sedimentary deposits may be preserved, seismic stratigraphic methods can distinguish between morainic material and proglacial deposits, thereby making it possible to reconstruct the glacial history of the lake. At favoured sites (such as Loch Muick in Scotland), the submerged terminal moraines from the last phase of glacial retreat can be recognized. As well as shedding light on past climate changes, lake sediments are sensitive indicators of recent environmental variability. Sediments of the lake floor commonly show alternating coarse and fine layers, which are sometimes attributed to varving from summer– winter cycles of change. It is known that in some lakes several varve successions develop during one year in response to freezing and refreezing in spring.

As most well-established lakes have been in existence for at least several thousand years, the coastal geomorphology of their shorelines has long since reached a mature condition. Pebble beaches may show growth and retreat of longshore ridges and bars in response to wind-induced wave action, but although some will also show development of spits where coastline directions change abruptly, few show more exciting features of coastal erosion. That these existed in the past can be demonstrated from examination of the shores of recent man-made reservoirs, where within 5 years substantial cliff systems form on glacial tills, and within 30 years complex systems of caves, stacks, and geos are seen to have developed on similar shore materials exposed to maximum wave activity.

John Mcmanus

Bibliography

Gierlowski-Kordesch, E. and and Kelts, K. (1994) Global geological record of lake basins, Vol. 1. Cambridge University Press.
Lerman, A., Imboden, D., and Gat, J. (eds) (1995) Physics and chemistry of lakes. (2nd edn). Springer-Verlag, Berlin.
McManus, J. and Duck, R. W. (eds) (1993) Geomorphology and sedimentology of lakes and reservoirs. John Wiley and Sons, Chichester.