pluvial lakes

pluvial lakes A lake is a natural inland body of water in which the water is generally standing. It has various characteristics and can, for example, be classified with regard to its size, shape, source of water, or water chemistry. A pluvial lake is one which has experienced large fluctuations in area and volume (i.e. changes in water balance) in response to climatic variations. For a lake to be classed as truly pluvial, changes in lake level should not be affected by any change in base level, nor by being dammed in any way, for example by ice, lava, or moraines. It has been noted that some lakes may have increased in size owing to the melting of ice; however, many pluvial lakes have not had glaciers in their catchments.

Pluvial lakes are so named because they reached their greatest extent during the ‘pluvial’ (from the Latin pluvius, meaning rain) periods of the Quaternary (the Quaternary being the last 1.6 million years). Within this time period there were significant changes in the Earth's climate, one of which was the greater availability of moisture at particular times in certain parts of the world. The term ‘pluvial’ was first used by Alfred Taylor in 1868 to indicate a period of increased moisture which resulted from greater precipitation or reduced evaporation, or a combination of the two. It was believed that these pluvial periods could be linked simply with the glacial fluctuations of the northern hemisphere, since initial studies in the American south-west indicated such a synchronicity. However, other, more recent studies, in East Africa for example, show that glacial advances in the northern hemisphere were coupled with very dry periods in some regions closer to the Equator. Since the simple temporal linking of the ‘glacial–pluvial’ theory is no longer acceptable, it is often suggested that the term ‘pluvial lake’ is not suitable; many therefore prefer the term ‘megalake’ or ‘ancient lake’.

Hydrology and basin form

Pluvial lakes occupied large, hydrologically closed basins. The catchments of these basins may have a very large surface area, collecting run-off from a vast region. Groundwater is important to many pluvial lakes, for it is commonly a major source of water for closed basins. As the basins are hydrologically closed, the lake level varies rapidly in response to changes in moisture, whether seasonally or over much longer periods of time. Pluvial periods kept lake levels high for thousands of years at a time and recharged the groundwater of the catchments.

Pluvial lakes were common in areas where geological processes such as tectonism created large basins and depressions. Many of these basins are characterized by an elongated shape, such as those in the south-western USA, reflecting the Basin and Range topography and tectonic regime of the area. Some of the basins are flat-bottomed, and increases in moisture thus result in a vast expansion of surface area with little increase in depth until the whole basin floor is covered. However, some lakes grew to such an extent that they flooded a single catchment and overflowed into adjacent basins. The result was the formation of chains of interconnected lakes, such as the Owens River valley system in California. This is a series of four lakes (Owens, China, Searles, and Paramint) with progressively lower base levels through the system, although each is separated by a ridge so that an increase in water level is required before the lake overflows. At the time of the last glacial maximum (about 18 000 years ago) over-flow linked Owens, China, and Searles lakes; although not connected, Paramint contained a lot of water. There was also a lake in Death Valley below this, known as Lake Manly, although this is not believed to have been connected to the Owens valley system.

Evidence of pluvial lakes

Pluvial lakes have been used in the reconstruction of palaeoclimates, using shoreline evidence and analysis of lake sediments to link lake levels to climate. When pluvial lakes have been at the same level for some time, shorelines will form which, as the lake recedes, are left abandoned. Lake stands will be recorded only during periods of lake regression, since a transgressing lake will rework previous shorelines. These relict features can be seen cut into the surrounding uplands of former pluvial lakes and they provide evidence for higher lake levels in the past. For example, shorelines that cut into the Wasatch mountains in Utah, USA are recognized as being the eastern borders of the pluvial lake, Lake Bonneville. In places where substantial parts of shorelines can be seen, they can be mapped to indicate the former maximum extent of the lake. It is also possible to see shoreline features on satellite images, where different vegetation bands can be identified occupying different shorelines.

Where there is no shoreline evidence, the existence of former lakes may be recorded by the lake sediments. The covering of the basin floor during periods of enhanced moisture conditions allows the deposition of lacustrine sediments such as marls and clays. At times of greater evaporation the water in pluvial lakes becomes more concentrated, and more saline deposits, for example, algal limestones, may be laid down. During the more arid periods of the Quaternary the surfaces of some pluvial lake basins were almost desiccated, allowing the deposition of evaporites such as gypsum and halite. If the lake bed became completely dry, material could have been lost through wind erosion or deflation. This could be recorded in the sediments as a section of aeolian deposits.

Location

The south-western United States has one of the greatest concentrations of pluvial lakes in the world. The Basin and Range province, caused by extensional tectonic activity, provided the closed basins and interior drainage required to support pluvial lakes. One of the most extensively studied pluvial lakes is Lake Bonneville. It was the subject of a classic study by G. K. Gilbert (during the 1890s), who mapped abandoned shorelines and reconstructed four past lake stands. The highest shoreline from Lake Bonneville is at 1565 m above sea level (a.s.l.) and is named the Bonneville shoreline. The others include the Provo shoreline (1470 m a.s.l.) and the Stansbury shoreline (1350 m a.s.l.). The fourth is a minor shoreline, 15 m above the present level of 1280 m a.s.l. and is named the Gilbert shoreline. Lake Bonneville was situated around the present Great Salt Lake, Utah Lake, and Sevier Lake (Fig. 1), which today represent about 5 per cent of its maximum size. At its greatest extent, Lake Bonneville covered more than 51 000 km² and was more than 330 m deep.

In Africa there is evidence of one of the largest pluvial lakes that is known to have existed: Lake Chad. At present, Lake Chad has a shoreline at 280 m a.s.l., whereas at its maximum extent (known as ancient Mega Chad) the shoreline was 335m a.s.l. Further east, the valleys of the East African Rift also exhibit evidence of former high lake stands. Lake Abhé, in Afar, reached a depth of more than 150 m and had a surface area of 6000 km². It is recognized that tectonic activity in the rift valleys may have disturbed the lake sediment records and altered the base level. This implies that the recorded lake levels will not reflect only changes in climate. The widespread conformity of the records from numerous lakes indicates, however, that the regional climate was a dominant factor in the fluctuations of these lakes.

The Middle East also has evidence of pluvial lakes. In the Dead Sea Rift Valley there was a large lake called Lisan Lake. The highest shoreline is recorded at a height of 220 m above the present Dead Sea. Again, it is thought that tectonic activity may have altered the base level, but that the climate was a significant factor in lake-level change.

Pluvial lakes today

Pluvial lakes were most commonly found between 30°N and 30°S, in areas that today are arid or semi-arid and where the climate is dominated by high-pressure systems. Owing to the drier nature of the climate in these areas today, former pluvial lakes are usually seen only as shallow saline pools, a fraction of their maximum size, or as predominantly dry playa basins. (Playa is a term given to a closed basin or a dry lake bed which has interior drainage, in an arid environment.) The similarity between this definition and that of pluvial lakes is notable, although a playa can form in many different ways, and the basin floor of a pluvial lake is just one example.

The basins left by pluvial lakes have proved useful to humans. Many lacustrine deposits which were laid down during low lake stands, such as chlorides, sulphates, and carbonates, are mined for their commercial value. Deposits from less stable phases of the lake may also be used, such as sands and gravels from the former lake margins. Where the exposed basins are large and flat, they can provide surfaces for highways or airfields. World record-breaking land-speed trials have taken place on the basin floor of Lake Bonneville.

A. Courtice

Bibliography

Goudie, A. (1992) Environmental change, (3rd edn). Clarendon Press, Oxford.
Lowe, J. J. and and Walker, M. J. C. (1984) Reconstructing Quaternary environments. Longman, Harlow.