Nature Of Sealevel Changes In Caspian

The Caspian Sea is a closed basin in the inland part of Eurasia and this sea's water level is below that of the world ocean. The sea basin stretches almost 1200 km from north to south and its width varies between 200 and 450 km. The total length of the coastline is about 7000 km. Its water surface area is about 390,600 km2 (as of January 1993). Water salinity in the northern part is 3 to 6%o, and reaches 12%o in the middle and southern parts.

Fluctuations in sea level for various lengths of time can be found in the data of geomorphological and historical studies of the record of the Caspian Sea (Fig. 2). Within the last 10,000 years, the amplitude of fluctuations of Caspian Sea level has been 15 m (varying from —20 to —35 m). During the period of instrumental observations (from 1830 onward), this value was only about 4m, varying from —25.3 m during the 1880s to —29 m in 1977. Annual increases in the level during this period met or exceeded 30 cm on three occasions (in 1867, 30 cm; in 1979, 32 cm; and in 1991, 39 cm). The mean annual increment in the level in the 1978 to 1991 period was 14.3 cm.

Natural factors are the primary cause of recent Caspian Sea level fluctuations (but not the only cause). Scientists have identified three distinct periods of level changes: 1830 to 1930, 1931 to 1977, and 1978 to the present. The first period of 100 years saw sea-level fluctuations not exceeding 1.5 m (5 ft). Researchers considered this period to have been relatively stable. The second period, from 1931 to 1977, is identified by a constant decline in level by 2.8 m (9.1 ft), and in 1977 the Caspian Sea reached its lowest level since the beginning of instrumental record-keeping in

As the sea level declined throughout the 1950s, 1960s, and early 1970s, Soviet scientists forecast that the decline would continue for at least a few decades into the future. Scientists have linked the reason for the decline to the regulation of Volga River flow. During these decades, major engineering activities were undertaken along the Volga, such as the construction of water diversion canals, reservoirs, the 1830s.

1840 1860 1880 1900 1920 1940 1960 1980 2000

Year

Figure 2 Caspian Sea level, 1835-1999, observed.

and dams. The construction of such engineering facilities diverted water away from the Caspian.

In response to this major drop in sea level, human settlements bordering the sea coast began to move toward the receding coastline. Fields and pasturelands were prepared for use, roads and rail lines were constructed, and housing and factories were built on the newly exposed seabed. During the Soviet era, many people emigrated from other parts of the region to settle along the border of the sea. Development of infrastructure along the coast took place to support the increasing population.

In an attempt to save the Caspian from drying out, Soviet scientists and engineers proposed the construction of a dam to block the flow of Caspian water to Kara-Bogaz-Gol Bay, a large desert depression in Turkmenistan adjacent to the sea's eastern shore. Political decisions made in the mid-1970s ordered the construction of the dam, but due primarily to bureaucratic inertia, the dam was not completed until the early 1980s. This was a few years after the Caspian's sea-level change had reversed direction. Before the dam was constructed, the bay took in 40 km3 (8.6 mi ) of Caspian water annually. It served as a huge evaporation pond, as well as a natural location for the accumulation of commercially useful mineral salts.

Another Soviet government response to the decline in the Caspian's sea level was a diversion of water into the Volga River from other Soviet rivers that flowed northward into the Arctic Ocean. River water flowing into the Arctic was viewed as wasted and without value to the Soviet Union because it was unused by human activity.

3 THE CASPIAN RISES

To the surprise of Soviet scientists, the level of the sea began to rise suddenly in 1978, the beginning of its third period of level changes. Since then, the Caspian has risen steadily by more than 2.5 m. One of the first actions the newly independent government of Turkmenistan took in 1992 was to tear down the dam in order to allow great amounts of water to flow into Kara-Bogaz-Gol Bay again and to replenish the supply of salts.

Scientists have proposed a variety of hypotheses about why the Caspian Sea level had increased so rapidly. These can be clustered into the following categories: tectonic plate movement on the seabed, climate fluctuations and change, and hydraulic construction along the Volga River, or some combination of these factors.

Tectonic Plate Movement Hypothesis

Tectonic movements over periods such as centuries and millennia have been the cause of many geologic changes in the Caspian basin. The region has been subjected to uplift, subsidence, overthrust of landforms, seabed mud-volcanic activity, and landslides, in addition to erosion processes and the accumulation on the Caspian seabed of river-transported sediments. However, it is difficult to see how tectonic

3 the caspian rises 889

movements could cause such sharp fluctuations in the Caspian's sea level over relatively short periods. Thus, it appears that such movements have had an insignificant impact on recent sea-level fluctuations.

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