The Great Caucasus Mountains stretch for 1100 km from northwest to southeast. The mountains are asymmetric in composition, formed by sedimentary rocks of the Jurassic, Cretaceous, Paleogenic, and Neogenic periods. Paleozoic rocks are exposed west of the axis of the mountain range (Zhulidov et al., 1997). Numerous mountain lakes are widespread over the Great Caucasus territory. Lakes vary in their origins and their water regimes. At present there are 1852 lakes with a total area of 95,8 km². The largest lakes are Kazenoyam, Abrau, Big Ritsa, Kelistba, Bazaleti (Table 1). Most of the mountain lakes (60%) have an area < 5000 m². These smaller lakes account for 11,5% of the total lake water surface (Efremov, 1993).

Most of the Caucasus lakes have a glacial (78,5%) or karst (10%) genesis. Narrow (shallow gully) tarns, depression tarns and moraine tarns were identified for the first time in Caucasus amongst glacial lakes (in the range of tarns). These tarns are different from each other by the type of dam that retains the lake water (Efremov, 1984). These mountain lakes have variable morphometric characteristics that may change depending on natural factors (geomorphologic, climatic etc.). Specific characteristics of each lake's physical, chemical and biological properties also depend upon natural factors (Efremov, 1993). Most of the lakes (78%) are situated on the north slope of the Great Caucasus in high-mountain areas near modern glaciation areas. A clear and regular pattern of lake distribution at high-altitudes is observed here. This fact allows us to identify "lake belts" at altitudes of 2500-3000 m on the north slope and 2000-2500 m on the south slope. River run-off begins at 2500-3000 m. Mountain lakes are very sensitive to changes occurring in their watersheds. Changes connected with climate, glaciation and river run-off are particularly important. Periglacial lakes are more sensitive to these changes than other lakes. According to existing ideas the formation and development of glacial lakes in high-mountain areas is a consequence of climatic variability, expressed in the process of glacier degradation. The area and quantity of glaciers are decreasing. Periglacial lakes are form in favorable geomorphologic conditions where glaciers have vanished. Studies have shown that most of the periglacial lakes appeared during retrogressive phases of glaciation: 2500 - 3000 years ago and in the 19-th century. Analysis of literature sources, topographic maps of 1881-1910 and topographic interpretation of aerial photos from different years have shown that glaciers existed to the end of the 19th century in many places now occupied by modern glacial lakes. Lake formation in the Great Caucasus is continuing today as glaciers recede. So, over the last 50 years about 100 new periglacial lakes have appeared in the West Caucasus (Table 2).

Lake shorelines increase by 1,5-15,0 m per year as glaciers recede (Efremov and Ilyichev, 1998). The glacial recession and the appearance of new periglacial lakes have a cyclical character. Periods of very intensive lake formation over the last 100 years correspond to periods of rapid glacier melting in 1890-1908, 1915-1929, 1935-1938, 1940-1955, 1960-1965 and 1975-1977 (Table 3). Similar regularities are observed in other mountain systems. Lateral tributaries separate from a main glacier during recession and new lakes appear between them. These lakes can periodically break. Good examples of such lakes are the Talsekva in Alaska (Stone, 1963) and the Mertsbakhera in Tien Shan.

Periglacial lakes can be reduced in size or completely disappear as a result of glacier activation. A few such events are known in the Great Caucasus when glaciers approached a lake area. An example is the lake situated near the edge of the East-Klukhorsky glacier (Lake N177). The lake appeared in 1880 and increased in size up to 1929. However, it almost vanished in 1935 as a result of the glacier's approach. N177 appeared again about 1945-1946 and has continued to become larger up to the present day. In some years (1977-1979, 1980-1982, 1986-1987) the area of N177 decreased, coinciding with the glacier's approach. Therefore, the presence of periglacial lakes always points to the gradual degradation of glaciers.

Observations conducted on several lakes in the West Caucasus (Karakel, Tumanlykel, Klukhorskoe, Zerkalnoe) have shown that a straight dependency between atmospheric precipitation and water level of the lakes exists. A dependency between air temperature and lake levels also exists. The levels of the periglacial lakes increase with increased temperature. The levels of lakes situated outside of glacial areas increase at the beginning of summer, as a result of melting of snow cover. After the snow has melted, changes in air temperature are basically reflected in the temperature of the water, but changes in lake levels are not observed in high-mountain areas. Levels of lakes situated in middle- and low-mountain areas may sometimes decrease in summer time during stable hot weather. Morphometric indexes of lakes also depend upon climatic conditions, as do water regime indexes. For instance, lake levels are raised when rainfall increases but so, in consequence, are depth, area, width, length of shoreline and others characteristics. We conclude that mountain lakes are indicators of environmental changes. However, specific relationships between indicators and climate and glaciation changes for individual lakes have not been studied in enough detail to make more than this generalized conclusion. We propose that this available information can form the basis for further studies.