Declining water levels

Falling water levels and the resulting shrinkage of lake areas are due to the overuse of water drawn from the lake itself or from inflowing or outflowing rivers.

The extreme case of the Aral Sea is now widely known. Similar situations are also reported for other Central Asian lakes such as the Caspian Sea in the middle decades of the twentieth century (Golubev, 1992), Lake Balkhash in Kazakhstan, Lake Qinghai in China, and some lakes in Iran. Lake Mono in California also lost about 30 per cent of its former area owing to a fall in the water level of 11 m, and suffers from raised salt concentration in the lake water (as does the Aral Sea), owing to the diversion of 85 per cent of its tributary river water to the city of Los Angeles.

Aside from such lakes in the arid zone, the water level in other lakes has often been significantly lowered by dredging their outlets in order to increase the capacity of hydroelectric power generation in outflowing rivers. This has often caused the temporary advance of eutrophication, as, for instance, was the case in Lake Sevan Armenia (Oganesian, 1991).

Fig. 3.1 The transparency of lake water in relation to lake water volume in 145 lakes and reservoirs of the world

Fig 3.2 Six major environmental problems of world lakes and reservoirs