Central Asia and the Amu Darya basin
In the Amu Darya basin in Central Asia, of the 19 to 21 km3 of drainage water run annually, 9.7 km3 is returned to the river. Over the last 30 years water salinity in the lower Amu Darya has increased to 3 g/L. Salinity in reservoirs of Central Asian rivers is also increasing. In a sample of 13 reservoirs, eight on the lowland had salt concentrations from 550 to 1200 mg/L, peaking at 1700 mg/L during the autumn-winter period.
While the salinity in mountain reservoirs is determined by the geology of the drained soils, in lowland reservoirs it is largely due to human influence, resulting from inputs of returned irrigation water. Reservoir water used for irrigation should not exceed a salinity of 1000 mg/L, a regulation beneficial for fish production. But reservoirs and lakes established from drainage discharges have salinities of 2.0 to 21.5 g/L making them increasingly unsuitable for freshwater fish. The negative impact on fish of the gradual increase in salinities in the drainage-receiving Lake Sarykamysh, off the Amu Darya, is well documented.
Wetlands of the Amu Darya delta
In the 1950s, wetlands covered nearly 800 000 ha in the Amu Darya delta but by the 1980s this was reduced to about 100 000 ha which are now supported by residual river flow and irrigation.
The major cause of wetland impoverishment has been the 3 to 8 metre drop of ground water as well as the end of floodplain inundation. At the beginning of the 1980s discharges to the Aral Sea declined due to the following water diversion as follows: Syr Darya from 18 to
1 km3 y-1, Amu Darya from 22 to 6 km3 y-1. In the Amu Darya delta 50 lakes totalling an area of 1 000 km2 dried out; wetland vegetation dried out or was damaged on some 10 000 km2.
Impact on fish stocks
Changes in river discharges and their regulation have considerably affected fish stocks. Spawning and nursery habitats in the major river deltas have substantially decreased, the total area of freshwater lakes in the delta has shrunk considerably and many lakes have disappeared, removing thousands of hectares of water for fishery purposes. Movement of migratory species has practically ceased and the Aral Sea is no longer of any importance to fisheries, with catches declining from a peak of 40 000 tonnes in good years prior to the decline in water level, to zero. As a consequence, fishing activities have been transferred to lakes (including terminal lakes receiving drainage water), reservoirs, and major irrigation canals.