Amudarya and Syrdarya Rivers and Their Deltas

Abstract

Characteristics of the annual flow of the Amudarya and Syrdarya rivers, which determines the water inflow to the Aral Sea and its level regime, are presented herein. On the basis of hydrological observations on the mentioned rivers and their tributaries the annual flow of the Amudarya and Syrdarya rivers in the zones of its formation (natural water resources), inflow to the delta summits and to the Aral Sea is presented over a 75-year timescale. For the same period information concerning water withdrawal from the rivers of the Amudarya and Syrdarya watersheds for economic reasons, including irrigation, is summarized. For the Amudarya and Syrdarya rivers natural flow variations for separate decades in the 75-year (1932–2006) period have been compared with the inflow to the delta summits which covers all kinds of anthropogenic influence in the catchment areas of these rivers. Up to the 1960s the deltas of the Amudarya and Syrdarya rivers, which had received a lot of water and sediments, were among the most dynamic in the world. These azonal objects resisted the deserts of Central Asia and were notable for their high biodiversity and biological productivity. As a result of dramatic man-induced reduction of river’s water inflow and a drop of Aral Sea level the deltas of the Amudarya and Syrdarya rivers, their hydrographic network and landscapes have undergone severe degradation. In many respects these deltas have lost their specific "deltaic" natural complex. At present they are artificially flooded only in part. After isolation of the Small Sea in 1987 its water was partly transferred to the Large Sea through the channel in the former Berg Strait. An earth-filled cross-dike was constructed in a channel in 1996 to maintain the level of the Small Sea; in 1999 it was destroyed, and in 2006 replaced with a solid dam. After this the level of the Small Sea began to depend on the inflow from the Syrdarya river. The level became stable at about 39.5 m (1990)–37.8 m (1991). It was even higher in some years, and the sea area increased (2003, 2005). Recently (in 2006–2008) the level of the Small Sea fluctuated within 41.2–41.7 m. Stabilization of the sea level due to rather small river water inflow was reflected on 2003–2006 space images: the growing mouth cape of the main channel of the Syrdarya river delta can be clearly seen. Thus, one could argue that restoration of delta-forming processes in the mouth of this river is quite possible. It has been established that the statistical parameters of natural river flow variation in the zones of flow formation during the period of stability of the Aral Sea level (until 1961) and during its very strong decrease (1971–2006) are practically invariable. The volumes of annual inflow to the delta summits and to the Aral Sea during different periods in the last 45 years are critically less than those before 1961. The functional relationship between the Aral Sea level variation and the water inflow to the Sea has been revealed. The dynamics of the Amudarya delta in different geological epochs and the Syrdarya delta during the last 70 years have been analyzed.