Salinization processes and sabkhat formation in the valleys and ancient deltas of the Murgab and Tedgen rivers in Central Asia

Abstract

The oases of Central Asia are sites of ancient civilizations. Irrigational activity in the Tedgen and Murgab oases began in the fourth millennium BC and continues into the present. Irrigational activity caused significant transformations in the relief of the valleys and ancient deltas of these rivers. In the deltas, one can observe traces of ancient irrigational constructions together with modern canals, bars, dams and embankments.

The relief of the Murgab oasis consists of many ramparts, irrigational canals and collectors, all of which are anthropogenic. Some represent cleared and straightened branches of the Murgab as is the case with the Sultan- Yab and Khurmuz-Fary canals and the Dgar and Kese-Yab collectors. These branches had natural levees, which became higher with the reconstruction of the riverbed. These levees are several kilometers long and 0.5-30 meters wide at their tops. At the bottom their width is 300-500 m. Between these levees there are shallow depressions with gentle slopes (0.003-0.001) and flat bottoms, the dimensions of which can reach 103 km². The depressions contain salinized lakes in the spring, and sabkhat form during dry periods.

The initial alluvial-deltaic relief remains only on the northwestern edge of the Murgab delta, which has low (up to 1 m), flat ramparts and shallow inter-stream depressions opened to the north. Irrigational relief is found in the central and southern regions, where ramparts attain their maximum heights and inter-stream depressions are closed.

Along the Murgab there are ravines. Young ravines have depths of 3-4 m and vertical slopes. Width of their bottoms in the mouth sometimes reaches 40-50 m. In the 150-200 m wide belt along the floodplain of the Murgab there are suffosion craters 1-1.5 m in diameter.

The Tedgen delta has typical deltaic relief with fan-shaped branches (most of them have been transformed into canals) and natural levees, composed of light material. Many branches are filled, but can be clearly distinguished on satellite photos. Sometimes there are lake depressions filled with clay sediments, which have transformed into solonchaks. At the borders of the modern delta there are barkhans and aeolian sand hills. There are many forms of anthropogenic relief in the Tedgen delta.

Presently new agro-irrigational relief is being formed on the ancient delta.