Development of the Volga Delta in Response to Caspian Sea-Level Fluctuation during Last 100 Years

The Volga Delta developed in the closed Caspian basin can be classified as a river-dominated type. The subaqueous part of the delta (avandelta) is characterized by an extremely gentle slope and may be subdivided into shallow and deep parts. The very low-angle slope of the shallow avandelta is a major factor to mitigate impacts of Caspian sea-level (CSL) changes on the Volga Delta during the last 100 years. The CSL has fluctuated 3.46 m in the last 100 years, with a high of -25.55 m relative to the Baku Datum (BD) in 1903 and a low of -29.01 m BD in 1977. Sea-level changes can be grouped into three stages: (1) slowly falling at an average rate of 1.1 cm yr(-1) in 1900-1929; (2) rapidly falling at an average rate of 6.3 cm yr(-1) in 1930-1977; (3) rapidly rising at an average rate of 8.8 cm yr(-1) in 1978-1999. During the rapid fall stage of sea level, the Volga delta-plain expanded at a rate of 2.3 km(2)yr(-1) to 180 km(2)yr(-1) because of alluvial accumulation and exposure of the shallow avandelta. Flourishing subaqueous vegetation promotes this expansion by trapping suspended sediments on the avandelta. The shallow avandelta is primarily covered by freshwater and its geomorphic and sedimentary features are controlled more by the river flow than by the CSL changes, waves or wind-induced currents. The shallow avandelta decelerated delta progradation during sea-level fall and delayed delta retreat during sea-level rise. Significant changes in the delta-plain area have not occurred in the rapid CSL rise stage. Moreover, the distributaries and their distal underwater channels underwent erosion by the river flow at least until the end of the last century. The CSL has risen with an average annual rate of 8.8 cm during the last 20 years, yet coastal erosion, coastal flooding, wetland loss and saltwater intrusion, observed in other deltas, have not occurred here. This may have resulted from the existence of the shallow avandelta and show that the world's deltas respond to sea-level rise in different ways.