Recent avulsion history of Sefidrud, south west of the Caspian Sea

摘要

The Sefidrud, a river flowing into south west Caspian Sea, is prone to avulsion. Previous studies of the river delta support the idea that several major avulsions have occurred during the past few thousand years. During the most recent major avulsion, a 23 km shift occurred along the coast. However, in literature the timing of the event is subject to uncertainty and therefore the causes of the avulsion remain unclear. Lying in the most populated area in Iran explains the importance to investigate the Sefidrud river avulsion. In this study, an 11 m-long sequence was obtained from an area where the river splits off into an old and a new distributary, and where a large preavulsion wetland had developed. Palaeo-environmental studies including sedimentology, macro-remain, and palynology were performed to detect changes in environment. Five radiocarbon dates were obtained to determine the avulsion age. In parallel an analysis of historical maps around the timing of the supposed avulsion and for more recent times provided a complementary way of refining the avulsion age. The results from this novel multidisciplinary study show that the last major avulsion occurred at a date after the early1800s and before 1929, during a period of rapid water level fall. Hence this is several centuries more recent than so far suggested. Studying aerial photographs, this research confirms the occurrence of minor avulsion between 1955 and 2014 where the avulsion point is around 2 Km from the coast line, and during a period of sea level fall. The results of this study also suggest that beyond the immediate effects on society and agriculture, avulsion increases the rate of coastal erosion near the abandoned distributary, as sediments are temporarily sequestered on the floodplain and causes intensive erosion. Avulsion has also major impact on coastal lagoons and inland wetlands by river diverting into the lagoons and wetlands.