Deposition and a decrease in the expected life of Mahabad dam and some indications of changes on environmental systems in its upstream area

Reports indicate that the Mahabad dam has been silting up more than it was expected. this research has been tried to study the causes and the agents of these changes. A duration of two years periods (that is from October 1986 to October 1987 and October 1991 to October 1992) was taken up to compare and then to determine the role of the sedimentation on these changes. Results show that the sedimentation in this dam has greatly increased in recent years. On the other hand، two satellite images، taken in 1987 and 1998، were selected to study the effects of land usage on the erosion acceleration; and they show the amount of the land usage، using vegetation cover index. The area changes on land usage، in 11 years، show that 12% of grass-land have turned to agricultural zoons. It is clear that the area of changes is so great to force one to think about the reasons of sediment production and precipitation into the dam. However، results from area estimation of land changes to dry farming show an increase in the susceptibility of these lands to landslides. As a result، the increase in mass wasting has made a lot of sediment accessible for transportation to the Mahabad dam and the decrease of its expected life.