Morphologic Change Assessment of Riverbed Before and after Dam Construction Using HEC RAS Modeland GIS (Case Study: Downstream of SatarkhanDam)

Rivers are important sources of water supply in the world. So reduction of their water canimpact on human life. This water supply is facilitated by dams as hugestructures that save water and make big reservoirsbehind. Dams are well known for influencing channel and vegetation dynamics downstream, but little work has focused on distinguishing effects of land use and channel responses to the impoundment. The impact of construction of dams and reservoirs on alluvial rivers extends both upstream and downstream of the dam. Downstream of dams, both the waterand sediment supplies can be altered leading to adjustmentsin the river channel geometry and ensuing changes in riparian and aquatic habitats.Construction oflarge water structuressuch as dams across rivers may lead to considerable decrease in the flood discharge proportionate to structure size which will in turn result in lower flood probability. Assessmentofthe vulnerability of river boundary is of great importance due to their rich soil and easy access to irrigationwater which makes such riverbank farms economically valuable especially in dry areas. The results of suchassessment can be used in land use planning.In this paper, we examinedthe interacting effects of Satarkhan dam on downstream changes in channel morphology of Aharchai River. Satarkhan dam was constructed on the main stem of the Aharchai in 1998 for water supply, flood control and sediment retention. This dam is located between Ahar and Varzaghan city, eastern Azarbaijan, northwest of Iran.