In semi arid area geomorphological impacts of dams are very major. Dams, as man- made structures, have major impacts on river hydrology, primarly through in the timing, magnitude, and frequency of low and high flows, ultimately producing a hydrologic regime differing significantly from the pre-impoundment natural flow regime. Despite the profiles of dams on river regulation for water resources management, they create artificial flow regimes downstream and also interrupt the transfer of sediments from headwater source area. These changes are started on acts of the fluvial processes at below dams, and caused adjustments of channel form. For study of effects of dams on upstream and downstream of semi arid catchments, Garangoo,s catchments is selected.This catchment is located on eastern slopes of Sahand Mt., (from 46° .27' to 47° .42 E., from 36° .58´ to 37° .44´N). In this paper emphasizes the downstream effects of Shand,s dam on Garangoo river, on the sedimentatation,s rate of dam and also, on the effects of dam of Sahand on downstream geomorphology. For assessing of rate of erosion ,amount of sediment, is used of bivariated method and GIS and then produced map of suitable area for erosion, sedimentation and landslides. The mapping is often performed through the identification and analysis instability factors .For mapping, firstly distribution map of factor produce and digitized of factors and then analyzed by bivariated methods and the next stage, weighted to factors and final stage, produced hazard donation map by using of Arc/View. This map shows that, upper part of Garangoo,s catchment, erosion is critical and about of all products of erosion processes and materials of resulted by the landslides occurrence, remove into streams, and the final stage, deposit in sea of dam. The results of study also show that, responses of rivers to damming are as linear erosion, lateral erosion, meander and deeping river's beds.