The trend of suspended sediment changes of Karaj and ‎Jajroud rivers during recent decades

Based on expert estimates, many Iranians believe that the amount of sediment yield is annually increasing. This study has attempted to assess the temporal trend in Suspended Sediment (SS) of Jajroud and Karaj rivers at Roudak and Sira stations. respectively with adequate data collected during four decades. The SS yield data and SS concentration as well as flow rate data were analyzed. Preliminary investigation showed that more than 92 percent of the SS at Roodak and Sira stations is transferred by top two and three flow deciles (equal to 20 and 30 percent) of the effective or flood discharges. Among these, the highest decile (10 percent) is responsible for carrying about 80 and 64 percent of SS accounted for the two study sites correspondingly. Therefore, mainly analyzing the highest decile flow data was considered. Four techniques were performed including: a) fitting regression lines to the measured SS concentrations and weights vs. time, and b) graphical comparisons of dependent variables (average discharge, average SS concentration and weight) during the four decades, c) using the Kruskal–Wallis test, and d) using Kendall rank correlation coefficient. All interpretation techniques showed decreasing trends vs. time which were significant based on the two nonparametric techniques. Since, the highest decile of daily flows of study sites have not shown any increasing trends during study period, it can be concluded that unlike the general publics’ believes the amount of SS has decreased. A possible explanation for this descending trend might be as a result of 43 and 25 conservation projects implemented during recent 30 years in the upstream basins of Amirkabir and Latian dams, respectively. The descending SS trend does not necessarily mean lower rates of erosion in these basins; but it rather means lesser sediment delivery ratios.