The establishment of stability in rivers is dependent on a variety of factors, and yet the established stability can be interrupted at any moment or time. One factor that can strongly disrupt the stability of rivers is the construction of dams. For this study, the identification and evaluation of morphological changes occurring to the Karkheh River, before and after the construction of the Karkheh Dam, along with determining the degree of changes to the width and length of the downstream meanders of the river, have been performed with the assistance of satellite images and by applying the CCHE2D hydrodynamic model. Results show that under natural circumstances the width of the riverbed increases downstream parallel to the decrease in the slope angle of the river. The average width of the river was reduced from 273 meters to 60 meters after dam construction. This 78% decrease in river width has made available 21 hectares of land across the river bank per kilometer length of the river. In the studied area, the average thalweg migration of the river is approximately 340 meters, while the minimum and maximum of river migration measured 53 and 768 meters, respectively. Evaluations reveal that nearly 56% of the migrations pertain to the western side of the river, while over 59% of these migrations take place outside the previous riverbed. By average, each year, the lateral migration rate of the river is 34 meters in the studied area which signifies the relevant instability of the region.

Use of general circulation models (GCMs) is common for forecasting of hydrometric and meteorological parameters, but the uncertainty of these models is high. This study developed a new approach for calculation of suspended sediment load (SSL) using historical flow discharge data and SSL data of the Idanak hydrometric station on the Marun River (in the southwest of Iran) from 1968 to 2014. This approach derived sediment rating relation by observed data and determined trend of flow discharge time series data by Mann-Kendall nonparametric trend (MK) test and Theil-Sen approach (TSA). Then, the SSL was calculated for future period based on forecasted flow discharge data by TSA. Also, one hundred annual and monthly flow discharge time series data (for the duration of 40 years) were generated by the Markov chain and the Monte Carlo (MC) methods and it calculated 90% of total prediction uncertainty bounds for flow discharge time series data by Latin Hypercube Sampling (LHS) on Monte Carlo (MC). It is observed that flow discharge and SSL will increase in summer and will reduce in spring. Also, The annual amount of SSL will reduce from 2,811.15 Ton/day to 1,341.25 and 962.05 Ton/day in the near and far future, respectively.

In this research, a combined method was developed to determine the erodibility of bends in the Karkheh River. For this purpose, a 40 km reach of the Karkheh River downstream of the Karkheh Dam was considered. The value of the shear stress was the calculated using the CCHE2D model. The results from the model show that in 1996 (before construction of the Karkheh dam), the length of the erodible reach was 1314 m; in 2011 (after construction of the Karkheh dam), this length was reduced to 840 m. Furthermore, the model illustrates that the location of the maximum shear stress is a function of the relative curvature (R/W) in the bends. For small values of the R/W (less than 1.5), the maximum shear stress occurs on the convex bank of a river bend. By increasing the R/W, the location of the maximum shear stress transfers to the concave bank of the river bend. Also, this location is displaced towards downstream by increasing the R/W.