Investigating the Relationships between Hydromorphological and Hydrological Characteristics on Habitat Suitability under Scenarios of Changing the Environmental Flow Regime based on Kordan River Ecosystem Restoration

River restoration and protection of the natural and ecological life of the river in besides to using it for various purposes is one of the most prominent areas of applied water resources science. Environmental flow provision is recognized as a prominent tool to reduce the negative effects of regulation river flow and conservation, restoration, and improvement of natural river habitat quality in watershed management and integrated water resources management. In this study, in order to evaluate and estimate the environmental flow regime of Kordan River, the Flow Duration Curve method based on hydrological statistics of the hydrometric station during the 35-year statistical period (1985-2020) and MesoHABSIM hydromorphocological model based on field studies and observations were used and accordingly, the available habitat suitability of the target fish species in different scenarios of the flow regime was examined. The results showed that the distribution of the minimum amount of ecological flow during the year is relatively uniform and within the river basin flow range (maximum of 1.8 m3/s in April and minimum of 0.3 m3/s in October). However, the optimal ecological flow distribution to provide optimal hydromorphological habitats throughout the year is unequal. Also, the maximum required amount of optimal ecological flow in May is equal to 3.6 m3/s, and the minimum amount of optimal ecological flow in October is equal to 0.7 m3/s. On the other hand, according to the analysis, at most times of the year, flow less than the average annual flow in the studied river which is equal to 3.95 m3/s, while these flows are insufficient to meet the ecological needs of aquatic. The results of the present study show that changes in the hydromorphology of Kordan River due to long-term regulation of river flow, lead to changes in the distribution of optimal morphological habitats. It is noteworthy that the MesoHABSIM model simulates the desirability and availability of fish species habitats for different scenarios of flow regime and river morphology and can also be used in river ecological restoration projects and environmental flow determination.