The effect of flood 2020 on the morphological changes of the Sadij River, Southeastern of Iran

River floods occur in the coastal plain of the Oman Sea every year. The flood on January 11, 2020 had the highest discharge recorded in the Jask hydrometric station that caused changes in Sadij River and floodplain in Iran. The aim of this research is to investigate the effect of this flood event on the lower part of the Sadij River. This is the first study has been conducted in relation to the impact of flooding on an ephemeral river morphology in the Iranian Coast of the Oman Sea. In this regard, the river channel was divided into four reaches (A, B, C, and D) based on the channel morphology. Then, the morphometric parameters of the river, central angle, curvature coefficient, River Network Change Index, as well as the pattern and the stream power were examined by comparing the Sentinel-2 satellite images before and after this flood. Also, the flood zoning was carried out using SMADA and HEC-RAS software in different flood return periods. The results showed that the average central angle and the average curvature coefficient were 85.61 and 1.42° for the four reaches before the flood and 77.72 and 1.22° for the post-flood period, respectively. Further, based on the River Network Change Index, the erosion was dominant process in the reaches A (2.35), B (1.27) and C (0.39) while deposition prevailed in the reach D (−0.08). The flood power of this flood was high and separating some of the meanders in reach B from the main river channel. Also, this flood with a return period of 100 years covers an area of 12,882.4 ha, which should be considered in planning and river management. Therefore, this study provides new insights into flood-induced morphological changes as well as flood zoning in semiarid ephemeral rivers that can be used for the future development plans and river restoration.