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Rivers are environmental features that react strongly to changes in their beds. Tectonics is the most important factor affecting the morphology of rivers, causing significant changes in river systems. In this study, we investigated the effect of tectonic activity on river systems in the eastern slopes of Mullah Daghi, southeast of Zanjan province. This area is drained by the Abhar River. We used the Digital Elevation Model (DEM), Geographic Information System (GIS), and Global Mapper 13 for our investigation. In the ArcGIS software environment, we used the DEM to prepare required maps, including geological maps and topography. We used the 1:250,000 Geological Map of Zanjan to create the fault map. To achieve our research objectives, we analyzed tectonic activity indicators such as mountain front sinuosity, basin shape, drainage density, longitudinal gradient of the river, river meanders, basin asymmetry, hypsometry integral, topographic symmetry of the basin, and width of the valley floor to the height. These indicators were combined to calculate the tectonic activity index (IAT). The results obtained through the IAT index indicate the activation of tectonic and neotectonic activities in the Ardejin and Abhar watersheds, and moderate activity in the Zareh Bash basin. These activities have different impacts on the river systems in the studied basins, such as river meanders in the Ardejin watershed basin, tilted watersheds in Abhar and Ardejin, and elongated basins in Zareh Bash. [ABSTRACT FROM AUTHOR]