ABSTRACT: Flooding is one of the natural disasters that occur with changes in meteorological conditions such as excessive precipitation and the rapid melting of snow. The major flooding formation factors are a region’s topography, geological structure, climatic conditions, any deterioration in the natural structures of rivers, and uncontrolled urbanization. To reduce the risk of flooding, this study conducted river rehabilitation work within the water collection basin of the Yazılıkaya River, located within the borders of the Nallıhan District of Ankara Province. Furthermore, a mortared check dam was designed and implemented in an area of 78.8 hectares (Ha). The study of the Yazılıkaya River in-stream rehabilitation project included a two-dimensional flood model, which was carried out with the help of HEC-RAS software (Hydrologic Engineering Centers River Analysis System) using obtained data (stream maps, flow rates and art structure cross-sections) and the DSI Synthetic method with Q10 and Q100 flow rates. To analyze the effects of a mortared check dam on the Yazılıkaya River, the flow rate values of Q10 and Q100 in both heavy and sudden rains were used; it was observed that while the speed of the water was 2-3.2 m/sec before the construction of the dam, it decreased to 0.5 – 0.75 m/sec after construction. After the completion of the mortared check dam, it was calculated that there were water reductions of 2 and 8 cm respectively in the water depth analysis in the settlements made according to the Q10 and Q100 flow rates. Generally, this study investigated the effect of upper basin flood control structures (mortared check dam) and the regulation of precipitation water flow regimes (water energy, speed, depth, etc.), on flood hazard in populated settlements and on the highway.