Dams are important structures that are mainly constructed for water and energy supply. Dam break creates a huge flow that leads to flooding in areas downstream. Therefore, determining characteristics of this flow, including the flow depth and wave propagation velocity, is of great importance. In this research, the simultaneous effects of reservoir geometries and downstream obstacles on hydrodynamic characteristics of the flow caused by dam break have been investigated using three-dimensional numerical modeling. For this purpose, six reservoirs with different geometries, including wide, trapezoidal, L-shaped, long, hexagonal, and octagonal reservoirs, with downstream dry beds have been considered. The results of three-dimensional numerical modeling indicate that the reservoir geometric shape has a severe effect on the flow, since it plays a determining role in the inlet flow to the downstream channel. Downstream obstacles also affect the flow caused by dam break, but their effects are local and significant only over a certain length behind obstacles. This length is related to the reservoir shape and varies within the range of 19.5 to 22.5 times the pier (obstacle) diameter. Thus, the largest length in which the local effects are significant is observed in the wide reservoir, which is approximately 22.5 times the pier diameter. Meanwhile, the minimum length is related to the long reservoir, which is 19.5 times the pier diameter.