The purpose of the analysis in this article was to determine the conditions for establishing the discharge curve of the old mill dam construction in comparison to the gage located above the bridge. The analysed object – Siekierka, is located on the Zwoleńka river. It is located in the closing section of a partial research catchment. The reconnaissance scope covered the section of the river necessary to assess the impact of the upstream and the downstream flow conditions on the weir waterflow. This included the flow area in the upper reach of the object, the spill-weir construction and the area adjacent to the downstream section. The weir sill raised above the river bed shapes the flow conditions in the upper water section and in the line of the bridge passing. The bridge has a river bed protected from the erosion and the weir width does not exceed 10 m. For this type of bridges the accumulation of the upper water level is allowed as well as the subcritical motion may occur on the crossing section. High bottom water conditions occurring below the weir indicate that the capacity of the object largely depends on the hydraulic conditions of the weir outlet. The article includes the results of studies on determining the impact of the lower water area and the construction of the weir sill on the outflow.

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