The objective of the present study was to determine the impact of two small bridges located within the main ‘R’ ditch of the Łączany drainage system on the capacity of the selected fragment of this ditch. A fragment of the ‘R’ ditch was selected for the study, 311.7 m long, with two small bridges (bridge No. 1 at km 3 + 455 and bridge No. 2 at km 3 + 365) and 11 crosssections. Having conducted hydraulic calculations, the capacity of the selected fragment of the ditch was ascertained, assuming the base flow (reliable discharge) determined in the detailed design of the drainage system. The base flow, amounting to 9.77 m3 • s–1, was also used to determine water damming in the examined bridges in accordance with the Regulation by the Minister of Transport and Maritime Economy. As a result of geodetic measurements, changes in the shape of the cross-sections of the ditch and changes to the bottom slope within the tested segment of the ditch were found. It was established that under the conditions of proper maintenance of the ditch, its capacity is not lower than the base flow. However, if the maintenance is neglected, the area before bridge No. 1 will be flooded, which is the result of the lowered capacity of the ditch. It was also found that the examined bridges did not contribute to the deterioration of the conditions of the base flow water through the ditch in the analysed fragment thereof.