Accumulation of sediment and silt in the drainage canals is undesirable, yet inevitable occurrence in the course of the use and operation of any drainage canal network. In this study, D-25 drainage canal group, taking place in the Nazilli irrigation system with an area of 1165 ha is the only system where all planned activities have been completed. It has been determined that the drainage system was constructed according to original drainage project. The depth of accumulated sediment in the drainage canals in the research area was determined from the difference between the measured elevation and the elevation given in the design projects. The reasons for siltation in the D-25 drainage canal group have been studied by looking at the results of the elevation measurements made in 2010-2012. The measurements made in D-25 drainage canal group showed that there were significant differences between the actual structure (bridge, culvert, and conduit) bottom elevations and the elevations given in the design projects. In addition, the length of some canals would not coincide with the design project either. 83.3% (93.3% in length) of the canals had differences in structure bottom elevations. Of the total 55 structures 45 (81.8%) had a 0.10 m or more difference in bottom elevation from the project. Of the erroneous structures 73.3% had an average of 0.40 m, and 26.7% had an average of -0.25 m difference in bottom elevations from the design projects.