In the past few years Slovenia has been facing frequent flooding. One of the main reasons is decreasing the bankfull discharge capacity. The graduation thesis compares the bankfull discharge of the Želimeljščica river in 1985 and 2015. The source of data from 1985 was the project Ureditev Želimeljščice made by the company Hidrotehnik d.d., the data for 2015 were gathered by the author of this thesis herself. Stream cross section profile was measured by using a tape measure and a meter ruler. The measurements of the stream flow were made using the Flo-tracer instruments, followed by the calculation of Mannings roughness coefficient from the acquired data. For hydrological calculations fort the years 1985 and 2015 the author used one-dimensional mathematical model of the hydraulic programming environment HEC-RAS. The results of the cross section chanell area, values of the Manning´s roughness coefficient and bankfull discharge capacity between the years 1985 in 2015 were compared.