Base flow, in addition to quick flow, is one of two main components that make up the entire drain of the stream. Base flow represents water from the underground storage and delayed sources (e.g. melting ice and snow). It is present even during drought periods, thus ensuring the flow of water in the stream. Based on the measured discharge, the base flow index (BFI) can be calculated. This is defined as the dimensionless ratio of the volume of the base flow and the total volume of water in the stream. BFI is a good indicator of the effects of geology on low flow rates and for this reason it is often used in a number of regional studies of low flows. First part of the thesis presents components of hydrograph and base flow on theoretical grounds. It shows the methods of elimination of base flow: graphical methods, automated procedures, physical approaches, chemical approaches. In addition is a detailed description of the program, BFI for Windows, by which is excreted base flow and calculated base flow index. The second part of the thesis presents the results of the analysis of 196 gauging stations of the national hydrological monitoring, obtained by using software BFI for Windows. Calculated mean BFI values of the individual gauging stations are divided into classes with an interval of 0.2 and shown on the map. The results led to the conclusion that BFI is depended on the geology in a great manner, but at the same time also on other factors, e.g. catchment slope.