The Surface Water Department of the Czech Hydrometeorological Institute in 2006 selected a new period for the evaluation of hydrological characteristics of mean daily discharge series. Initially, two fortyyear long periods were considered: 1961-2000 and 1966-2005 and the period of 1961-2005 was finally accepted. Comparison of the still used 1931-1980 period with the new one was carried out using the data from 50 Czech gauging stations that had continuous data time series. The mean long-term runoff of Czech rivers changed very little from 1931-1980 to 1961-2005. Comparison of the variation coefficients brought similar results. However, among specific M-day discharges the low flow values (e.g. Q355d) showed a significant increase. This might reflect not only natural causes but could suggest anthropogenic factors as well. Trend analyses started with testing mean annual runoff at 65 Czech gauging stations that were not influenced by human activity. 34 stations showed an increasing trend and 31 a decreasing trend. However, the trend was statistically significant only at one station. Trends of individual months dramatically differentiate. In January through March the increasing trends prevail. March trends show the highest number of stations with a significant increase. The three following months are complete opposites in which decreasing trends dominate. In May and June, not a single increase was recorded and many decreases were statistically significant. Almost no significant trends were examined in the months of the second half of the year. and V posledních měsících se oddělení povrchových vod ČHMÚ zabývalo výběrem období pro výpočet nových hydrologických charakteristik M-denních průtoků na českých tocích. Uvažována byla čtyřicetiletí 1961-2000 a 1966-2005, nakonec však bylo přijato období 1961-2005. Na datech z 50 vodoměrných stanic, které měly nepřerušené řady pozorování, bylo uskutečněno první srovnání statistických charakteristik období 1931-1980 (doposud používané) a 1961-2005. U nového období byly u neovlivněných stanic vyhodnoceny trendy průměrných ročních průtoků a průtoků jednotlivých měsíců.