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A series of annual natural runoff recorded at eight hydrological stations in the Yellow River basin (YRB) and distributed at the main channel and two sub-basins (Weihe basin and Fenhe basin) are studied during the period from 1951 to 1998. The trends and the beginning points (or abrupt changes) of these series are detected with Mann-Kendall test (M-K test) and its progressive application. The results show that there are similar trends for different stations. The annual natural runoff series, except for stations of Lanzhou, Hekouzhen, and Longmen, have a downward tendency at some stations whose beginning points of the trend or abrupt changes occurred in the early or mid 1990s. However, their initial years of decrease are different. The decreasing trend of runoff has led to drying-up in lower reaches of the YRB, a water resources scarcity crisis, and a deterioration of the ecosystem. The reasons for the trends have been analyzed in this paper The trends of precipitation in the corresponding regions are similar to those of annual natural runoff It is an important aspect for runoff decrease. In addition, human activities have changed the hydrological cycle and lead to a reduction in runoff to a certain degree. Finally, two countermeasures for the runoff decrease are put forward. One is to develop water savings in irrigation regions, and the other is to plan South-to-North water transfer projects for the YRB. However the conflict between supply and demand will still exist for a long time because of the water scarcity in the future. [ABSTRACT FROM AUTHOR]