Probability behaviours and trends of hydrological extremes, the 7-day high flow (HF7) in this study, of the Poyang Lake basin are thoroughly investigated using 11 probability distribution functions and Mann-Kendall trend test method. Parameters and goodness-of-fit of the probability functions are estimated using the L-moment technique and Kolmogorov-Smirnov statistic D (K-S D). The hydrological data are extracted from six major hydrological stations located in the five main tributaries of the Poyang Lake basin. The results indicate that: (1) the Wakeby distribution is the good choice for description of the statistical behaviours of HF7 in the study region; (2) changing properties of HF7 are varying from on station to another, implying different fluvial geomorphological features, climate changes, and human activities within different tributaries. Generally, the HF7 events are increasing after the construction of water reservoirs except the Wan'an water reservoir. The analysis results demonstrate considerable influences of water reservoirs on high flow changes. Relatively low design standards of the water reservoirs greatly limit the functions of the water reservoirs in terms of flood control. Besides, most water reservoirs were built in the tributaries of the Poyang Lake basin, exerting limited influences on the streamflow extremes; (3) higher frequency of hydrological extremes may cause higher probability of floods, posing increasingly serious challenges for flood control. In this sense, more efforts should be paid to enhance human mitigation to natural hazards such as floods in the Poyang Lake basin. Copyright © 2011 John Wiley & Sons, Ltd.