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Topography–climate–vegetation–runoff relationships are important issues in hydrological studies. In this paper, based on analyzing water isotope characteristics of river water, the influence of these variables on the relative contribution of rain to river water was investigated during one rain event in the Heishui Valley of the upper Yangtze River in China. During one rain event on August 19, 2005, a total number of 182 river water samples were collected at 13 sampling sites located along the principal river course and its tributaries. The analysis of water isotopes in the principal river course and its tributaries showed that new rain water and secondary evaporation precipitation caused great variation in values of δD and high d-excess increased with altitude. Based on calculations of two-component hydrograph separation using δ 18 O, the results showed that the biggest relative contribution of new rain to river water (43%) was found in tributary B, while the smallest contribution (less than 5%) was found in tributary I. According to stepwise linear regression analysis, topography (elevation and slope) was the most important factor affecting the contributions of new rain to river water. When only vegetation variables were considered in the regression model, alpine shrub coverage proved to be negatively correlated with the contributions of new rain to river water, while alpine meadow coverage was positively correlated with the contributions of new rain. This would imply that increasing the relative coverage of alpine shrubs in this mountainous region of China may decrease the risk of flooding. [ABSTRACT FROM AUTHOR]