Water in river is important for human beings. For better understanding the variations of nitrate concentrations in five main rivers in northern Anhui Province, China, the NH4+-N concentrations of the 168 river water samples collected during 2010 have been analyzed by probability graph. The results indicate that the concentrations of NH4+-N vary significantly, which implying the multi contributions of nitrate in the river. In comparison with the water quality standards of China, most of the samples (76.6%) can be used for agricultural and industrial purpose, and more than half of the samples (53.6%) can be used for drinking purpose. Moreover, probability plots, in combination with quantile and density plots have subdivided the concentrations of NH4+-N into five populations with four thresholds (0.28, 0.70, 1.64 and 6.14 mg/l), the concentrations <0.28 and 1.64 mg/l are considered to be representative of pristine and current environmental background, respectively, and concentrations >1.64 mg/l are considered to be representative of anthropogenic pollution.