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Previous studies showed that continuous exposure to ammonia nitrogen (AN) contributed to regional losses of benthic invertebrate diversity in China. Yet, the overall ecological risk of AN to aquatic organisms in major riverine systems of China has not been appropriately studied. Our research then investigated temporal (seasonally/yearly) and spatial distributions of AN and un-ionized ammonia (NH₃) in major Chinese river basins using historic data generated between 2007 and 2014, and developed risk assessment criteria. Our results showed that the highest average AN concentrations occurred during winter (0.82–2.76 mg/L) and the lowest during summer (0.36–0.78 mg/L). NH₃ exhibited the opposite trend with the highest average concentrations mostly observed during spring (15.13–92.84 μg/L) and the lowest concentrations mainly during winter (10.53–45.43 μg/L). Both AN and NH₃ concentrations …