The NO3-N concentrations in riverine water showed significant seasonal variations. The NO3-N was ascribed to sewage in spring by δ15N-NO3 and Cl− variations. The NO3-N was induced by nitrification from synthetic fertilizers in early summer. The NO3-N in late summer and autumn was from soil organic matter or sewage/manure. Denitrification and nitrification processes controlled the river NO3-N contents.