Abstract: Small mountainous rivers play an important role in the global sediment budget between the land and the sea. Previous studies mostly focus on sediment flux to the oceans in addition to estuarine environment, but rare on the related water cycle. The seasonal compositions of δD and δ¹⁸O of river water from small rivers (such as Jiaojiang and Mulanxi rivers) and their estuaries in Zhejiang and Fujian provinces are investigated in this study. Combined with the precipitation and river hydrological data, we discussed the temporal and spatial variations in δD and δ¹⁸O of river water and their influence factors. It is found that the δ¹⁸O of river water are very different depending upon water sources. In the study areas, river water is mainly sourced from precipitation in summer and from groundwater in winter. Spatially, the δ¹⁸O of mainstream water in this area is relatively constant in summer while increases from the inland to the estuary in winter due to the evaporation effect. Under the effects of precipitation and the latitude effect on groundwater, the δ¹⁸O of river water decreases from south to north in winter. In the estuary area, however, the δ¹⁸O values correlated well with the salinity. Together with the data from other estuaries in eastern China, this paper offers a new reference for further study of the spatial and temporal variation of coastal current in the east coast of China.