Abstract: This study investigated the effects of water fluctuations induced by reservoir operations on hyporheic exchanges in an island in Manwan Reservoir of Langcan River. In-situ measurements of water level and temperature were conducted synchronously in the study area. The lateral hyporheic exchange flux was calculated, and the relationship between water temperature and water level was established. The dissolved oxygen and soluble carbon were also analysed along the hyporheic flow path. Results showed that periodic fluctuations of water level due to reservoir operations enhanced lateral hyporheic exchange in the island. The maximum variation of water level was 2.2 m, and the amount of water flowing in and out of the island reached 3 956 m³ during one filling-releasing cycle. The water exchange at the edge of the island was 4 to 5 times higher than that at the island centre. During water level rising, water temperature in island decreased and the temperature gradient was larger at the middle-bottom layer; while during water level falling, water temperature in island increased and the temperature gradient was larger in the upper-middle layer. The contents of dissolved oxygen, dissolved organic carbon and inorganic carbon decreased from 3.27 mg/L, 7.3 mg/L, 66.0 mg/L to 0.17 mg/L, 2.4 mg/L and 40.6 mg/L along the hyporheic flow path from reservoir to island, respectively. These results indicated that reservoir operations could enhance hyporheic exchange between reservoir and island, which imposed potential influences on the biogeochemical cycle of elements in the river.