The Hailuogou River, on the south-eastern edge of the Tibetan Plateau, receives a substantial portion of its flow from meltwater. The stable isotopic composition and water temperature were observed for waters collected from the main stream and selected tributaries. The results indicate that the river water is generally more depleted in 18O and 2H than groundwater, but more enriched than meltwater. The river water in the upper reaches is characterised by more negative isotopic values, the isotopic fingerprint of meltwaters, and lower temperatures. Both δ18O and δ2H increase significantly during passage from the upper basin downward, at least partly due to enrichment from tributary-inputs. There are also an altitude δ18O-effect of −0.1‰ per 100-m rise, and an altitude δ2H-effect of −0.8‰ per 100-m rise, along the main stream. The positive correlation between isotopic composition and water temperature further highlights the role of meltwater in regulating the river's isotope hydrology. The fraction of meltwater inputs over the total river flow ranges from 67% at Section 11 to 89% at Section 7, in the proglacial headwater region. The tributary-input enrichments and ice-snow melting are the main mechanisms controlling the isotopic composition of river runoff, but depending strongly on altitude.