During the observation period, the volume-weighted mean values of δ¹⁸O for streamflow at the catchment outlet, for the precipitation at the two sample sites, and for glacial meltwater were −11.9‰, −11.3‰, and −15.6‰, respectively. Calculations using the isotope hydrograph separation method, Equation (4), show that the contribution of glacial melt runoff to total runoff was 15.0%. Uncertainty results for hydrograph separation were shown in Table 1. Calculated by Equation (7), the uncertainty of the separation was ± 3.7% at the confidence level of 95%. Therefore, the contribution of glacial melt runoff was 15.0 ± 3.7% (the confidence level of 95%) at the Gaerqu River catchment. As the total runoff during the observation period was 2.55 × 10⁸ m³, the glacial melt runoff can be calculated as (0.38 ± 0.09) × 10⁸ m³.