This study focused on the hydrological and runoff formation processes of river water by using stable isotope tracing in the source regions of the Yangtze River during different ablation episodes in 2016 and the ablation period from 2016 to 2018. The effects of altitude on stable isotope characteristics for the river in the glacier permafrost area were greater than for the main stream and the permafrost area during the ablation period in 2016. There was a significant negative correlation (at the 0.01 level) between precipitation and δ¹⁸O, while a significant positive correlation was evident between precipitation and d-excess. More interestingly, significant negative correlations appeared between δ¹⁸O and temperature, relative humidity, and evaporation. A mixed segmentation model for end-members was used to determine the proportion of the contributions of different water sources to the target water body. The proportions of precipitation, supra-permafrost water, and glacier and snow meltwater for the main stream were 41.70 %, 40.88 %, and 17.42 %, respectively. The proportions of precipitation, supra-permafrost water, and glacier and snow meltwater were 33.63 %, 42.21 %, and 24.16 % for the river in the glacier permafrost area and 20.79 %, 69.54 %, and 9.67 %, respectively, for that in the permafrost area. The supra-permafrost water was relatively stable during the different ablation periods, becoming the main source of runoff in the alpine region, except for precipitation, during the ablation period.

As the transition zone between the Tibetan Plateau and arid region, the Qilian Mountains are important ecological barriers and source regions of inland rivers in northwest China. In recent decades, drastic changes in the cryosphere have had a considerable impact on the formation process of water resources in the Qilian Mountains. In this study, 2164 environmental isotope samples were used to quantify the runoff components of 11 major rivers in the Qilian Mountains and investigate the influence of cryosphere changes on mountain runoff. The results show that the mountain runoff primarily originates in the cryosphere belt, which contributes approximately 82 %, 71 %, and 80 % to the Hexi inland water system, the upper stream of the Yellow River system, and the Qinghai inland river system, respectively. The maximum contribution ratio of glacier and snow meltwater to runoff occurred in May. The contribution of supra-permafrost water to runoff gradually increased from May and reached approximately 40 % in some rivers in October. Cryosphere degradation in the Qilian Mountains after the 1990s has caused a rapid increase in runoff, changes in the peak runoff time, and an increase in winter runoff. These changes in the hydrological processes bring opportunities and challenges to managing inland river water resources, and various adaptive measures to seek advantages and avoid disadvantages have been proposed. The findings from the environmental isotope analysis provide insights into understanding water resources and realizing the nexus of life, agriculture, industry, and ecological water use.