This study systematically analyzed 2321 water samples to determine the spatiotemporal supply patterns of different runoff types in the source region of the Three Rivers from 2019 to 2021 using stable isotope tracing and endmember-element mixed runoff segmentation modeling. The results indicate that the delta 18O and delta D values of the tributary were more negative than those of the mainstream. The mainstream and tributary were primarily recharged by precipitation (61 %) and supra-permafrost water (53 %), respectively. Furthermore, with increasing altitude, the proportions of precipitation and glacial meltwater in the mainstream decreased and increased, respectively. Three river-water supply patterns were observed: (1) water was stored in glaciers and snow after solid precipitation events occurred, and the runoff was directly replenished by the melting of glaciers and snow; (2) various surface water bodies were mixed to recharge the supra-permafrost water, which replenished the runoff; and (3) lake water and wetlands were supplied by various surface water bodies, including surface runoff. These findings reveal the evolutionary mechanism of runoff in cold regions under climatic warming and provide crucial information for water resource management and sustainable development.