Characteristics of Runoff Changes during the Freeze–Thaw Period and the Response to Environmental Changes in a High-Latitude Water Tower

Runoff in high-latitude water towers is crucial for ecological and human water demands during freeze–thaw periods but is highly sensitive to climate change and human activities. This study focuses on Changbai Mountain, the source of the Songhua, Tumen, and Yalu rivers, analyzing runoff variation and its environmental responses using the modified Mann–Kendall method and the water–energy balance equation. The results show significant non-stationarity in runoff trends, with an increasing trend in the Yalu River basin (p < 0.05), a decreasing trend in the Tumen River basin (p < 0.05), and complex trends in the Songhua River basin. Additionally, the relationship between runoff and driving factors during freeze–thaw periods was quantized. When the snowfall, potential evapotranspiration (E0), and subsurface changes increased by 1%, the snowmelt runoff changes were 1.58~1.96%, −0.58~−1.96%, and −0.86~−1.11% in the Yalu River basin; 2.16~2.35%, −1.04~−1.35%, and −1.56~−1.95% in the Tumen River basin; and 1.44~2.41%, −0.44~−1.41%, and −0.72~−1.62% in the Songhua River basin. The increased snowfall was the most prominent reason for the increase in snowmelt runoff during spring. The results of this study will benefit ecosystem conservation and the stability of downstream water supply in this high-latitude water tower.