<p>In mountainous areas within seasonally frozen regions, groundwater level dynamics is complex due to climate, landforms and geological conditions. Understanding the mechanism of groundwater level dynamics is crucial for restricting groundwater resource exploitation and protecting the mountain ecological environment. Previous studies have revealed that there is an abnormal groundwater level dynamics in river valleys during the freezing period. However, the mechanism of abnormal groundwater level dynamics in river valleys is still unclear. This study explored groundwater level dynamics mechanism in different positions of river valleys based on in-situ groundwater level monitoring and numerical simulation, and the groundwater level dynamics patterns in river valleys with different characteristics were analysed. The findings are as follows: (1) For the first time, it is discovered that an abnormal groundwater dynamic zone exists near river channel under certain hydraulic characteristics, where groundwater level rises above the initial value during the freezing period. (2) The mechanism of groundwater level variation within the abnormal zone has been clarified: aqueous medium permeability evolution in hyporheic zone primarily drive abnormal groundwater level fluctuation during the freezing period near the river channel, with river water level variations influencing the width of the abnormal zone. (3) For river valleys with different hydraulic characteristics, there are differences in groundwater level dynamic patterns between the far and near river channel areas, and the fundamental reason lies in the differences in groundwater recharge/discharge. In the far river channel area, deep groundwater level is only affected by lateral recharge in the freezing period, showing consistent groundwater level dynamics across valley shapes. In the near river channel area, when groundwater table depth is shallow (appearing in flat river valleys), the freeze‒thaw process of the topsoil affects the vertical groundwater migration and causes groundwater level fluctuations. When the groundwater level is lower than the maximum depth affected by topsoil freezing (appearing in steep river valleys), freeze‒thaw affect the lateral recharge intensity and control groundwater level fluctuations. The dynamic patterns of groundwater level in different river valley shapes under freeze‒thaw effects provide a new perspective for the study of groundwater level dynamics in river valley areas.</p>