The Tibetan Plateau (TP), a key amplifier of global climate change, has undergone widespread lake expansion, altering regional hydrology through intensified lake—atmosphere interactions. This effect is particularly pronounced during the autumn (October—November) after the monsoon; however, its mechanisms and downstream impacts remain underexplored. This study integrates high-resolution Weather Research and Forecasting (WRF) model and reanalysis data (1980—2019) to quantify how TP lake clusters regulate moisture transport and water availability via two competing mechanisms: retention (local moisture recycling) and compensation (downwind moisture export). In strong lake-effect (LE) years, intensified thermal convection and weak westerlies increase precipitation over lakes (retention), reducing moisture transport to the Salween, Mekong, Yangtze and Yellow River source regions, thereby decreasing downstream LE precipitation and water availability. In contrast, weak LE years, characterised by stronger westerlies, promote efficient LE moisture export (compensation), increasing LE precipitation and water availability in the downstream basins. A long-term decline in LE water availability over the lake region (−0.18 Gt per decade, p < 0.05) is correlated with an increase in river basins (+0.04 Gt per decade, p < 0.1), driven by enhanced lake evaporation and subsequent downwind precipitation. Extreme years (e.g. 1997 vs 1999) demonstrate the basin-scale impacts of these mechanisms, with the Yellow River source facing a 0.45 Gt deficit in retention-dominated years but a 0.73 Gt surplus during compensation phases. These findings redefine TP lakes as critical retainers and redistributors of water resources, directly affecting dry-season water security across river basins under climate change.