Hydrological effects of the cascade reservoir in the Middle Yarlung Zangbo River

High-altitude regions, with their unique climatic and geographical conditions, render the Yarlung Zangbo River basin an ideal natural laboratory for investigating river system responses to global climate change and anthropogenic activities. Focusing on the Zangmu-Jiacha cascade reservoirs in the middle reaches, we analyzed observed hydrological and water temperature data from 2000—2022 to dissect downstream impacts. Using the upstream Yangcun station as a reference and defining pre- and post-dam periods by reservoir commissioning, we systematically examined alterations in temporal dynamics, intra-annual distribution patterns, and hydro-thermal relationships. Our results reveal that following Zangmu Dam operation, annual, flood-season, and non-flood-season discharge, along with annual minimum flows, increased downstream at Nuxia station, while annual maximum flows slightly decreased. Concurrently, annual and flood-season thermal flux increased downstream, while the annual water temperature range converged towards upstream reference conditions—a phenomenon amplified after the subsequent Jiacha Dam operation. Intriguingly, reservoir operations induced contrasting seasonal patterns: the downstream intra-annual flow distribution flattened （reduced seasonality） relative to upstream, whereas the water temperature distribution became more peaked, exhibiting increased upstream-downstream synchronization characterized by amplified warming during rising temperature periods and intensified cooling during falling temperature periods. Increased discharge during November to May was primarily linked to reservoir regulation, whereas increases from June to October reflected combined influences of regulation and augmented glacier meltwater contributions. Furthermore, the development of the cascade intensified both the asynchrony in flow—water temperature correlations and the time lag in hydro—thermal relationships between upstream and downstream sites.