Emergence of wet-season hydrological droughts and the development of a “brittle” river system in the Vietnamese Mekong Delta

River regulation works (RRWs) are a globally implemented strategy to enhance water security, primarily by increasing dry-season flows. However, this intervention can trigger unintended and often paradoxical consequences, fundamentally altering the nature of hydrological risk. This study investigates the profound impact of RRW development on drought characteristics in the Vietnamese Mekong Delta, a region of immense agricultural importance and ecological sensitivity. Building upon the established understanding of the Mekong’s altered flow regimen, we analyze how this engineered shift has transformed drought dynamics. Using the standardized precipitation index-3 and standardized streamflow index (SSI-3) from 2000 to 2024, we uncover a critical paradigm shift. While the frequency of traditional dry-season hydrological droughts has significantly decreased as intended, the system’s vulnerability to severe, prolonged wet-season droughts has increased dramatically. Correlation analysis reveals a significant “decoupling” between regional rainfall and streamflow in the current mega-dam era (2010–2024). The most alarming finding is the emergence of wet-season hydrological droughts, with the 2019–2020 event being the most extreme in the 25-year record, during which SSI values dropped to below −2.0 despite only a moderate meteorological drought. We introduce and define the concept of a “brittle” river system: By dampening the natural flood pulse, intensive regulation has removed the river’s inherent buffering capacity against monsoon rainfall deficits, making the delta extremely vulnerable to climatic shocks. These findings challenge the conventional understanding of drought in the Vietnamese Mekong Delta and have urgent implications for water management, emphasizing the need for strategies that enhance wet-season resilience while addressing growing risks.