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The 4.2 ka event is widely presumed to be a globally widespread aridity event and has been linked to several episodes of societal changes across the globe. Whether this climate event impacted the cultural development in south-central China remains uncertain due to a lack of regional paleorainfall records. We present here stalagmite stable carbon isotope and trace element–based reconstruction of hydroclimatic conditions from south-central China. Our data reveal a sub–millennial scale (~5.6 to 4.3 ka) drying trend in the region followed by a gradual transition to wetter conditions during the 4.2 ka event (4.3–3.9 ka). Together with the existing archaeological evidence, our data suggest that the drier climate before 4.3 ka may have promoted the Shijiahe culture, while the pluvial conditions during the 4.2 ka event may have adversely affected its settlements in low-lying areas. While military conflicts with the Wangwan III culture may have accelerated the collapse of Shijiahe culture, we suggest that the joint effects of climate and the region's topography also played important causal roles in its demise. 

Abstract Global expansion of cyanobacterial harmful algal blooms, driven by anthropogenic nutrient loading and climate change, poses escalating threats to water security. While altered precipitation regimes are recognized as critical climate multipliers, their hierarchical controls on cyanobacterial harmful algal bloom dynamics remain poorly quantified, particularly in stratified reservoirs. Here, we integrate a 60‐yr (1961–2020) trend analysis of rainfall extremes with 4 yr (2017–2020) of high‐frequency monitoring of hydrology, nutrients, and phytoplankton through manual sampling and autonomous buoys in Lake Qiandaohu, a deep subtropical reservoir vital for drinking water in eastern China. Using Mann–Kendall trend analysis, random forest, and partial least squares path modeling, we elucidate the hierarchical mechanisms linking altered rainfall regimes to phytoplankton community shifts, with a focus on cyanobacterial harmful algal blooms. Our results reveal significant increases in both annual rainfall and rainstorm frequency over the past six decades, with peak events coinciding notably with early summer agricultural fertilization. Intensified early summer rainstorms triggered disproportionate phosphorus loading, significantly lowering N/P ratios and transiently suppressing cyanobacterial dominance. However, a critical approximately 20‐d lag period followed these events, wherein rapid thermal stratification recovery, combined with persistent legacy nutrients, selectively favored cyanobacterial proliferation. Furthermore, subsequent continuous dry‐hot spells amplified cyanobacterial dominance by enhancing water column stability and elevating temperatures. These findings reveal a paradoxical post‐storm vulnerability window that facilitates bloom formation in climate‐sensitive reservoirs. Given projected intensification of rainfall extremes under climate change, this study highlights the urgent need for integrated management strategies combining watershed nutrient control, meteorological forecasting, and adaptive reservoir operation to mitigate cyanobacterial harmful algal bloom risks.