Algal blooms often worsen during droughts, threatening water quality. We analyzed climate and river data in South Korea’s Nakdong River to understand how drought affects phytoplankton growth. Using various drought indices and a Bayesian model, we found that a hydrological drought index (SSI) best explained changes in chlorophyll-a levels, which indicate algal biomass. Tributary areas were especially sensitive, with blooms peaking during dry spring and summer seasons. Our study helps improve early-warning systems for managing blooms as climate extremes intensify.