Are hydropower dams and sand mining responsible for hydrological change in the Red river (Asia)?

• This study presents up-to-date discharge data from seven sites, covering up to 2023. • Upstream discharge decline is due to cascading hydropower dams along the main river. • Midland groundwater discharge increase is due to sand mining and reduced sediment. The Red River (RR) in Southeast Asia provides water for food and energy production, building materials in the form of sand mining and facilitates the movement of people and goods for millions of inhabitants in the Yunnan province in China and provinces in northern Vietnam. Since the late 20th century, rapid economic development in the region has led to significant human-induced changes to the RR. In this study, we applied multiple statistical tests (Mann-Kendall, Pettitt, Mann-Whitney and principal component analysis) to analyse daily water flow data from seven hydro-meteorological stations in the lower section of the RR, aiming to identify the primary factors altering the water flow regime. Our findings indicate that the Hoa Binh reservoir, commissioned in 1989, and the Son La reservoir, commissioned in 2010, have significantly modified the flow regime, reducing the annual mean water flow of the RR main tributary from 1763 m 3 s −1 between 1986 and 2008 to 1334 m 3 s −1 between 2009 and 2023. This study is the first to identify that widespread and loosely regulated sand mining in the lower RR region has not only lowered the riverbed and water levels, as reported in recent studies, but also increased groundwater discharge into the river. As a result, despite the reduced upstream water flow from the mountains, the river discharge in the downstream region has remained statistically unchanged over the past decade in the RR.