Evolution of river regime in the Mekong River basin over eight decades and role of dams in recent hydrologic extremes

Abstract. Flow regimes in major global river systems are undergoing rapid alterations due to unprecedented stress from climate change and human activities. The Mekong River Basin (MRB) was, until recently, among the last major global rivers relatively unaltered by humans, but this is changing alarmingly in the last decade due to booming dam construction. Numerous studies have examined the MRB’s flood pulse and its alterations in recent years; however, a mechanistic quantification at the basin scale attributing these changes to either climatic or human drivers is lacking. Here, we present the first results of the changes in natural hydrologic regimes in the MRB over the past eight decades and the impacts of dams in recent decades by examining 83 years (1940–2022) of river regime characteristics simulated by a river-floodplain hydrodynamic model that includes 126 major dams in the MRB. Results indicate that while the Mekong’s river flow has undergone substantial decadal trends and variabilities, the operation of dams in recent years is causing a fundamental shift in the seasonal volume and timing of river flow and extreme hydrological conditions. Even though the dam-induced impacts are small so far and most pronounced in areas directly downstream of major dams, dams are intensifying the natural variations in the Mekong’s mainstream wet season flow. Further, dams have exacerbated drought conditions by substantially delaying the MRB’s wet season onset, especially in years when the natural wet season durations are already shorter than in normal years (e.g., 2019 and 2020). Further, dams have shifted up to 20 % of the mainstream annual volume between dry and wet seasons in recent years; while this has minimal impact on the MRB’s annual flow volume, the flood occurrence in many major areas of the Tonle Sap Lake and Mekong Delta have been largely altered. This study provides critical insights on the long-term hydrologic variabilities and impacts of dams on the Mekong’s flow regimes, which can help improve water resources management in light of intensifying hydrologic extremes.