Human barriers fragment three-quarters of all rivers in the Mekong basin

Rapid expansion of human barriers threatens many of the remaining free-flowing rivers of the world. The Mekong Basin is an exemplar in this regard with previous studies indicating high levels of river fragmentation caused by barriers, despite such estimates being limited dues to incomplete data. The present study offers the first comprehensive evaluation of river fragmentation in the Mekong Basin by leveraging a comprehensive basin-scale barrier database containing 13,054 unique barriers, and by employing five dendritic connectivity index-derived indices, one barrier density-derived index, and two newly-developed basin-scale connectivity assessment methods. The results revealed striking fragmentation across the Mekong Basin, with the Thailand and Vietnam regions exhibited the most pronounced levels, characterized by lowest dendritic connectivity values, due to high numbers of irrigation facilities. Only a limited number of sub-basins in the Upper Mekong (3.4 % of all sub-basins) and Lower Mekong (10.7 %) regions remain free-flowing. Environmental correlation analysis of river barrier construction suggested densely populated low-elevation rural areas that are more likely to suffer from diminished river connectivity. This assessment enhances our current understanding of river fragmentation in the Mekong Basin and help in the formulation of strategies for future connectivity restoration efforts across the entire basin.