The role of dams as sources and sinks of plastics in global rivers

The late 20^th century witnessed a sharp proliferation in the construction of dams and in-stream barriers to address the rising global demands for urban supply, energy, and food production. Nowadays, around 16% of all global annual river discharge is regulated by approximately 58,700 large dams (>3 hm³ storage capacity) with a global storage capacity of 7,000-8,300 km³. While essential for food security and socioeconomic development, dams impose significant trade-offs by fragmenting river systems, disrupting fluvial regimes, trapping sediments and altering water quality in impounded areas. In the last decade, plastic pollution in freshwater and marine environments has garnered growing scientific attention. While extensive research has been conducted in riverine and marine environments, studies on plastic contamination in reservoir-dam systems remains scarce due to the logistical and hydrological complexities involved in the study of these systems. This work aims to provide insights into the influence of reservoir-dam systems and their operational regime on the fate of plastic pollution. We propose a conceptual model of the dynamics and hydrosedimentary processes of micro- and macroplastics within hydropower, water supply and flood control reservoirs. This is complemented with a comprehensive review of 43 research articles focused on microplastic pollution in reservoirs published between 2015 and 2024 covering up to 62 large reservoir-dam systems worldwide with an average storage capacity of 3.2 hm³, of which 62% belong to cascading dam schemes. The meta-analysis evidenced the pervasiveness of plastic pollution. Microplastics were detected in water and sediments from all studies, including the impounded regions and the upstream and downstream sections of the reservoirs. Microplastics concentration within the set of reservoirs exhibits considerable heterogeneity, with preliminary values ranging from very low to highly polluted levels (3 to 87,000 MPs/m³ in water and 20-20,070 MPs/kg in sediments). Such values are highly conditioned by land use, topography, seasonal rainfall patterns and sampling periods and the different settling rates of microplastics among others. The concentration of plastics decreases by several orders of magnitude downstream of dams in a significant proportion of case studies (65%), particularly in cascading dam systems, suggesting that reservoirs act primarily as key sinks for plastics due to their low-energy hydrodynamics. This is highly relevant for the formulation of transport models estimating plastic input to the ocean. However, growing evidence indicates that water management operations can significantly accelerate the transport of plastics towards downstream regions. Water discharge and sediment flushing operations can release substantial quantities of low-density plastics and microfibers downstream, thereby accelerating their flux to estuaries. Similarly, reservoirs that convey water directly to purification plants, irrigation canals or pumping stations can contribute to the removal of plastics from the fluvial system. These findings are particularly important for understanding the role of reservoir regulation in mediating plastic transport from inland water systems to the ocean, as well as for developing more effective strategies to mitigate microplastic contamination.