Rivers do not carry plastic into the ocean in a slow, steady stream. Instead, they release it in powerful bursts especially during floods. A new study shows that when rivers swell after heavy rainfall, the amount of microplastics and mesoplastics in the water can rise dramatically, suggesting that scientists may be significantly underestimating how much plastic ultimately reaches the sea.
Researchers found that plastic transport is heavily concentrated during short periods of high water flow. If monitoring only takes place during calm, low-flow conditions, the most important part of the story may be missed.
Measuring Plastic in the Middle of a Storm
Most previous studies on river plastic pollution have focused on normal flow conditions, largely because sampling during floods is more difficult and potentially dangerous. However, floods turn rivers into high-speed conveyor belts, capable of sweeping large amounts of debris downstream.
Plastic waste that begins as larger trash on land eventually breaks down into smaller fragments microplastics (less than 5 millimeters) and mesoplastics (between 5 and 25 millimeters). These tiny particles spread across ecosystems, enter food chains, and have even been detected in human bodies.
To better understand how floods affect plastic transport, researchers conducted an intensive field campaign across four rivers in Japan. The rivers ran through mixed landscapes that included urban areas, farmland, and forests.
The team monitored six rainfall events ranging from light to heavy storms, with total precipitation between 8.8 and 117.9 millimeters. Instead of collecting just a few samples, the researchers gathered surface water every hour for 12 to 15 hours during each storm. This approach allowed them to track plastic concentrations during both the rising phase of flooding and the falling phase as water levels receded.
At the same time, they measured turbidity a common indicator of how much material is suspended in the water.
“Previous studies have shown that microplastic concentration increases significantly in flooded rivers, likely due to runoff from urban roads that carry plastic into rivers through sewer pipes and other channels,” said lead author Momoru Tanaka of Tokyo University of Science.
“In this study, for the first time in the world, we clarified this transport by directly collecting river water samples during flood events from four Japanese rivers at six times, capturing how plastic transport changes as water levels rise and fall.”
Plastic Levels Spike During Floods
The results were striking. During high-flow conditions, concentrations of microplastics and mesoplastics increased by one to four orders of magnitude compared to low-flow conditions.
In practical terms, this means that plastic levels during floods can be ten times, one hundred times, or even one thousand times higher than during calm days.
Heavy rain mobilizes debris that typically sits undisturbed on land fragments from worn tires and plastic products, litter from urban streets, and waste flushed through drainage systems. Floodwaters do not simply move more water; they gather and transport far more plastic.
Short Windows, Massive Impact
The researchers also developed a load–discharge relationship a method commonly used in hydrology to estimate how plastic load scales with river discharge. This relationship makes it possible to estimate annual plastic export using river flow records, even when continuous sampling is not feasible.
What they discovered was eye-opening: most plastic export happens during relatively short periods of high flow.
In one of the rivers studied, 90 percent of the annual mesoplastic load was transported in just 43 days. Microplastics followed a similar, though slightly less extreme, pattern.
This finding suggests that routine sampling during low-flow conditions could dramatically underestimate total plastic discharge into the ocean.
Tool for Better Monitoring
The study also found a strong correlation between turbidity and plastic concentrations. Since turbidity and suspended sediment levels are already monitored in many rivers, this connection could offer a practical and cost-effective way to estimate plastic transport more accurately.
“These findings offer valuable insights for understanding riverine transport of plastic debris during floods. Anyone can easily estimate their plastic waste volume, thereby visualizing the burden on rivers numerically,” said senior author Yasuo Nihei.
“Using the identified load–discharge relationship, the annual plastic discharge can be better visualized, supporting improved monitoring and more effective policy decisions.”
Rethinking Plastic Pollution Estimates
The study highlights a critical gap in current monitoring strategies. Floods are not a minor detail in the plastic pollution story they may be one of the main delivery mechanisms sending plastic waste from land to sea.
While the research does not fully explain why different rivers show different transport patterns, it makes one conclusion clear: to understand how plastic reaches the ocean, scientists must measure rivers during storms, not just during calm weather.
As extreme rainfall events are expected to intensify in many regions due to climate change, the role of floods in transporting plastic pollution may become even more significant in the years ahead.