Rivers and tidal currents keep 80% of microfibers from reaching oceans, study suggests

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Every time we do a load of laundry, tiny fibers of polyester escape from our clothes and slip down the drain. These microfibers, so small they can be invisible to the naked eye, are among the most common forms of microplastic in the ocean. Yet, new research published in Journal of Geophysical Research: Oceans shows that most of them may not make it that far.

Up to 80% of polyester microfibers released from wastewater treatment plants in the Salish Sea—a network of coastal waters spanning British Columbia and Washington state—are caught close to shore in rivers, estuaries, and shallow bays before they can drift into the open ocean.

Using a combination of field measurements near the Fraser River, Canada, and high-resolution computer models, Ph.D. researcher Jose Valentí-Muelas and colleagues mapped the journey of these fibers from laundry pipes to the sea floor.

The scientists found that polyester microfibers accumulate close to their sources and behind natural barriers like sills and channels, with only a tiny fraction (approximately 0.13%) escaping into the Pacific Ocean, while the rest either settle in sediments or wash up along the coastline.

How fibers move in water

Tracking fibers in water is more complicated than it sounds. Despite being denser than seawater, polyester microfibers sink more slowly because of their long, thin shape, sometimes remaining suspended near the surface for days.

To account for this behavior, the researchers used the SalishSeaCast computer model, which simulates the currents, tides, and river flows of the Salish Sea in three dimensions. Virtual particles representing millions of microfibers were released at wastewater treatment plant outlets and their movements were tracked hour by hour.

The simulations revealed that estuarine circulation—the complex movement of water driven by tides, river discharge, and ocean inflow—creates microfiber retention zones. In these areas, sinking particles accumulate in sediments or along beaches, forming natural traps that keep them from drifting farther out to sea. In channels and basins with slower currents, fibers can remain for days or weeks, giving them more time to settle onto the seabed.

To ensure their simulations reflected reality, the team collected water samples at 10 locations along the Fraser River and measured the number of polyester microfibers in the upper layers of the water column. The results matched the model predictions within an order of magnitude, confirming that most fibers indeed accumulate near their sources.

Coastlines as microfiber sinks

Beaching plays a surprisingly large role in microfiber retention, with 14% of the simulated particles washing up along coastlines. The study shows that some fibers are pushed onto the shore by tides and horizontal mixing, where they remain until eventually resuspended or buried. Rivers also influence the rate at which microfibers reach the shore.

During periods of higher freshwater flow, surface water moves faster and can carry fibers more efficiently towards beaches. Conversely, in calmer conditions, fibers may linger longer in the water column, traveling only a short distance from their source.

Sedimentation (the process by which fibers sink to the seafloor) is even more important, accounting for 31% of the particles in the simulations settling into sediments.

The combination of sedimentation and beaching forms the primary sinks for these microplastics, leaving only a small fraction available for export to the open ocean. Interestingly, fibers released in fast-moving coastal areas like the mouth of Juan de Fuca Strait traveled farther than those from slow-moving estuaries like Puget Sound, emphasizing how local hydrodynamics shape microfiber pathways.

Implications for pollution management

These findings carry important implications for environmental management. Even with high retention near their sources, microfibers don't disappear. Sediments can act as long-term reservoirs, releasing fibers back into the water during storms or strong tidal events.

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Over time, this could make coastal sediments a persistent source of pollution, emphasizing the need for continued research into sediment dynamics, resuspension mechanisms, and the long-term effects of microfiber accumulation on marine ecosystems.

But by identifying where microfibers are most likely to accumulate, scientists and policymakers can target monitoring and clean-up efforts more effectively. Estuaries and shallow bays act as natural filters, but they also become hotspots of microfiber pollution, potentially affecting local wildlife and sediment health, as well as local populations.

Reducing microfiber emissions at the source through improved laundry filtration, wastewater treatment, or changes in textile production remains critical.

The study also highlights a broader lesson: understanding the journey of microplastics requires looking beyond the open ocean. Coastal dynamics, river flows, and tidal currents shape the distribution and fate of these pollutants, meaning that local solutions could have global consequences.

Written for you by our author Hannah Bird, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

J. Valentí‐Muelas et al, Polyester Microfiber Dynamics in an Estuarine Semi‐Enclosed Basin, Journal of Geophysical Research: Oceans (2026). DOI: 10.1029/2025jc023366

Journal information: Journal of Geophysical Research: Oceans

Key concepts

microplastic contaminationcontaminant transportsediment transportcoastal ecosystemsestuarine ecosystems

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