A river in the north-west of England had the highest recorded levels of microplastic pollution in the world.
Microplastics are very small pieces of plastic which enter the water network from synthetic clothing or microbeads in cosmetics.
Researchers at the University of Manchester looked at rural streams and urban rivers at 40 sites across Greater Manchester, and discovered tiny plastic beads, fibres and fragments in all parts of the network.
This included a site on the River Tame in Denton, which the team said had the worst levels of contamination so far recorded anywhere in the world.
The scientists are calling for tighter regulations on waste flowing into urban waterways, and say pollution in these channels is a "major contributor" to contamination in the oceans.
After major flooding hit the Greater Manchester area, the researchers re-tested all of the sites - and found that much of it had been washed away.
About 70% of the microplastics stored on the river bed had been removed, they said, suggesting large quantities had been flushed into the ocean.
Prof Woodward added: "We are only beginning to understand the extent of the microplastic contamination problem in the world's rivers.
"To tackle the problem in the oceans, we have to prevent microplastics entering river channels.
"We also need to reduce our use of plastics large and small."
Earlier this year, a UK-wide ban on adding microbeads into rinse-off cosmetics and personal care products such as face scrubs, toothpastes and shower gels came into force.
Theresa May has also pledged to eliminate all avoidable plastic waste within 25 years.