The tiny polyethylene and polypropylene beads are an emerging concern among scientists and environmentalists. The beads come mostly from personal care products such as facial exfoliants and body washes. They are not biodegradable, however, and because they are not removed easily by wastewater treatment plants, they flow out to sea and enter the food chain.

“Microplastic is now a ubiquitous contaminant in the Pacific Ocean — and seas around the world,” said Eriksen, a scientist with the 5 Gyres Institute, a nonprofit dedicated to researching plastics in the world’s waterways. “We believe that 80% of it comes from coastal watersheds like Los Angeles.”

Eriksen is just starting to test the Los Angeles River to determine if it holds microbeads, and if so, their source. On Friday, he found what he was looking for in about 10 minutes.

Near the confluence of the river and Arroyo Seco, about three miles north of downtown, Eriksen found a handful of algae and wriggling leeches speckled with tiny filaments, shards and beads that could have come from myriad sources: laundry wastewater, degraded plastic bags, stir sticks, personal care products.

“The scary thing is that the beads sponge up toxins, then get consumed by organisms from shellfish to crabs to fish” later eaten by humans, he said.

Scientists are only beginning to understand the hazards posed by microplastic pollution in the world’s oceans and inland waterways. In 2012, Eriksen and a team of researchers discovered large amounts of microbeads and other microplastic pollution in the Great Lakes. Those findings prompted a coalition of mayors of Great Lakes cities to ask the U.S. Environmental Protection Agency to determine the possible health risks to lake ecosystems and humans.