According to the Royal Horticultural Society, duckweed species “form large mats that cover the water’s surface”.
The plants “grow and multiply rapidly in warm, nutrient-rich, still or slowly moving water”, it stated.
People walking by the River Foss where the amount of duckweed was at its height said they were certainly worried about the effect it was having.
Student Ruby Reay said: “It’s just not very pleasant because it’s also got to affect the wildlife.”
Ms Reay described the duckweed growth as “thick, almost phlegmy-looking”.
Meanwhile, David Bayliss said: “It looks like a stagnant and mucky river. I wouldn’t even fish in there.”
Mr Bayliss’ wife, Corrine, said: “It’s not a flagship for York is it? It looks like pea soup.”