The River Foss changed colour after tiny, rounded leaves known as duckweed gathered on the surface of the water.
An Environment Agency spokeswoman said: “It is a common duckweed and it’s naturally occurring. It thrives in slow flowing and nutrient-rich rivers such as the River Foss.
“It’s quite a common sight at this time of the year. It’s also not toxic or harmful to wildlife, so it shouldn’t be a concern at all.”
The Royal Horticultural Society says the free-floating aquatic perennials multiply rapidly and fill any open surface of water unless regularly cleared, forming a “green carpet” on the surface of the water.
York Council workers have been called out this week to try to clear the duckweed from the river.
The Environment Agency has said that duckweed is “not a sign of pollution” and it is usually flushed away when rainfall increases and flow of the river improves.
It can be treated with a chemical herbicide, but this often uses up a lot of the oxygen in the water and can cause fish and other aquatic species to