Stretches of the Leeds and Liverpool Canal, including Burnley's famed 'Straight Mile', have been enveloped in a thick green duckweed for most of the summer.
Walkers, cyclists and barge owners will have noticed the duckweed which has persisted for several months now on stretches of the canal in Burnley, including at Rosegrove and the Burnley Embankment, known locally the Straight Mile.
Although the duckweed is not classed as dangerous, it has proved unsightly recently as it is so thick any litter dropped in the canal flows on its surface.
The Canal and River Trust, which looks after Britain's canal network, has advised people to enjoy the canal as normal.
A spokesman for the Trust said: “Duckweed is the world’s smallest flowering plant, which multiplies rapidly in hot weather.
"It is not harmful to humans or animals and the Trust encourages everyone to continue to enjoy the big outdoors along the Leeds and Liverpool Canal, but urges people with young children or dogs to take extra care when they see the weed.
"We’re monitoring the situation and we’re hopeful with the cold weather starting it should disperse shortly."
Heavy rain at the weekend has appeared to disperse much of the duckweed on the Straight Mile which carries the canal 60ft above the town.
It was built between 1796 and 1801. Though costly to construct, it meant the valley could be traversed by the canal without the need for two systems of locks.
The embankment, an innovative solution to the problems of canal engineering in its time, remains an impressive construction today. It is widely regarded as one of the "seven wonders" of the British Waterway System.