Photos released by Angling Trust Yorkshire show an area of water at Sprotbrough Lock covered by lush greenery – making it look like the area is dry land and safe to walk on.
But the plants are in fact debris gathered behind the raft across the river.
A spokesman said the patch was a potential hazard to anglers, dog walkers and “especially children.”
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An Angling Trust Yorkshire spokesman said: “This is a build-up of rubbish and debris with water fern and floating pennywort in abundance.
"It appears that Himalayan balsam had taken root on the raft. For all intents and purposes, the area looks like dry land.
“This is not the case and will potentially tear open if walked upon. The matter has been raised with the Canals and Rivers Trust for their attention.”