Alien duckweed water ferns are being blamed for turning a river red.
Dog walkers and nature lovers have noticed the changing colour of the water because of the plant at Gypsy Corner in Cuckmere Valley.
The fern is spreading across rivers in Sussex and is virtually impossible to kill.
Confused sheep have been known to drown thinking they could walk across the thick mat of brick-red weed which originally comes from North America.
Fran Southgate, an officer at the voluntary Sussex Otters and Rivers Project, said: "We have got native duckweeds that can form dense mats, but they're not as bad as this one."
She said the weed was now so widespread it could never be eliminated, only controlled, and called on people not to put plants from their ponds or gardens in the wild as they often disturbed the ecosystem.
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