Thick weeds which have already caught small boats in their grasp are a "danger to life" alarmed boaters have warned.
The dangerous water plants are lurking in a stretch of the River Yare in Thorpe St Andrew between The Town House and Thorpe Western Junction Railway Bridge.
It is understood the Broads Authority has been speaking to water-based businesses in the area and is hoping to resolve the issue in the next 10 days.
Mark Appel, owner of Bishy Barney Day Boats in Thorpe St Andrew, has been located at the River Green for five years and said he has never seen the weeds as bad before.
Mr Appel said: "There is a risk to life. The weed is so thick and heavy that if someone did fall in they would struggle to get out of it - especially if you are under it.
"That is not being overdramatic.
"The weather was suddenly so sunny and dry that the weeds had a massive head start and came to full bloom in April."
Thorpe St Andrew county councillor Ian Mackie (Cons) said: "It’s not just a question of inconvenience in the summer but one of public safety.
"I urge the Broads Authority to act as soon as possible."
The president of Yare Boat Club, who did not wish to be named, added: "We often get weeds growing at this time of the year particularly the other side of the river from us where it is shallower."
A joint statement from the Environment Agency and Thorpe St Andrew Town Council said: "The Broads Authority have been made aware of more water-plants currently in the area than may have been present in previous months.
"During warmer weather these plants are likely to be growing more vigorously due to favourable temperatures and conditions, although after October they will die down naturally.
"The Broads Authority’s water-plant cutter does several cuts of the River Yare between April and October and will be heading to Thorpe St Andrew within the next few weeks."
It comes as the river has had problems with sewage spewing into the water in Thorpe St Andrew's River Green due to a misconnection into the surface water system.
The Broads Authority has been contacted for comment.
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