Warnings have been issued following reports of people jumping and swimming in the River Witham in Boston.
Peter Hunn, Boston Borough Council’s community safety manager, said there are a host of hidden dangers which could end in tragedy for people looking to cool off.
“I don’t want to be a killjoy, but every year people die in open water because they have gone in unprepared for the dangers they face,” he said.
“If it’s hot and you want to cool off, go to a swimming pool where it’s safe.”
Coun Stephen Woodliffe, Boston Borough Council’s portfolio holder for community safety said: “We’ve had scorching weather this weekend, but rivers, drains, ponds, lakes, and open water seldom get warm in this country, even during a British summer heat wave.
“The effects of immersion in cold water can overcome even the fittest and strongest of swimmers, so resist the temptation to go in, no matter how hot you are or how temptingly cooling the water looks.”
It is easy to misjudge your swimming ability in cold open water, and young people are especially at risk from this, the council notes.
Cold water quickly saps stamina and strength and if a swimmer is a long way out from the bank they can quickly be in serious difficulty, it added.
A spokesman said: “Drowning is the third most common cause of accidental death among the under-16s.
“Objects under the water which may be unseen from the riverbank can entrap or injure swimmers.
“Boats regularly manoeuvre in the Grand Sluice and Anton’s Gowt area. There is a real danger of becoming entangled in propellers or even being caught in the wash of a boat and dragged under by the displacement of the water created by the boats’ movement.
“One of the biggest problems in this area is presented by the lock gates, a regular place where children and adults swim in Boston. The gates operate on an automated system. No warning is given prior to the gates operating. Anyone swimming in this area when this process is commencing will be drawn through the gates and washed down river.”
Other health hazards, the council notes, include ecoli, Weils Disease and even asthma and eczema.
Swimming is also prohibited in these areas and those caught may be liable to a £100 fine.