The 28-year-old man was taken to hospital after jumping into the River Swale in Richmond, North Yorkshire, on Thursday afternoon.
It's understood the man, who is from County Durham, was trying to cool off as the UK basked some of its hottest temperatures on record, and hit his head while jumping into the water.
He is being treated for serious injuries although his condition is not thought to be life-threatening.
A spokesman for North Yorkshire Police said they were called to The Falls at Riverside Road at 1.20pm, joining volunteers from Swaledale Mountain Rescue, ambulance staff and firefighters.
He said: “I would urge people to be sensible in the sun and water during the current hot weather.
“There is still a lot of water in the River Swale following the recent downpours and the river can be a dangerous place.”
Insp Gee thanked members of the public who provided refreshments to police and other emergency service workers at the scene today.
“We train for incidents such as this in the River Swale and this was another successful multi-agency operation.”
Meanwhile, fire brigades across the region have been reiterating the warning to not risk swimming in rivers, reservoirs or other potentially dangerous bodies of water.
Ronnie Goldwater, Station Firefighter at Bingley Fire Station, said: “Firefighters are specially trained in water rescue and attend a variety of different scenarios where people have got into trouble in the water.
“However as the temperatures increase there are also people who go into water deliberately, perhaps to cool off on a hot day, who then find themselves in trouble. Children can also get into difficulty even in very shallow water, especially if there are under currents that can knock them off their feet.”