A teenage girl is in a serious condition in hospital after being swept away by a river on Dartmoor, Devon.
The girl, whose friends raised the alarm by mobile phone after losing sight of her at Walla Brook near Watern Tor, was in a party of 10 young people.
Police said the teenagers were among 85 groups of pupils training for the annual Ten Tors Expedition event.
In a separate incident, a man and a woman were killed when they were washed off a Cornwall harbour wall.
The couple were walking along the pier at Mullion Cove when a wave washed them into the sea in gale force winds.
An RNLI lifeboat and Royal Navy rescue helicopter were dispatched to recover them, but the pair were pronounced dead at Treliske hospital.
In the incident on Dartmoor, two other teams of young people also had to be rescued, a number of whom were suffering from hypothermia, police said.
Another teenager suffered leg injuries.
Supt Barry Marsden said it had been a major operation to rescue the teenagers from the moor in deteriorating weather.
The police, Dartmoor rescue services and the Royal Naval Air Service were involved, he said.
The alarm was raised by the first group at about 1405 GMT on Sunday.
The girl who had fallen in the river was located within about 20 minutes by a rescue helicopter and airlifted to Derriford hospital, where she was resuscitated but is described as "very poorly".
The subsequent two groups were also rescued by helicopter - members from all three teams were taken to Okehampton Battle Camp for checks, then driven by ambulance to hospital.
Duty Inspector Julie Challoner said: "The Ten Tors is something they do every year, go up and down 10 big groups of rock called tors."
"It's a really big tradition in Devon across Dartmoor."
It was not known if any teachers or trainers were with the children at the time.
Weather conditions in the area were poor, with heavy rain over a 12-hour period that had swollen rivers.
The helicopter scrambled to rescue the teenager was the same one which later recovered the bodies of the man and woman from the sea at Mullion Cove.
In that incident, the alarm was raised to coastguards shortly after 1700 GMT.