A woman in her fifties has died after plunging over Niagara Falls, it emerged today.

Tourists watched in horror as the woman, who had been swimming in the river above the Horseshoe Falls, got caught in the rapids and was thrown over to her death.

Witnesses said the woman appeared to be alive when she went over yesterday afternoon but had died by the time rescuers reached her in the river below.

Tourists called authorities when they saw the woman, who has not yet been identified, being swept towards the waterfall's edge.

Witnesses on the famous Maid of the Mist tourist boat said rescuers were on the scene as the woman neared the Falls and were forced to watch helplessly as she plummeted over them.

She will have fallen 53 metres to the river below, at a speed of about 32 kilometres per hour.

The woman's body was pulled from the river at about 4.45pm on Tuesday by crew members on board the Maid of the Mist.

Constable Nilan Dave told WIVB two workers on board the boat helped to recover the body.

The circumstances which led to the woman being in the water are not yet clear but foul play is not suspected, police said.

The death is being investigated by the Niagara Parks police and Niagara Regional Police.

On Monday, two men came within 700 feet of the waterfall edge before they were pulled to safety in their small boat.

The motor had failed on the boat, allowing the current to carry them ominously towards the brink.

Rescuers managed to hook the craft away from danger just in time with a rescue line.