Police made the discovery in the River Mersey in Chorlton, close to Darley Avenue and Barlow Moor Road, at about 09:45 GMT.
Officers were working to identify the victim and inform her family.
The death is not being treated as suspicious and police believe the woman may have suffered medical complications.

Second quarter operating profit rose nearly 5% to 3.49bn euros (Â£2.47bn).
But it listed a raft of potential problems, including "fierce competition, interest rate and exchange rate volatility, and fluctuations in raw materials prices".
The results come one day after news VW had overtaken Toyota to become the world's biggest vehicle maker.
But VW noted that robust growth in the global economy lost momentum in the first half 2015 and it said it expected the same level of growth for this year as in 2014.
VW cited risks to global financial markets resulting from "the strained debt situation of many countries".
It expects the strongest growth to come from Asian emerging economies, with moderate rates of expansion from major industrialised nations.
The company's shares are down more than 2% in Frankfurt.

Foreign Minister Mevlut Cavusoglu said US warplanes and drones were now arriving at Turkish airbases.
"We will soon launch a comprehensive battle against Islamic State all together," he said.
Last month Turkey launched its first strikes on the militants since the group began its advance through Iraq and Syria in 2013.
It had previously been reluctant to take military action, but that position changed after a number of attacks within Turkey that were blamed on IS.
Syria said on Wednesday that any military action not co-ordinated with Damascus would breach its sovereignty.
Turkey agreed to let the US military use the Incirlik airbase, in the south-west of the country, in July.
"As part of our agreement with the US we have made progress regarding the opening up of our bases, particularly Incirlik," Mr Cavusoglu told reporters as he met US Secretary of State John Kerry in Malaysia on Wednesday.
"We're seeing that manned and unmanned American planes are arriving and soon we will launch a comprehensive battle against Islamic State all together."
'More effective strikes' - Selin Girit, BBC News, Istanbul
Coalition planes currently take off from bases in the Gulf to hit IS targets in Syria. The use of Turkish bases, especially Incirlik, will potentially make the strikes much more effective.
Because of Turkey's proximity, coalition warplanes will be able to reach targets in Syria in less than an hour and spend up to six hours in Syrian airspace.
One Turkish official said the operations could start any moment now. That might please many in Turkey, and outside, who have been calling for a tougher stance against IS.
But there is serious opposition to any Turkish boots on the ground in Syria and concern about the proposed "safe zone" there.
Many suggest that as Turkey only has an interim government - since the June general election - it should not take such serious decisions as engaging in a "comprehensive battle".
The US military is familiar with the southern Turkish base, and its recent history is tied closely with recent US military operations.
Tensions increased after a series of attacks on the Turkish side of the Syrian border.
A Turkish soldier was killed in an exchange of fire with IS fighters near the Syrian border in July.
Just days earlier a suicide bombing blamed on IS killed 32 people in the predominantly Kurdish town of Suruc.
Turkish police have arrested hundreds of people in raids against IS and Kurdish militants across the country in recent weeks.
The country has also carried out a series of air strikes, saying they were targeting militants from the Kurdistan Workers' Party (PKK) in northern Iraq as well as Islamic State militants.
But observers say PKK fighters have been on the receiving end of far more attacks than IS.
Turkish officials deny that the campaign against the Islamic State group is a cover to prevent Kurdish gains.
The government has previously faced criticism at home and abroad for not doing enough against IS, despite being part of the international coalition fighting it.
The body of an elderly woman has been found in a river in Manchester.
[NEXT_CONCEPT]
The German carmaker Volkswagen reported a rise in its quarterly profit, but noted slowing demand in key markets.
[NEXT_CONCEPT]
Turkey has said it plans to escalate its attacks on Islamic State (IS) militants in Syria.