VOLUNTEERS from the Environment Trust found two pistols and a knife during a clean-up of the riverbed in Twickenham.
They had expected to clear the waterway of rubbish and weeds, but instead found a bag containing the weapons.
They took their unusual finds straight to the local police station.
A police spokeswoman said: "The volunteers found two rusty handguns in a bag during their clean-up. They were handed in to Twickenham police, where an officer made them 'safe'. They will be 'forensicated' and any results will be issued in due course."
The clearing was made possible thanks to the Port of London's draw-off of water, which lowers the river to normal levels to allow lock maintenance.
It also gave the volunteers a chance to explore the ecology and archaeology beneath the surface of the water.
The trust's ecologist, Joe Pecorelli, said: "We are delighted to have found a healthy diversity of aquatic animals, including larvae of the mayfly, a creature which will only inhabit pollution-free waterways."
But he said he was disappointed to see an increase in the number of zebra mussels, the greatest density he had seen in 10 years working the Tideway.
The zebra mussel (dreissena polymorpha) is native to the freshwater lakes of Russia, and is thought to have travelled to the UK in the ballast water of cargo ships.
It spreads rapidly, causing disruption to river ecology. Now the trust will work with the Environment Agency to assess the spread of the species.
The trust's next event, at Richmond riverside, is on Saturday. Volunteers should meet at the Bridge Boathouse at 9.30am.
For more details visit www.environmenttrust.co.uk