The illegal dumping of sofas, fridges and mattresses in a south London river has put 450 residential and commercial properties at risk of flooding.
The Environment Agency (EA) has begun a massive clean up operation in Tooting and South Wimbledon that will cost over £1 million.
EA officers have the task of clearing debris from an 800 metre stretch of the River Graveney, which in parts of Tooting runs underground.
In the event of a flood, these blockages could have a potentially devastating effect.
EA Improvements team leader Matthew Graham said: "Fly-tipping is a serious problem in any area, but dumping into rivers can impact its ability to cope with flooding - increasing the risk for properties and public in the area.
"This work is extremely expensive due to the high-risk nature of the clearance, and it is frustrating that valuable resources need to be spent rectifying this sort of problem."